//! Immutable and mutable collections of I/O buffers.
//!
//! [`IoBufs`] and [`IoBufsMut`] present multiple buffers as one logical byte
//! stream. [`Builder`] assembles an [`IoBufs`] from encoded bytes and existing
//! byte segments, and [`EncodeExt`] encodes values into [`IoBufs`] through a
//! [`Builder`].

use super::{
    buf::{IoBuf, IoBufMut},
    panic_advance,
};
use bytes::{Buf as _, BufMut, Bytes, BytesMut};
use commonware_codec::{Buf, BufsMut, EncodeSize, Input, OwnedBytes, Write};
use std::{collections::VecDeque, io::IoSlice};

/// Container for one or more immutable buffers.
#[derive(Clone, Debug)]
pub struct IoBufs {
    inner: IoBufsInner,
}

/// Internal immutable representation.
///
/// - Representation is canonical and minimal for readable data:
///   - `Single` is the only representation for empty data and one-chunk data.
///   - `Chunked` is used only when four or more readable chunks remain.
/// - `Pair`, `Triple`, and `Chunked` never store empty chunks.
#[derive(Debug)]
enum IoBufsInner {
    /// Single buffer (fast path).
    Single(IoBuf),
    /// Two buffers (fast path).
    Pair([IoBuf; 2]),
    /// Three buffers (fast path).
    Triple([IoBuf; 3]),
    /// Four or more buffers.
    Chunked(VecDeque<IoBuf>),
}

// Clones `Pair` and `Triple` chunks one at a time so each stays in registers
// until it is stored into the result. The derived clone of a chunk array builds
// a temporary array, to drop the chunks already cloned if a later clone
// unwinds, and then copies it into place. That copy reloads the array with
// loads wider than the stores that filled it and stalls store forwarding.
impl Clone for IoBufsInner {
    #[inline]
    fn clone(&self) -> Self {
        match self {
            Self::Single(buf) => Self::Single(buf.clone()),
            Self::Pair([a, b]) => Self::Pair([a.clone(), b.clone()]),
            Self::Triple([a, b, c]) => Self::Triple([a.clone(), b.clone(), c.clone()]),
            Self::Chunked(bufs) => Self::Chunked(bufs.clone()),
        }
    }
}

impl Default for IoBufs {
    fn default() -> Self {
        Self {
            inner: IoBufsInner::Single(IoBuf::default()),
        }
    }
}

impl IoBufs {
    /// Build canonical immutable chunk storage from readable chunks.
    ///
    /// Empty chunks are removed before representation selection.
    fn from_chunks_iter(chunks: impl IntoIterator<Item = IoBuf>) -> Self {
        let mut iter = chunks.into_iter().filter(|buf| !buf.is_empty());
        let first = match iter.next() {
            Some(first) => first,
            None => return Self::default(),
        };
        let second = match iter.next() {
            Some(second) => second,
            None => {
                return Self {
                    inner: IoBufsInner::Single(first),
                };
            }
        };
        let third = match iter.next() {
            Some(third) => third,
            None => {
                return Self {
                    inner: IoBufsInner::Pair([first, second]),
                };
            }
        };
        let fourth = match iter.next() {
            Some(fourth) => fourth,
            None => {
                return Self {
                    inner: IoBufsInner::Triple([first, second, third]),
                };
            }
        };

        let mut bufs = VecDeque::with_capacity(4);
        bufs.push_back(first);
        bufs.push_back(second);
        bufs.push_back(third);
        bufs.push_back(fourth);
        bufs.extend(iter);

        Self {
            inner: IoBufsInner::Chunked(bufs),
        }
    }

    /// Re-establish canonical immutable representation invariants.
    fn canonicalize(&mut self) {
        let inner = std::mem::replace(&mut self.inner, IoBufsInner::Single(IoBuf::default()));
        self.inner = match inner {
            IoBufsInner::Single(buf) => {
                if buf.is_empty() {
                    IoBufsInner::Single(IoBuf::default())
                } else {
                    IoBufsInner::Single(buf)
                }
            }
            IoBufsInner::Pair([a, b]) => Self::from_chunks_iter([a, b]).inner,
            IoBufsInner::Triple([a, b, c]) => Self::from_chunks_iter([a, b, c]).inner,
            IoBufsInner::Chunked(bufs) => Self::from_chunks_iter(bufs).inner,
        };
    }

    /// Returns a reference to the single contiguous buffer, if present.
    ///
    /// Returns `Some` only when all remaining data is in one contiguous buffer.
    pub const fn as_single(&self) -> Option<&IoBuf> {
        match &self.inner {
            IoBufsInner::Single(buf) => Some(buf),
            _ => None,
        }
    }

    /// Consume this container and return the single buffer if present.
    ///
    /// Returns `Ok(IoBuf)` only when all remaining data is already contained in
    /// a single chunk. Returns `Err(Self)` with the original container
    /// otherwise.
    pub fn try_into_single(self) -> Result<IoBuf, Self> {
        match self.inner {
            IoBufsInner::Single(buf) => Ok(buf),
            inner => Err(Self { inner }),
        }
    }

    /// Number of bytes remaining across all buffers.
    #[inline]
    pub fn len(&self) -> usize {
        self.remaining()
    }

    /// Number of non-empty readable chunks.
    #[inline]
    pub fn chunk_count(&self) -> usize {
        // This assumes canonical form.
        match &self.inner {
            IoBufsInner::Single(buf) => {
                if buf.is_empty() {
                    0
                } else {
                    1
                }
            }
            IoBufsInner::Pair(_) => 2,
            IoBufsInner::Triple(_) => 3,
            IoBufsInner::Chunked(bufs) => bufs.len(),
        }
    }

    /// Returns the readable chunk at the given index without advancing the buffers.
    ///
    /// Returns `None` when `index` is greater than or equal to [`Self::chunk_count`].
    pub fn chunk_at(&self, index: usize) -> Option<&[u8]> {
        match &self.inner {
            IoBufsInner::Single(buf) => (index == 0 && !buf.is_empty()).then(|| buf.as_ref()),
            IoBufsInner::Pair(bufs) => bufs.get(index).map(AsRef::as_ref),
            IoBufsInner::Triple(bufs) => bufs.get(index).map(AsRef::as_ref),
            IoBufsInner::Chunked(bufs) => bufs.get(index).map(AsRef::as_ref),
        }
    }

    /// Iterates over the readable buffer owners without coalescing them.
    pub(crate) fn iter(&self) -> impl ExactSizeIterator<Item = &IoBuf> + Clone {
        (0..self.chunk_count()).map(|index| match &self.inner {
            IoBufsInner::Single(buf) => {
                assert_eq!(index, 0);
                buf
            }
            IoBufsInner::Pair(bufs) => &bufs[index],
            IoBufsInner::Triple(bufs) => &bufs[index],
            IoBufsInner::Chunked(bufs) => &bufs[index],
        })
    }

    /// Whether all buffers are empty.
    #[inline]
    pub const fn is_empty(&self) -> bool {
        // Only Single can be empty. All other variants contain readable bytes.
        match &self.inner {
            IoBufsInner::Single(buf) => buf.is_empty(),
            IoBufsInner::Pair(_) | IoBufsInner::Triple(_) | IoBufsInner::Chunked(_) => false,
        }
    }

    /// Whether this contains a single contiguous buffer.
    ///
    /// When true, `chunk()` returns all remaining bytes.
    #[inline]
    pub const fn is_single(&self) -> bool {
        matches!(self.inner, IoBufsInner::Single(_))
    }

    /// Visit each readable chunk in order without coalescing.
    #[inline]
    pub fn for_each_chunk(&self, mut f: impl FnMut(&[u8])) {
        match &self.inner {
            IoBufsInner::Single(buf) => {
                let chunk = buf.as_ref();
                if !chunk.is_empty() {
                    f(chunk);
                }
            }
            IoBufsInner::Pair(pair) => {
                for buf in pair {
                    let chunk = buf.as_ref();
                    if !chunk.is_empty() {
                        f(chunk);
                    }
                }
            }
            IoBufsInner::Triple(triple) => {
                for buf in triple {
                    let chunk = buf.as_ref();
                    if !chunk.is_empty() {
                        f(chunk);
                    }
                }
            }
            IoBufsInner::Chunked(bufs) => {
                for buf in bufs {
                    let chunk = buf.as_ref();
                    if !chunk.is_empty() {
                        f(chunk);
                    }
                }
            }
        }
    }

    /// Prepend a buffer to the front.
    ///
    /// Empty input buffers are ignored.
    pub fn prepend(&mut self, buf: IoBuf) {
        if buf.is_empty() {
            return;
        }
        let inner = std::mem::replace(&mut self.inner, IoBufsInner::Single(IoBuf::default()));
        self.inner = match inner {
            IoBufsInner::Single(existing) if existing.is_empty() => IoBufsInner::Single(buf),
            IoBufsInner::Single(existing) => IoBufsInner::Pair([buf, existing]),
            IoBufsInner::Pair([a, b]) => IoBufsInner::Triple([buf, a, b]),
            IoBufsInner::Triple([a, b, c]) => {
                let mut bufs = VecDeque::with_capacity(4);
                bufs.push_back(buf);
                bufs.push_back(a);
                bufs.push_back(b);
                bufs.push_back(c);
                IoBufsInner::Chunked(bufs)
            }
            IoBufsInner::Chunked(mut bufs) => {
                bufs.push_front(buf);
                IoBufsInner::Chunked(bufs)
            }
        };
    }

    /// Append a buffer to the back.
    ///
    /// Empty input buffers are ignored.
    pub fn append(&mut self, buf: IoBuf) {
        if buf.is_empty() {
            return;
        }
        let inner = std::mem::replace(&mut self.inner, IoBufsInner::Single(IoBuf::default()));
        self.inner = match inner {
            IoBufsInner::Single(existing) if existing.is_empty() => IoBufsInner::Single(buf),
            IoBufsInner::Single(existing) => IoBufsInner::Pair([existing, buf]),
            IoBufsInner::Pair([a, b]) => IoBufsInner::Triple([a, b, buf]),
            IoBufsInner::Triple([a, b, c]) => {
                let mut bufs = VecDeque::with_capacity(4);
                bufs.push_back(a);
                bufs.push_back(b);
                bufs.push_back(c);
                bufs.push_back(buf);
                IoBufsInner::Chunked(bufs)
            }
            IoBufsInner::Chunked(mut bufs) => {
                bufs.push_back(buf);
                IoBufsInner::Chunked(bufs)
            }
        };
    }

    /// Splits the buffer(s) into two at the given index.
    ///
    /// Afterwards `self` contains bytes `[at, len)`, and the returned
    /// [`IoBufs`] contains bytes `[0, at)`.
    ///
    /// Whole chunks are moved without copying. If the split point lands inside
    /// a chunk, the chunk is split zero-copy via [`IoBuf::split_to`].
    ///
    /// # Panics
    ///
    /// Panics if `at > len`.
    pub fn split_to(&mut self, at: usize) -> Self {
        if at == 0 {
            return Self::default();
        }

        let remaining = self.remaining();
        assert!(
            at <= remaining,
            "split_to out of bounds: {:?} <= {:?}",
            at,
            remaining,
        );

        if at == remaining {
            return std::mem::take(self);
        }

        // A prefix within the front chunk of a chunked representation splits
        // that chunk in place, without moving the deque out and back. The
        // smaller representations handle that case in their own arms below.
        if matches!(self.inner, IoBufsInner::Chunked(_))
            && let Some(prefix) = self.split_front(at)
        {
            return Self::from(prefix);
        }

        let inner = std::mem::replace(&mut self.inner, IoBufsInner::Single(IoBuf::default()));
        match inner {
            IoBufsInner::Single(mut buf) => {
                // Delegate directly and keep remainder as single
                let prefix = buf.split_to(at);
                self.inner = IoBufsInner::Single(buf);
                Self::from(prefix)
            }
            IoBufsInner::Pair([mut a, mut b]) => {
                let a_len = a.remaining();
                if at < a_len {
                    // Split stays entirely in chunk `a`.
                    let prefix = a.split_to(at);
                    self.inner = IoBufsInner::Pair([a, b]);
                    return Self::from(prefix);
                }
                if at == a_len {
                    // Exact chunk boundary: move `a` out, keep `b`.
                    self.inner = IoBufsInner::Single(b);
                    return Self::from(a);
                }

                // Split crosses from `a` into `b`.
                let b_prefix_len = at - a_len;
                let b_prefix = b.split_to(b_prefix_len);
                self.inner = IoBufsInner::Single(b);
                Self {
                    inner: IoBufsInner::Pair([a, b_prefix]),
                }
            }
            IoBufsInner::Triple([mut a, mut b, mut c]) => {
                let a_len = a.remaining();
                if at < a_len {
                    // Split stays entirely in chunk `a`.
                    let prefix = a.split_to(at);
                    self.inner = IoBufsInner::Triple([a, b, c]);
                    return Self::from(prefix);
                }
                if at == a_len {
                    // Exact boundary after `a`.
                    self.inner = IoBufsInner::Pair([b, c]);
                    return Self::from(a);
                }

                let mut remaining = at - a_len;
                let b_len = b.remaining();
                if remaining < b_len {
                    // Split lands inside `b`.
                    let b_prefix = b.split_to(remaining);
                    self.inner = IoBufsInner::Pair([b, c]);
                    return Self {
                        inner: IoBufsInner::Pair([a, b_prefix]),
                    };
                }
                if remaining == b_len {
                    // Exact boundary after `b`.
                    self.inner = IoBufsInner::Single(c);
                    return Self {
                        inner: IoBufsInner::Pair([a, b]),
                    };
                }

                // Split reaches into `c`.
                remaining -= b_len;
                let c_prefix = c.split_to(remaining);
                self.inner = IoBufsInner::Single(c);
                Self {
                    inner: IoBufsInner::Triple([a, b, c_prefix]),
                }
            }
            IoBufsInner::Chunked(mut bufs) => {
                // Count the chunks the prefix takes whole. The split lands
                // inside the next one when bytes remain.
                let mut remaining = at;
                let mut whole = 0;
                for buf in &bufs {
                    let avail = buf.remaining();
                    if remaining == 0 || remaining < avail {
                        break;
                    }
                    remaining -= avail;
                    whole += 1;
                }

                // Split in place the chunk that the prefix ends inside. The
                // bounds check above guarantees that chunk exists.
                let piece = (remaining > 0).then(|| bufs[whole].split_to(remaining));

                // Move out the side with fewer chunks, counting the piece
                // toward the prefix, and keep the deque for the other, so the
                // copying and any new deque scale with the shorter side. A
                // prefix of fewer than four chunks from a large container
                // needs no new deque.
                let prefix_chunks = whole + usize::from(piece.is_some());
                let suffix_chunks = bufs.len() - whole;
                if prefix_chunks <= suffix_chunks {
                    let prefix =
                        Self::from_exact_chunks(bufs.drain(..whole).chain(piece), prefix_chunks);
                    self.inner = Self::from_chunk_deque(bufs).inner;
                    prefix
                } else {
                    self.inner = Self::from_exact_chunks(bufs.drain(whole..), suffix_chunks).inner;
                    bufs.extend(piece);
                    Self::from_chunk_deque(bufs)
                }
            }
        }
    }

    /// Builds canonical storage from a deque of non-empty chunks and keeps the
    /// deque when it holds four or more.
    fn from_chunk_deque(bufs: VecDeque<IoBuf>) -> Self {
        if bufs.len() >= 4 {
            Self {
                inner: IoBufsInner::Chunked(bufs),
            }
        } else {
            Self::from_chunks_iter(bufs)
        }
    }

    /// Builds canonical storage from `len` non-empty chunks and allocates a
    /// deque only when there are four or more.
    fn from_exact_chunks(chunks: impl Iterator<Item = IoBuf>, len: usize) -> Self {
        if len >= 4 {
            Self {
                inner: IoBufsInner::Chunked(chunks.collect()),
            }
        } else {
            Self::from_chunks_iter(chunks)
        }
    }

    /// Splits off the first `len` bytes as a view of the front chunk when that chunk holds them
    /// all, and returns `None` when they span chunks.
    ///
    /// A prefix that leaves part of the front chunk returns right after its split. One that
    /// consumes the whole front chunk of a multi-chunk representation restores the canonical form
    /// through the outlined [`Self::remove_front`].
    #[inline(always)]
    fn split_front(&mut self, len: usize) -> Option<IoBuf> {
        let front = match &mut self.inner {
            IoBufsInner::Single(buf) => return (buf.len() >= len).then(|| buf.split_to(len)),
            IoBufsInner::Pair([front, ..]) | IoBufsInner::Triple([front, ..]) => front,
            IoBufsInner::Chunked(bufs) => bufs.front_mut()?,
        };
        if front.len() > len {
            return Some(front.split_to(len));
        }
        if front.len() < len {
            return None;
        }
        let prefix = std::mem::take(front);
        self.remove_front();
        Some(prefix)
    }

    /// Removes the empty front chunk of a multi-chunk representation and
    /// restores the canonical form.
    fn remove_front(&mut self) {
        match &mut self.inner {
            IoBufsInner::Single(_) => {}
            IoBufsInner::Pair(_) | IoBufsInner::Triple(_) => self.canonicalize(),
            IoBufsInner::Chunked(bufs) => {
                bufs.pop_front();
                if bufs.len() <= 3 {
                    self.canonicalize();
                }
            }
        }
    }

    /// Takes a prefix that spans chunks, copying it once.
    // Kept out of line so the inlined front-chunk fast path stays small.
    #[inline(never)]
    fn take_spanning<T: OwnedBytes>(&mut self, len: usize) -> T {
        // A prefix that spans chunks is copied once, into one `IoBuf` when `T` claims `IoBuf`
        // views and into `Bytes` otherwise. Claiming an empty view tells which case applies.
        if T::claim(IoBuf::default()).is_ok() {
            T::claim(self.split_to(len).coalesce())
                .unwrap_or_else(|view| T::from_bytes(view.into()))
        } else {
            T::from_bytes(self.copy_to_bytes(len))
        }
    }

    /// Coalesce all remaining bytes into a single contiguous [`IoBuf`].
    ///
    /// Zero-copy if only one buffer. Copies into one native heap allocation
    /// if multiple buffers, so the result supports zero-copy
    /// [`IoBuf::try_into_mut`].
    #[inline]
    pub fn coalesce(self) -> IoBuf {
        match self.inner {
            IoBufsInner::Single(buf) => buf,
            inner => {
                let bufs = Self { inner };
                let mut out = IoBufMut::with_capacity(bufs.remaining());
                bufs.for_each_chunk(|chunk| out.put_slice(chunk));
                out.freeze()
            }
        }
    }
}

impl Buf for IoBufs {
    /// Returns the prefix as a view of the front chunk when that chunk holds it all, and copies it
    /// once when it spans chunks. A `T` that claims [`IoBuf`] views receives the [`IoBuf`], and
    /// any other `T` is built from its bytes.
    // Inlined with `split_front` so a front-chunk view is built in registers at the call site. A
    // view returned through a stack slot can be reloaded with loads wider than the stores that
    // filled it, and that stalls store forwarding.
    #[inline(always)]
    fn take_owned<T: OwnedBytes>(&mut self, len: usize) -> T {
        if let Some(view) = self.split_front(len) {
            return T::claim(view).unwrap_or_else(|view| T::from_bytes(view.into()));
        }
        self.take_spanning(len)
    }
}

impl bytes::Buf for IoBufs {
    #[inline]
    fn has_remaining(&self) -> bool {
        !self.is_empty()
    }

    #[inline]
    fn remaining(&self) -> usize {
        match &self.inner {
            IoBufsInner::Single(buf) => buf.remaining(),
            IoBufsInner::Pair([a, b]) => a.remaining().saturating_add(b.remaining()),
            IoBufsInner::Triple([a, b, c]) => a
                .remaining()
                .saturating_add(b.remaining())
                .saturating_add(c.remaining()),
            IoBufsInner::Chunked(bufs) => bufs
                .iter()
                .map(|b| b.remaining())
                .fold(0, usize::saturating_add),
        }
    }

    #[inline]
    fn chunk(&self) -> &[u8] {
        match &self.inner {
            IoBufsInner::Single(buf) => buf.chunk(),
            IoBufsInner::Pair([a, b]) => {
                if a.remaining() > 0 {
                    a.chunk()
                } else if b.remaining() > 0 {
                    b.chunk()
                } else {
                    &[]
                }
            }
            IoBufsInner::Triple([a, b, c]) => {
                if a.remaining() > 0 {
                    a.chunk()
                } else if b.remaining() > 0 {
                    b.chunk()
                } else if c.remaining() > 0 {
                    c.chunk()
                } else {
                    &[]
                }
            }
            IoBufsInner::Chunked(bufs) => {
                for buf in bufs.iter() {
                    if buf.remaining() > 0 {
                        return buf.chunk();
                    }
                }
                &[]
            }
        }
    }

    #[inline]
    fn chunks_vectored<'a>(&'a self, dst: &mut [IoSlice<'a>]) -> usize {
        if dst.is_empty() {
            return 0;
        }

        match &self.inner {
            IoBufsInner::Single(buf) => {
                let chunk = buf.chunk();
                if !chunk.is_empty() {
                    dst[0] = IoSlice::new(chunk);
                    return 1;
                }
                0
            }
            IoBufsInner::Pair([a, b]) => fill_vectored_from_chunks(dst, [a.chunk(), b.chunk()]),
            IoBufsInner::Triple([a, b, c]) => {
                fill_vectored_from_chunks(dst, [a.chunk(), b.chunk(), c.chunk()])
            }
            IoBufsInner::Chunked(bufs) => {
                fill_vectored_from_chunks(dst, bufs.iter().map(|buf| buf.chunk()))
            }
        }
    }

    #[inline]
    fn advance(&mut self, cnt: usize) {
        let should_canonicalize = match &mut self.inner {
            IoBufsInner::Single(buf) => {
                buf.advance(cnt);
                false
            }
            IoBufsInner::Pair(pair) => advance_small_chunks(pair.as_mut_slice(), cnt),
            IoBufsInner::Triple(triple) => advance_small_chunks(triple.as_mut_slice(), cnt),
            IoBufsInner::Chunked(bufs) => {
                advance_chunked_front(bufs, cnt);
                bufs.len() <= 3
            }
        };

        if should_canonicalize {
            self.canonicalize();
        }
    }

    #[inline]
    fn copy_to_bytes(&mut self, len: usize) -> Bytes {
        let (result, needs_canonicalize) = match &mut self.inner {
            IoBufsInner::Single(buf) => return buf.copy_to_bytes(len),
            IoBufsInner::Pair(pair) => {
                copy_to_bytes_small_chunks(pair, len, "IoBufs::copy_to_bytes: not enough data")
            }
            IoBufsInner::Triple(triple) => {
                copy_to_bytes_small_chunks(triple, len, "IoBufs::copy_to_bytes: not enough data")
            }
            IoBufsInner::Chunked(bufs) => {
                copy_to_bytes_chunked(bufs, len, "IoBufs::copy_to_bytes: not enough data")
            }
        };

        if needs_canonicalize {
            self.canonicalize();
        }

        result
    }
}

/// Zero-copy: wraps the buffer as the single chunk.
impl From<IoBuf> for IoBufs {
    fn from(buf: IoBuf) -> Self {
        Self {
            inner: IoBufsInner::Single(buf),
        }
    }
}

/// Zero-copy: freezes the buffer and wraps it as the single chunk.
impl From<IoBufMut> for IoBufs {
    fn from(buf: IoBufMut) -> Self {
        Self {
            inner: IoBufsInner::Single(buf.freeze()),
        }
    }
}

/// Zero-copy via `From<Bytes> for IoBuf`.
impl From<Bytes> for IoBufs {
    fn from(bytes: Bytes) -> Self {
        Self::from(IoBuf::from(bytes))
    }
}

/// Zero-copy via `From<BytesMut> for IoBuf`.
impl From<BytesMut> for IoBufs {
    fn from(bytes: BytesMut) -> Self {
        Self::from(IoBuf::from(bytes))
    }
}

/// Zero-copy via `From<Vec<u8>> for IoBuf`.
impl From<Vec<u8>> for IoBufs {
    fn from(vec: Vec<u8>) -> Self {
        Self::from(IoBuf::from(vec))
    }
}

/// Zero-copy: collects the chunks, dropping empty ones.
impl From<Vec<IoBuf>> for IoBufs {
    fn from(bufs: Vec<IoBuf>) -> Self {
        Self::from_iter(bufs)
    }
}

/// Zero-copy: collects the chunks, dropping empty ones.
impl<const N: usize> From<[IoBuf; N]> for IoBufs {
    fn from(bufs: [IoBuf; N]) -> Self {
        Self::from_iter(bufs)
    }
}

/// Zero-copy: collects the chunks, dropping empty ones.
impl FromIterator<IoBuf> for IoBufs {
    fn from_iter<T: IntoIterator<Item = IoBuf>>(bufs: T) -> Self {
        Self::from_chunks_iter(bufs)
    }
}

/// Zero-copy: creates a static view with no owner.
impl<const N: usize> From<&'static [u8; N]> for IoBufs {
    fn from(array: &'static [u8; N]) -> Self {
        Self::from(IoBuf::from(array))
    }
}

/// Zero-copy: creates a static view with no owner.
impl From<&'static [u8]> for IoBufs {
    fn from(slice: &'static [u8]) -> Self {
        Self::from(IoBuf::from(slice))
    }
}

/// Container for one or more mutable buffers.
///
/// # Capacity retention
///
/// The intended usage is fill-then-read: write into the container (through
/// [`BufMut`], [`Self::copy_from_slice`], or
/// [`Blob::read_at_buf`](crate::Blob::read_at_buf)), then consume it through
/// [`bytes::Buf`]. Caller-reserved write capacity generally survives read
/// operations, with three exceptions:
/// - The deque-backed read paths (four or more chunks) skip past a chunk
///   with no readable bytes by popping it, so a never-filled chunk ordered
///   before readable data loses its capacity when a read crosses it.
/// - The same paths pop any chunk whose readable bytes a read fully
///   consumes, releasing its writable tail. The two and three-chunk shapes
///   advance in place and retain such tails, so `remaining_mut()` after an
///   identical read can differ by shape.
/// - A nonzero `copy_to_bytes` that exactly drains the front chunk's
///   readable bytes consumes that chunk's whole handle on every shape (see
///   [`IoBufMut`]'s `copy_to_bytes` doc), so spare capacity behind an
///   exactly-drained chunk is released rather than retained.
///
/// Interleaving writes after reads is outside this contract. A read can
/// leave a drained chunk's surviving tail ordered before later readable
/// bytes, and a subsequent [`BufMut`] write fills that tail first.
#[derive(Debug)]
pub struct IoBufsMut {
    inner: IoBufsMutInner,
}

/// Internal mutable representation.
///
/// Construction and canonicalization keep every chunk that still owns
/// storage (`capacity() > 0`), readable or not, so caller-reserved write
/// capacity generally survives read operations. The reader-facing rules and
/// the accepted exceptions are documented on [`IoBufsMut`] under "Capacity
/// retention". Only fully-drained chunks (capacity consumed by `advance`)
/// and empty defaults are removed as the shape collapses.
#[derive(Debug)]
enum IoBufsMutInner {
    /// Single buffer (common case, no allocation).
    Single(IoBufMut),
    /// Two buffers (fast path, no VecDeque allocation).
    Pair([IoBufMut; 2]),
    /// Three buffers (fast path, no VecDeque allocation).
    Triple([IoBufMut; 3]),
    /// Four or more buffers.
    Chunked(VecDeque<IoBufMut>),
}

impl Default for IoBufsMut {
    fn default() -> Self {
        Self {
            inner: IoBufsMutInner::Single(IoBufMut::default()),
        }
    }
}

impl IoBufsMut {
    /// Build mutable chunk storage from already-filtered chunks.
    ///
    /// This helper intentionally does not filter. Callers route through
    /// [`Self::from_writable_chunks_iter`] so storage-owning chunks are kept.
    fn from_chunks_iter(chunks: impl IntoIterator<Item = IoBufMut>) -> Self {
        let mut iter = chunks.into_iter();
        let first = match iter.next() {
            Some(first) => first,
            None => return Self::default(),
        };
        let second = match iter.next() {
            Some(second) => second,
            None => {
                return Self {
                    inner: IoBufsMutInner::Single(first),
                };
            }
        };
        let third = match iter.next() {
            Some(third) => third,
            None => {
                return Self {
                    inner: IoBufsMutInner::Pair([first, second]),
                };
            }
        };
        let fourth = match iter.next() {
            Some(fourth) => fourth,
            None => {
                return Self {
                    inner: IoBufsMutInner::Triple([first, second, third]),
                };
            }
        };

        let mut bufs = VecDeque::with_capacity(4);
        bufs.push_back(first);
        bufs.push_back(second);
        bufs.push_back(third);
        bufs.push_back(fourth);
        bufs.extend(iter);
        Self {
            inner: IoBufsMutInner::Chunked(bufs),
        }
    }

    /// Build canonical mutable chunk storage from writable chunks.
    ///
    /// Keeps chunks that still own storage, readable or not (`capacity()`
    /// covers both readable bytes and the writable tail), so a never-filled
    /// chunk's reserved capacity is not discarded. Fully-drained chunks
    /// (capacity consumed by `advance`) and empty defaults are removed.
    fn from_writable_chunks_iter(chunks: impl IntoIterator<Item = IoBufMut>) -> Self {
        Self::from_chunks_iter(chunks.into_iter().filter(|buf| buf.capacity() > 0))
    }

    /// Re-establish canonical mutable representation invariants.
    ///
    /// Uses the same storage-keeping filter as construction, so chunks that
    /// were drained of readable bytes but still own writable capacity survive.
    /// Reads then leave `remaining_mut()` unchanged, apart from the exceptions
    /// listed under "Capacity retention" on [`IoBufsMut`].
    fn canonicalize(&mut self) {
        let inner = std::mem::replace(&mut self.inner, IoBufsMutInner::Single(IoBufMut::default()));
        self.inner = match inner {
            IoBufsMutInner::Single(buf) => IoBufsMutInner::Single(buf),
            IoBufsMutInner::Pair([a, b]) => Self::from_writable_chunks_iter([a, b]).inner,
            IoBufsMutInner::Triple([a, b, c]) => Self::from_writable_chunks_iter([a, b, c]).inner,
            IoBufsMutInner::Chunked(bufs) => Self::from_writable_chunks_iter(bufs).inner,
        };
    }

    #[inline]
    fn for_each_chunk_mut(&mut self, mut f: impl FnMut(&mut IoBufMut)) {
        match &mut self.inner {
            IoBufsMutInner::Single(buf) => f(buf),
            IoBufsMutInner::Pair(pair) => {
                for buf in pair.iter_mut() {
                    f(buf);
                }
            }
            IoBufsMutInner::Triple(triple) => {
                for buf in triple.iter_mut() {
                    f(buf);
                }
            }
            IoBufsMutInner::Chunked(bufs) => {
                for buf in bufs.iter_mut() {
                    f(buf);
                }
            }
        }
    }

    /// Returns a reference to the single contiguous buffer, if present.
    ///
    /// Returns `Some` only when this is currently represented as one chunk.
    pub const fn as_single(&self) -> Option<&IoBufMut> {
        match &self.inner {
            IoBufsMutInner::Single(buf) => Some(buf),
            _ => None,
        }
    }

    /// Returns a mutable reference to the single contiguous buffer, if present.
    ///
    /// Returns `Some` only when this is currently represented as one chunk.
    pub const fn as_single_mut(&mut self) -> Option<&mut IoBufMut> {
        match &mut self.inner {
            IoBufsMutInner::Single(buf) => Some(buf),
            _ => None,
        }
    }

    /// Consume this container and return the single buffer if present.
    ///
    /// Returns `Ok(IoBufMut)` only when readable data is represented as one
    /// chunk. Returns `Err(Self)` with the original container otherwise.
    pub fn try_into_single(self) -> Result<IoBufMut, Self> {
        match self.inner {
            IoBufsMutInner::Single(buf) => Ok(buf),
            inner => Err(Self { inner }),
        }
    }

    /// Number of bytes remaining across all buffers.
    #[inline]
    pub fn len(&self) -> usize {
        self.remaining()
    }

    /// Whether all buffers are empty.
    #[inline]
    pub fn is_empty(&self) -> bool {
        match &self.inner {
            IoBufsMutInner::Single(buf) => buf.is_empty(),
            IoBufsMutInner::Pair([a, b]) => a.is_empty() && b.is_empty(),
            IoBufsMutInner::Triple([a, b, c]) => a.is_empty() && b.is_empty() && c.is_empty(),
            IoBufsMutInner::Chunked(bufs) => bufs.iter().all(IoBufMut::is_empty),
        }
    }

    /// Whether this contains a single contiguous buffer.
    ///
    /// When true, `chunk()` returns all remaining bytes.
    #[inline]
    pub const fn is_single(&self) -> bool {
        matches!(self.inner, IoBufsMutInner::Single(_))
    }

    /// Freeze into immutable [`IoBufs`].
    ///
    /// Each chunk freezes through [`IoBufMut::freeze`], so a chunk whose spare
    /// capacity is too large to record in its handle allocates its owner record.
    pub fn freeze(self) -> IoBufs {
        match self.inner {
            IoBufsMutInner::Single(buf) => IoBufs::from(buf.freeze()),
            IoBufsMutInner::Pair([a, b]) => IoBufs::from_chunks_iter([a.freeze(), b.freeze()]),
            IoBufsMutInner::Triple([a, b, c]) => {
                IoBufs::from_chunks_iter([a.freeze(), b.freeze(), c.freeze()])
            }
            IoBufsMutInner::Chunked(bufs) => {
                IoBufs::from_chunks_iter(bufs.into_iter().map(IoBufMut::freeze))
            }
        }
    }

    /// Coalesce all buffers into a single contiguous [`IoBufMut`].
    ///
    /// Zero-copy if only one buffer. Otherwise copies into one allocation of
    /// exactly the readable length.
    pub fn coalesce(self) -> IoBufMut {
        match self.inner {
            IoBufsMutInner::Single(buf) => buf,
            inner => {
                let mut bufs = Self { inner };
                let mut out = IoBufMut::with_capacity(bufs.len());
                bufs.for_each_chunk_mut(|buf| out.put_slice(buf.as_ref()));
                out
            }
        }
    }

    /// Returns the total capacity across all buffers.
    pub fn capacity(&self) -> usize {
        match &self.inner {
            IoBufsMutInner::Single(buf) => buf.capacity(),
            IoBufsMutInner::Pair([a, b]) => a.capacity().saturating_add(b.capacity()),
            IoBufsMutInner::Triple([a, b, c]) => a
                .capacity()
                .saturating_add(b.capacity())
                .saturating_add(c.capacity()),
            IoBufsMutInner::Chunked(bufs) => bufs
                .iter()
                .map(|b| b.capacity())
                .fold(0, usize::saturating_add),
        }
    }

    /// Sets the length of the buffer(s) to `len`, distributing across chunks
    /// while preserving the current chunk layout.
    ///
    /// This is useful for APIs that must fill caller-provided buffer structure
    /// in place (for example [`Blob::read_at_buf`](crate::Blob::read_at_buf)).
    ///
    /// # Safety
    ///
    /// Caller must initialize all `len` bytes before the buffer is read.
    ///
    /// # Panics
    ///
    /// Panics if `len` exceeds total capacity.
    pub(crate) unsafe fn set_len(&mut self, len: usize) {
        let capacity = self.capacity();
        assert!(
            len <= capacity,
            "set_len({len}) exceeds capacity({capacity})"
        );
        let mut remaining = len;
        self.for_each_chunk_mut(|buf| {
            let cap = buf.capacity();
            let to_set = remaining.min(cap);
            // SAFETY: forwarded from this method's contract. The caller
            // initializes all `len` bytes before any read.
            unsafe { buf.set_len(to_set) };
            remaining -= to_set;
        });
    }

    /// Copy data from a slice into the buffers.
    ///
    /// # Panics
    ///
    /// Panics if the slice length doesn't match the total buffer length.
    pub fn copy_from_slice(&mut self, src: &[u8]) {
        assert_eq!(
            src.len(),
            self.len(),
            "source slice length must match buffer length"
        );
        let mut offset = 0;
        self.for_each_chunk_mut(|buf| {
            let len = buf.len();
            buf.as_mut().copy_from_slice(&src[offset..offset + len]);
            offset += len;
        });
    }
}

impl Input for IoBufsMut {
    type Buf = IoBufs;
}

/// Freezes every chunk (see [`IoBufsMut::freeze`]).
impl From<IoBufsMut> for IoBufs {
    fn from(bufs: IoBufsMut) -> Self {
        bufs.freeze()
    }
}

impl bytes::Buf for IoBufsMut {
    #[inline]
    fn has_remaining(&self) -> bool {
        !self.is_empty()
    }

    #[inline]
    fn remaining(&self) -> usize {
        match &self.inner {
            IoBufsMutInner::Single(buf) => buf.remaining(),
            IoBufsMutInner::Pair([a, b]) => a.remaining().saturating_add(b.remaining()),
            IoBufsMutInner::Triple([a, b, c]) => a
                .remaining()
                .saturating_add(b.remaining())
                .saturating_add(c.remaining()),
            IoBufsMutInner::Chunked(bufs) => bufs
                .iter()
                .map(|b| b.remaining())
                .fold(0, usize::saturating_add),
        }
    }

    #[inline]
    fn chunk(&self) -> &[u8] {
        match &self.inner {
            IoBufsMutInner::Single(buf) => buf.chunk(),
            IoBufsMutInner::Pair([a, b]) => {
                if a.remaining() > 0 {
                    a.chunk()
                } else if b.remaining() > 0 {
                    b.chunk()
                } else {
                    &[]
                }
            }
            IoBufsMutInner::Triple([a, b, c]) => {
                if a.remaining() > 0 {
                    a.chunk()
                } else if b.remaining() > 0 {
                    b.chunk()
                } else if c.remaining() > 0 {
                    c.chunk()
                } else {
                    &[]
                }
            }
            IoBufsMutInner::Chunked(bufs) => {
                for buf in bufs.iter() {
                    if buf.remaining() > 0 {
                        return buf.chunk();
                    }
                }
                &[]
            }
        }
    }

    #[inline]
    fn chunks_vectored<'a>(&'a self, dst: &mut [IoSlice<'a>]) -> usize {
        if dst.is_empty() {
            return 0;
        }

        match &self.inner {
            IoBufsMutInner::Single(buf) => {
                let chunk = buf.chunk();
                if !chunk.is_empty() {
                    dst[0] = IoSlice::new(chunk);
                    return 1;
                }
                0
            }
            IoBufsMutInner::Pair([a, b]) => fill_vectored_from_chunks(dst, [a.chunk(), b.chunk()]),
            IoBufsMutInner::Triple([a, b, c]) => {
                fill_vectored_from_chunks(dst, [a.chunk(), b.chunk(), c.chunk()])
            }
            IoBufsMutInner::Chunked(bufs) => {
                fill_vectored_from_chunks(dst, bufs.iter().map(|buf| buf.chunk()))
            }
        }
    }

    #[inline]
    fn advance(&mut self, cnt: usize) {
        let should_canonicalize = match &mut self.inner {
            IoBufsMutInner::Single(buf) => {
                buf.advance(cnt);
                false
            }
            IoBufsMutInner::Pair(pair) => advance_small_chunks(pair.as_mut_slice(), cnt),
            IoBufsMutInner::Triple(triple) => advance_small_chunks(triple.as_mut_slice(), cnt),
            IoBufsMutInner::Chunked(bufs) => {
                advance_chunked_front(bufs, cnt);
                bufs.len() <= 3
            }
        };

        if should_canonicalize {
            self.canonicalize();
        }
    }

    #[inline]
    fn copy_to_bytes(&mut self, len: usize) -> Bytes {
        // Zero-length drains must not disturb chunk state. The deque-backed
        // path pops chunks with no readable bytes, and popping a never-filled
        // chunk would discard its reserved capacity.
        if len == 0 {
            return Bytes::new();
        }
        let (result, needs_canonicalize) = match &mut self.inner {
            IoBufsMutInner::Single(buf) => return buf.copy_to_bytes(len),
            IoBufsMutInner::Pair(pair) => {
                copy_to_bytes_small_chunks(pair, len, "IoBufsMut::copy_to_bytes: not enough data")
            }
            IoBufsMutInner::Triple(triple) => {
                copy_to_bytes_small_chunks(triple, len, "IoBufsMut::copy_to_bytes: not enough data")
            }
            IoBufsMutInner::Chunked(bufs) => {
                copy_to_bytes_chunked(bufs, len, "IoBufsMut::copy_to_bytes: not enough data")
            }
        };

        if needs_canonicalize {
            self.canonicalize();
        }

        result
    }
}

// SAFETY: Delegates to IoBufMut which implements BufMut safely.
unsafe impl BufMut for IoBufsMut {
    #[inline]
    fn remaining_mut(&self) -> usize {
        match &self.inner {
            IoBufsMutInner::Single(buf) => buf.remaining_mut(),
            IoBufsMutInner::Pair([a, b]) => a.remaining_mut().saturating_add(b.remaining_mut()),
            IoBufsMutInner::Triple([a, b, c]) => a
                .remaining_mut()
                .saturating_add(b.remaining_mut())
                .saturating_add(c.remaining_mut()),
            IoBufsMutInner::Chunked(bufs) => bufs
                .iter()
                .map(|b| b.remaining_mut())
                .fold(0, usize::saturating_add),
        }
    }

    #[inline]
    unsafe fn advance_mut(&mut self, cnt: usize) {
        // SAFETY: The caller guarantees `cnt <= self.remaining_mut()`, so each
        // delegated advance remains within the writable chunks.
        unsafe {
            // On failure, every writable byte was consumed before the chunks ran
            // out, so the advanced amount (`cnt - remaining`) is exactly what was
            // available.
            let mut remaining = cnt;
            let advanced = match &mut self.inner {
                IoBufsMutInner::Single(buf) => {
                    buf.advance_mut(cnt);
                    return;
                }
                IoBufsMutInner::Pair(pair) => advance_mut_in_chunks(pair, &mut remaining),
                IoBufsMutInner::Triple(triple) => advance_mut_in_chunks(triple, &mut remaining),
                IoBufsMutInner::Chunked(bufs) => {
                    let (first, second) = bufs.as_mut_slices();
                    advance_mut_in_chunks(first, &mut remaining)
                        || advance_mut_in_chunks(second, &mut remaining)
                }
            };
            if !advanced {
                panic_advance(cnt, cnt - remaining);
            }
        }
    }

    #[inline]
    fn chunk_mut(&mut self) -> &mut bytes::buf::UninitSlice {
        match &mut self.inner {
            IoBufsMutInner::Single(buf) => buf.chunk_mut(),
            IoBufsMutInner::Pair(pair) => {
                if pair[0].remaining_mut() > 0 {
                    pair[0].chunk_mut()
                } else if pair[1].remaining_mut() > 0 {
                    pair[1].chunk_mut()
                } else {
                    bytes::buf::UninitSlice::new(&mut [])
                }
            }
            IoBufsMutInner::Triple(triple) => {
                if triple[0].remaining_mut() > 0 {
                    triple[0].chunk_mut()
                } else if triple[1].remaining_mut() > 0 {
                    triple[1].chunk_mut()
                } else if triple[2].remaining_mut() > 0 {
                    triple[2].chunk_mut()
                } else {
                    bytes::buf::UninitSlice::new(&mut [])
                }
            }
            IoBufsMutInner::Chunked(bufs) => {
                for buf in bufs.iter_mut() {
                    if buf.remaining_mut() > 0 {
                        return buf.chunk_mut();
                    }
                }
                bytes::buf::UninitSlice::new(&mut [])
            }
        }
    }
}

/// Zero-copy: wraps the buffer as the single chunk.
impl From<IoBufMut> for IoBufsMut {
    fn from(buf: IoBufMut) -> Self {
        Self {
            inner: IoBufsMutInner::Single(buf),
        }
    }
}

/// Convert a [`Vec<u8>`] into a single-buffer [`IoBufsMut`].
///
/// Adopts the vec's allocation and capacity via `From<Vec<u8>> for IoBufMut`.
impl From<Vec<u8>> for IoBufsMut {
    fn from(vec: Vec<u8>) -> Self {
        Self {
            inner: IoBufsMutInner::Single(IoBufMut::from(vec)),
        }
    }
}

/// Reuses the allocation via `From<BytesMut> for IoBufMut` when no other
/// handle shares it and copies otherwise. Either way at least the reserved
/// capacity is kept.
impl From<BytesMut> for IoBufsMut {
    fn from(bytes: BytesMut) -> Self {
        Self {
            inner: IoBufsMutInner::Single(IoBufMut::from(bytes)),
        }
    }
}

/// Zero-copy: collects the chunks, dropping zero-capacity ones.
impl From<Vec<IoBufMut>> for IoBufsMut {
    fn from(bufs: Vec<IoBufMut>) -> Self {
        Self::from_iter(bufs)
    }
}

/// Zero-copy: collects the chunks, dropping zero-capacity ones.
impl<const N: usize> From<[IoBufMut; N]> for IoBufsMut {
    fn from(bufs: [IoBufMut; N]) -> Self {
        Self::from_iter(bufs)
    }
}

/// Zero-copy: collects the chunks, dropping zero-capacity ones.
impl FromIterator<IoBufMut> for IoBufsMut {
    fn from_iter<T: IntoIterator<Item = IoBufMut>>(bufs: T) -> Self {
        Self::from_writable_chunks_iter(bufs)
    }
}

/// Copies via `From<[u8; N]> for IoBufMut`.
impl<const N: usize> From<[u8; N]> for IoBufsMut {
    fn from(array: [u8; N]) -> Self {
        Self {
            inner: IoBufsMutInner::Single(IoBufMut::from(array)),
        }
    }
}

/// Drain `len` readable bytes from a small fixed chunk array (`Pair`/`Triple`).
///
/// Returns drained bytes plus whether the caller should canonicalize afterward.
#[inline]
fn copy_to_bytes_small_chunks<B: bytes::Buf, const N: usize>(
    chunks: &mut [B; N],
    len: usize,
    not_enough_data_msg: &str,
) -> (Bytes, bool) {
    let total = chunks
        .iter()
        .map(|buf| buf.remaining())
        .fold(0, usize::saturating_add);
    assert!(total >= len, "{not_enough_data_msg}");

    if chunks[0].remaining() >= len {
        let bytes = chunks[0].copy_to_bytes(len);
        return (bytes, chunks[0].remaining() == 0);
    }

    let mut out = BytesMut::with_capacity(len);
    let mut remaining = len;
    for buf in chunks.iter_mut() {
        if remaining == 0 {
            break;
        }
        let to_copy = remaining.min(buf.remaining());
        out.extend_from_slice(&buf.chunk()[..to_copy]);
        buf.advance(to_copy);
        remaining -= to_copy;
    }

    // Slow path always consumes past chunk 0, so canonicalization is required.
    (out.freeze(), true)
}

/// Drain `len` readable bytes from a deque-backed chunk representation.
///
/// Returns drained bytes plus whether the caller should canonicalize afterward.
#[inline]
fn copy_to_bytes_chunked<B: bytes::Buf>(
    bufs: &mut VecDeque<B>,
    len: usize,
    not_enough_data_msg: &str,
) -> (Bytes, bool) {
    while bufs.front().is_some_and(|buf| buf.remaining() == 0) {
        bufs.pop_front();
    }

    if bufs.front().is_none() {
        assert_eq!(len, 0, "{not_enough_data_msg}");
        // The deque is empty now, so the container must collapse back to the
        // canonical Single representation.
        return (Bytes::new(), true);
    }

    if bufs.front().is_some_and(|front| front.remaining() >= len) {
        let front = bufs.front_mut().expect("front checked above");
        let bytes = front.copy_to_bytes(len);
        if front.remaining() == 0 {
            bufs.pop_front();
        }
        return (bytes, bufs.len() <= 3);
    }

    let total = bufs
        .iter()
        .map(|buf| buf.remaining())
        .fold(0, usize::saturating_add);
    assert!(total >= len, "{not_enough_data_msg}");

    let mut out = BytesMut::with_capacity(len);
    let mut remaining = len;
    while remaining > 0 {
        let front = bufs
            .front_mut()
            .expect("remaining > 0 implies non-empty bufs");
        let to_copy = remaining.min(front.remaining());
        out.extend_from_slice(&front.chunk()[..to_copy]);
        front.advance(to_copy);
        if front.remaining() == 0 {
            bufs.pop_front();
        }
        remaining -= to_copy;
    }

    (out.freeze(), bufs.len() <= 3)
}

/// Advance across a [`VecDeque`] of chunks by consuming from the front.
#[inline]
fn advance_chunked_front<B: bytes::Buf>(bufs: &mut VecDeque<B>, mut cnt: usize) {
    while cnt > 0 {
        let front = bufs.front_mut().expect("cannot advance past end of buffer");
        let avail = front.remaining();
        if avail == 0 {
            bufs.pop_front();
            continue;
        }
        if cnt < avail {
            front.advance(cnt);
            break;
        }
        front.advance(avail);
        bufs.pop_front();
        cnt -= avail;
    }
}

/// Advance across a small fixed set of chunks (`Pair`/`Triple`).
///
/// Returns `true` when one or more chunks became (or were) empty, so callers
/// can canonicalize once after the operation.
#[inline]
fn advance_small_chunks<B: bytes::Buf>(chunks: &mut [B], mut cnt: usize) -> bool {
    let mut idx = 0;
    let mut needs_canonicalize = false;

    while cnt > 0 {
        let chunk = chunks
            .get_mut(idx)
            .expect("cannot advance past end of buffer");
        let avail = chunk.remaining();
        if avail == 0 {
            idx += 1;
            needs_canonicalize = true;
            continue;
        }
        if cnt < avail {
            chunk.advance(cnt);
            return needs_canonicalize;
        }
        chunk.advance(avail);
        cnt -= avail;
        idx += 1;
        needs_canonicalize = true;
    }

    needs_canonicalize
}

/// Advance writable cursors across `chunks` by up to `*remaining` bytes.
///
/// Returns `true` when the full request has been satisfied.
///
/// # Safety
///
/// Forwards to [`BufMut::advance_mut`], so callers must ensure the advanced
/// region has been initialized according to [`BufMut`]'s contract.
#[inline]
unsafe fn advance_mut_in_chunks<B: BufMut>(chunks: &mut [B], remaining: &mut usize) -> bool {
    if *remaining == 0 {
        return true;
    }

    for buf in chunks.iter_mut() {
        let avail = buf.chunk_mut().len();
        if avail == 0 {
            continue;
        }
        if *remaining <= avail {
            // SAFETY: Upheld by this function's safety contract.
            unsafe { buf.advance_mut(*remaining) };
            *remaining = 0;
            return true;
        }
        // SAFETY: Upheld by this function's safety contract.
        unsafe { buf.advance_mut(avail) };
        *remaining -= avail;
    }
    false
}

/// Fill `dst` with `IoSlice`s built from `chunks`.
///
/// Empty chunks are skipped. At most `dst.len()` slices are written.
/// Returns the number of slices written.
#[inline]
fn fill_vectored_from_chunks<'a, I>(dst: &mut [IoSlice<'a>], chunks: I) -> usize
where
    I: IntoIterator<Item = &'a [u8]>,
{
    let mut written = 0;
    for chunk in chunks
        .into_iter()
        .filter(|chunk| !chunk.is_empty())
        .take(dst.len())
    {
        dst[written] = IoSlice::new(chunk);
        written += 1;
    }
    written
}

/// Assembles [`IoBufs`] from a mix of inline writes and zero-copy pieces.
///
/// All inline writes go into a single buffer, and [`BufsMut::push`] records
/// the inline offset at which each pushed piece belongs. [`Builder::finish`]
/// freezes the buffer once, splits it at those offsets with
/// [`IoBuf::split_to`], and interleaves the pushed chunks with the inline
/// segments.
///
/// A pushed [`IoBuf`] becomes a chunk as is. Any other pushed value converts
/// into [`Bytes`], and the chunk owns that `Bytes` without copying.
///
/// The inline buffer has a fixed capacity set at construction and will not
/// grow. Callers must ensure the capacity accounts for all inline
/// (non-pushed) bytes that will be written. Exceeding it will panic.
///
/// ```text
/// builder.put_u16(99);                   // inline
/// builder.push(shard_payload.clone());   // zero-copy: shares the Bytes
/// builder.put_u32(checksum);             // inline
/// let output = builder.finish();
///
/// // inline buffer  [ 99 | checksum ]           one allocation, frozen once
/// // pushed Bytes   [ ----- 1 MB shard ----- ]  the caller's, never copied
/// //
/// // output         [ 99 ][ ----- 1 MB shard ----- ][ checksum ]
/// //                 inline      pushed Bytes           inline
/// ```
pub struct Builder {
    // Single working buffer for all inline writes.
    buf: IoBufMut,
    // Each entry is (offset_in_buf, pushed_chunk) recording where a push
    // interrupts the inline byte stream.
    pushes: Vec<(usize, IoBuf)>,
}

impl Builder {
    /// Creates a builder whose inline buffer holds exactly `capacity` bytes.
    ///
    /// A zero capacity allocates nothing, so the builder accepts only pushed
    /// chunks.
    ///
    /// # Panics
    ///
    /// Panics if `capacity` exceeds `isize::MAX`. Writing more than `capacity`
    /// inline bytes panics at the write.
    pub fn new(capacity: usize) -> Self {
        Self {
            buf: IoBufMut::with_capacity(capacity),
            pushes: Vec::new(),
        }
    }

    /// Freezes the inline buffer and assembles [`IoBufs`] by splitting it at
    /// the recorded push boundaries.
    pub fn finish(self) -> IoBufs {
        if self.pushes.is_empty() {
            return IoBufs::from(self.buf.freeze());
        }

        // Segments split off the owned frozen buffer. A first split of a
        // descriptor stores the shared record with a plain write, and the
        // remainder moves into the result without taking another reference.
        let mut frozen = self.buf.freeze();
        let mut result = IoBufs::default();
        let mut pos = 0;

        for (offset, pushed) in self.pushes {
            if offset > pos {
                result.append(frozen.split_to(offset - pos));
            }
            result.append(pushed);
            pos = offset;
        }

        result.append(frozen);
        result
    }
}

// SAFETY: All methods delegate directly to `self.buf`, an `IoBufMut` with a
// sound `BufMut` implementation. The inline buffer has fixed capacity. Writes
// that exceed it panic in bytes' `BufMut` trait defaults (which check
// `remaining_mut`) or, for a direct `advance_mut`, in `IoBufMut::advance_mut`.
unsafe impl BufMut for Builder {
    #[inline]
    fn remaining_mut(&self) -> usize {
        self.buf.remaining_mut()
    }

    #[inline]
    unsafe fn advance_mut(&mut self, cnt: usize) {
        // SAFETY: The caller guarantees that `cnt` is within the writable chunk.
        unsafe {
            self.buf.advance_mut(cnt);
        }
    }

    #[inline]
    fn chunk_mut(&mut self) -> &mut bytes::buf::UninitSlice {
        self.buf.chunk_mut()
    }
}

impl BufsMut for Builder {
    fn push(&mut self, chunk: impl Into<Bytes> + 'static) {
        let chunk = IoBuf::claim(chunk).unwrap_or_else(|chunk| IoBuf::from(chunk.into()));
        if !chunk.is_empty() {
            self.pushes.push((self.buf.len(), chunk));
        }
    }
}

/// Extension trait for encoding values that implement [`Write`] and
/// [`EncodeSize`] into [`IoBufs`].
pub trait EncodeExt: EncodeSize + Write {
    /// Encode this value into [`IoBufs`] through a [`Builder`].
    ///
    /// Override [`Write::write_bufs`] to avoid copying large byte fields.
    ///
    /// # Panics
    ///
    /// Panics if [`Write::write_bufs`] writes more inline bytes than
    /// [`EncodeSize::encode_inline_size`], or if [`EncodeSize::encode_size`]
    /// does not match the total bytes written.
    fn encode_iobufs(&self) -> IoBufs {
        let len = self.encode_size();
        let mut builder = Builder::new(self.encode_inline_size());
        self.write_bufs(&mut builder);
        let bufs = builder.finish();
        assert_eq!(
            bufs.remaining(),
            len,
            "write_bufs() must write exactly encode_size() bytes"
        );
        bufs
    }
}

impl<T: EncodeSize + Write> EncodeExt for T {}

#[cfg(all(test, not(feature = "loom")))]
#[commonware_macros::test_group("miri")]
mod tests {
    use super::*;
    use bytes::{Bytes, BytesMut};
    use commonware_codec::{Decode, Encode, types::lazy::Lazy};
    use commonware_utils::range::NonEmptyRange;
    use std::{
        alloc::{GlobalAlloc, Layout, System},
        cell::Cell,
        collections::{BTreeMap, HashMap},
        sync::Arc,
    };

    /// Counts this thread's allocations during [`allocation_calls_during`] and delegates every
    /// call to the system allocator.
    struct TrackingAllocator;

    std::thread_local! {
        /// This thread's allocation count, or `None` while tracking is off.
        static ALLOCATION_CALLS: Cell<Option<usize>> = const { Cell::new(None) };
    }

    #[global_allocator]
    static ALLOCATOR: TrackingAllocator = TrackingAllocator;

    /// Counts an allocation when tracking is enabled on this thread.
    fn record_allocation() {
        ALLOCATION_CALLS.with(|calls| {
            if let Some(count) = calls.get() {
                calls.set(Some(count + 1));
            }
        });
    }

    // SAFETY: Every operation delegates to the system allocator with unchanged
    // arguments. Recording an allocation only updates a thread-local cell.
    unsafe impl GlobalAlloc for TrackingAllocator {
        unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
            record_allocation();
            // SAFETY: The caller supplies the layout required by GlobalAlloc.
            unsafe { System.alloc(layout) }
        }

        unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
            record_allocation();
            // SAFETY: The caller supplies the layout required by GlobalAlloc.
            unsafe { System.alloc_zeroed(layout) }
        }

        unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
            // SAFETY: This wrapper delegates every allocation to System, so the
            // pointer came from System with this layout.
            unsafe { System.dealloc(ptr, layout) }
        }

        unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
            record_allocation();
            // SAFETY: The pointer and layout came from System through this wrapper,
            // and the caller supplies the new size required by GlobalAlloc.
            unsafe { System.realloc(ptr, layout, new_size) }
        }
    }

    /// Runs `f` and returns its result with the number of allocations it made on this thread.
    fn allocation_calls_during<T>(f: impl FnOnce() -> T) -> (T, usize) {
        ALLOCATION_CALLS.with(|calls| assert!(calls.replace(Some(0)).is_none()));
        let result = f();
        let calls = ALLOCATION_CALLS.with(|calls| {
            calls
                .replace(None)
                .expect("allocation tracking must stay enabled")
        });
        (result, calls)
    }

    /// A prefix held by the front chunk is handed out as a view of that chunk, in every
    /// representation, and consuming the chunk keeps the container canonical. Only the first
    /// partial view of an unshared chunk allocates, for the chunk's owner record. A prefix that
    /// spans chunks is copied once into the requested type.
    #[test]
    fn test_take_owned_front_chunk() {
        let chunk = |byte: u8| IoBuf::copy_from_slice(&[byte; 8]);
        let mut bufs = IoBufs::default();
        for byte in 1..=5 {
            bufs.append(chunk(byte));
        }
        assert!(matches!(bufs.inner, IoBufsInner::Chunked(_)));

        // Chunked: a partial take leaves the chunk in place, a take of the remainder removes it.
        // The partial take shares the unshared chunk, which allocates its owner record.
        let front_ptr = bufs.chunk().as_ptr();
        let (view, calls) = allocation_calls_during(|| bufs.take_owned::<IoBuf>(3));
        assert_eq!(calls, 1);
        assert_eq!(view, [1u8; 3]);
        assert_eq!(view.as_ptr(), front_ptr);
        let (view, calls) = allocation_calls_during(|| bufs.take_owned::<IoBuf>(5));
        assert_eq!(calls, 0);
        assert_eq!(view, [1u8; 5]);
        assert!(matches!(bufs.inner, IoBufsInner::Chunked(ref chunks) if chunks.len() == 4));

        // Consuming the next chunk collapses to the small representation.
        let (view, calls) = allocation_calls_during(|| bufs.take_owned::<IoBuf>(8));
        assert_eq!(calls, 0);
        assert_eq!(view, [2u8; 8]);
        assert!(matches!(bufs.inner, IoBufsInner::Triple(_)));

        // Triple and Pair front takes are views too, and a `Bytes` field gets the view's bytes.
        let (view, calls) = allocation_calls_during(|| bufs.take_owned::<IoBuf>(8));
        assert_eq!(calls, 0);
        assert_eq!(view, [3u8; 8]);
        assert!(matches!(bufs.inner, IoBufsInner::Pair(_)));
        let front_ptr = bufs.chunk().as_ptr();
        let bytes = bufs.take_owned::<Bytes>(4);
        assert_eq!(bytes, vec![4u8; 4]);
        assert_eq!(bytes.as_ptr(), front_ptr);
        assert!(matches!(bufs.inner, IoBufsInner::Pair(_)));

        // A view keeps the chunk's native owner and converts in place once it is the last holder.
        drop(bufs);
        assert!(view.try_into_mut().is_ok());

        // Spanning prefixes are copied once. An IoBuf lands in one native buffer (one data
        // allocation, plus the owner record that splitting the second chunk allocates), Bytes in
        // one shared buffer, and the remainder stays canonical.
        let mut bufs = IoBufs::from(chunk(1));
        bufs.append(chunk(2));
        let (copied, calls) = allocation_calls_during(|| bufs.take_owned::<IoBuf>(12));
        assert_eq!(calls, 2);
        assert_eq!(copied.as_ref(), [[1u8; 8].as_slice(), &[2u8; 4]].concat());
        assert!(copied.try_into_mut().is_ok());
        assert!(matches!(bufs.inner, IoBufsInner::Single(_)));
        assert_eq!(bufs.len(), 4);

        let mut bufs = IoBufs::from(chunk(1));
        bufs.append(chunk(2));
        let (copied, calls) = allocation_calls_during(|| bufs.take_owned::<Bytes>(12));
        assert_eq!(calls, 1);
        assert_eq!(copied, [[1u8; 8].as_slice(), &[2u8; 4]].concat());
        assert_eq!(bufs.len(), 4);

        // Zero-length takes are empty views that leave the container untouched.
        let empty = bufs.take_owned::<IoBuf>(0);
        assert!(empty.is_empty());
        assert_eq!(bufs.len(), 4);
        assert!(IoBufs::default().take_owned::<IoBuf>(0).is_empty());
    }

    /// An unshared native buffer allocates only its data until its first share, which allocates
    /// its owner record once. It lives here, next to the tracking allocator that counts the calls.
    #[test]
    fn test_iobuf_owner_record_allocations() {
        // Allocate, write, freeze, recover, write, freeze, and drop: one data allocation.
        let ((), calls) = allocation_calls_during(|| {
            let mut buf = IoBufMut::with_capacity(64);
            buf.put_slice(b"payload");
            let mut buf = buf.freeze().try_into_mut().unwrap();
            buf.put_slice(b"!");
            drop(buf.freeze());
        });
        assert_eq!(calls, 1);

        // Moves, whole splits, and empty views allocate nothing.
        let mut source = IoBuf::copy_from_slice(&[7u8; 64]);
        let (mut buf, calls) = allocation_calls_during(|| {
            let mut moved = std::mem::take(&mut source);
            assert!(moved.slice(0..0).is_empty());
            assert!(moved.split_to(0).is_empty());
            moved.split_to(64)
        });
        assert_eq!(calls, 0);

        // The first share allocates the record, and later shares reuse it.
        let (first, calls) = allocation_calls_during(|| buf.clone());
        assert_eq!(calls, 1);
        let (later, calls) =
            allocation_calls_during(|| (buf.slice(1..3), buf.split_to(8), buf.clone()));
        assert_eq!(calls, 0);
        drop((first, later));

        // Spare capacity too large for the handle allocates the record at freeze instead.
        let mut big = IoBufMut::with_capacity((1 << 17) + 1);
        big.put_u8(1);
        let (frozen, calls) = allocation_calls_during(|| big.freeze());
        assert_eq!(calls, 1);
        let (_view, calls) = allocation_calls_during(|| frozen.clone());
        assert_eq!(calls, 0);

        // Whole conversions and drains into Bytes move the descriptor into the Bytes box without
        // promoting it, so each allocates only that box.
        let mut advanced = IoBuf::copy_from_slice(&[7u8; 16]);
        advanced.advance(2);
        let (_bytes, calls) = allocation_calls_during(|| Bytes::from(advanced));
        assert_eq!(calls, 1);
        let mut drained = IoBuf::copy_from_slice(&[7u8; 16]);
        let (_bytes, calls) = allocation_calls_during(|| drained.copy_to_bytes(16));
        assert_eq!(calls, 1);
        let mut drained = IoBufMut::with_capacity(16);
        drained.put_slice(&[7u8; 8]);
        let (_bytes, calls) = allocation_calls_during(|| drained.copy_to_bytes(8));
        assert_eq!(calls, 1);
    }

    fn assert_encode_iobufs_matches_encode<T: Encode + EncodeExt>(value: &T) {
        let mut encoded = value.encode_iobufs();
        let baseline = value.encode();
        let mut encoded_bytes = vec![0u8; encoded.remaining()];
        encoded.copy_to_slice(&mut encoded_bytes);
        assert_eq!(encoded_bytes, baseline.as_ref());
    }

    #[test]
    fn test_iobufs_from_iter_and_array_shapes() {
        fn check<const N: usize>() {
            let data = b"abcde";
            let chunks: [IoBuf; N] = std::array::from_fn(|i| IoBuf::from(&data[i..i + 1]));
            for bufs in [IoBufs::from(chunks.clone()), chunks.into_iter().collect()] {
                assert!(matches!(
                    (&bufs.inner, N),
                    (IoBufsInner::Single(_), 0 | 1)
                        | (IoBufsInner::Pair(_), 2)
                        | (IoBufsInner::Triple(_), 3)
                        | (IoBufsInner::Chunked(_), 4..)
                ));
                assert_eq!(bufs.chunk_count(), N);
                for index in 0..N {
                    assert_eq!(
                        bufs.chunk_at(index).unwrap().as_ptr(),
                        data[index..].as_ptr()
                    );
                }
                assert_eq!(bufs.coalesce().as_ref(), &data[..N]);
            }
        }

        check::<0>();
        check::<1>();
        check::<2>();
        check::<3>();
        check::<4>();
        check::<5>();
    }

    #[test]
    fn test_iobufs_from_iter_and_array_drop_empty_buffers() {
        fn check<const N: usize>(chunks: [&'static [u8]; N], expected: &[u8]) {
            let chunks = chunks.map(IoBuf::from);
            for bufs in [IoBufs::from(chunks.clone()), chunks.into_iter().collect()] {
                // Each nonempty input contains one byte.
                assert_eq!(bufs.chunk_count(), expected.len());
                assert!(matches!(
                    (&bufs.inner, expected.len()),
                    (IoBufsInner::Single(_), 0 | 1)
                        | (IoBufsInner::Pair(_), 2)
                        | (IoBufsInner::Triple(_), 3)
                        | (IoBufsInner::Chunked(_), 4..)
                ));
                assert_eq!(bufs.coalesce().as_ref(), expected);
            }
        }

        check([b"", b"", b""], b"");
        check([b"", b"a", b""], b"a");
        check([b"", b"a", b"b"], b"ab");
        check([b"a", b"", b"b"], b"ab");
        check([b"a", b"b", b""], b"ab");
        check([b"a", b"", b"b", b"c"], b"abc");
        check([b"a", b"b", b"", b"c", b"d"], b"abcd");
        check([b"", b"a", b"", b"b", b"c", b"d", b"e", b""], b"abcde");
    }

    #[test]
    fn test_iobufs_from_iter_error_releases_buffers() {
        const BUFFER_COUNT: usize = 6;

        // Cover failures before and after the transition to deque storage,
        // always leaving at least one unconsumed buffer in the iterator.
        for fail_at in 0..=4 {
            let owners: Vec<Arc<[u8]>> = (0..BUFFER_COUNT).map(|_| Arc::from(&b"x"[..])).collect();
            let bufs: Vec<_> = owners
                .iter()
                .map(|owner| IoBuf::from(Bytes::from_owner(owner.clone())))
                .collect();
            // The test and the buffer each hold one reference to the owner.
            assert!(owners.iter().all(|owner| Arc::strong_count(owner) == 2));

            let mut items_pulled = 0;
            let result = bufs
                .into_iter()
                .enumerate()
                .map(|(index, buf)| {
                    items_pulled += 1;
                    if index == fail_at {
                        Err("injected")
                    } else {
                        Ok(buf)
                    }
                })
                .collect::<Result<IoBufs, _>>();

            assert_eq!(result.unwrap_err(), "injected");
            assert_eq!(items_pulled, fail_at + 1);
            // Only the test's reference remains, including for unconsumed buffers.
            for owner in owners {
                assert_eq!(Arc::strong_count(&owner), 1);
            }
        }
    }

    #[test]
    fn test_iobufsmut_from_iter_and_array_shapes() {
        fn check<const N: usize>() {
            let chunks =
                || -> [IoBufMut; N] { std::array::from_fn(|_| IoBufMut::with_capacity(1)) };
            for bufs in [IoBufsMut::from(chunks()), chunks().into_iter().collect()] {
                assert!(matches!(
                    (&bufs.inner, N),
                    (IoBufsMutInner::Single(_), 0 | 1)
                        | (IoBufsMutInner::Pair(_), 2)
                        | (IoBufsMutInner::Triple(_), 3)
                        | (IoBufsMutInner::Chunked(_), 4..)
                ));
                assert!(bufs.is_empty());
                assert_eq!(bufs.capacity(), N);
                assert_eq!(bufs.remaining_mut(), N);
            }
        }

        check::<0>();
        check::<1>();
        check::<2>();
        check::<3>();
        check::<4>();
        check::<5>();
    }

    #[test]
    fn test_iobufsmut_from_iter_and_array_preserve_capacity() {
        fn check(convert: impl FnOnce([IoBufMut; 4]) -> IoBufsMut) {
            let empty = IoBufMut::with_capacity(2);
            let mut partial = IoBufMut::with_capacity(2);
            partial.put_slice(b"a");
            let full = IoBufMut::from(b"bc");
            let pointers = [
                empty.as_ref().as_ptr(),
                partial.as_ref().as_ptr(),
                full.as_ref().as_ptr(),
            ];

            let mut bufs = convert([IoBufMut::default(), empty, partial, full]);
            assert!(matches!(bufs.inner, IoBufsMutInner::Triple(_)));
            assert_eq!(bufs.len(), 3);
            assert_eq!(bufs.capacity(), 6);
            assert_eq!(bufs.remaining_mut(), 3);

            // Fill the empty buffer with XY and the partial buffer's tail with Z.
            bufs.put_slice(b"XYZ");
            assert_eq!(bufs.capacity(), 6);
            assert_eq!(bufs.remaining_mut(), 0);
            let frozen = bufs.freeze();
            for (index, pointer) in pointers.into_iter().enumerate() {
                assert_eq!(frozen.chunk_at(index).unwrap().as_ptr(), pointer);
            }
            assert_eq!(frozen.coalesce(), b"XYaZbc");
        }

        check(IoBufsMut::from);
        check(|chunks| chunks.into_iter().collect());
    }

    #[test]
    fn test_iobufs_emptiness_after_consumption() {
        fn assert_state(bufs: &IoBufs, expected: &[u8]) {
            assert_eq!(bufs.remaining(), expected.len());
            assert_eq!(bufs.is_empty(), expected.is_empty());
            assert_eq!(bufs.has_remaining(), !expected.is_empty());
            let mut actual = Vec::new();
            bufs.for_each_chunk(|chunk| actual.extend_from_slice(chunk));
            assert_eq!(actual, expected);
        }

        let data = b"abcdefgh";
        let source = IoBufs::from(vec![
            IoBuf::default(),
            IoBuf::from(b"ab"),
            IoBuf::default(),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
            IoBuf::default(),
            IoBuf::from(b"gh"),
            IoBuf::default(),
        ]);

        // Prefixes exercise every representation, including an empty Single.
        for end in 0..=data.len() {
            let mut suffix = source.clone();
            let prefix = suffix.split_to(end);
            assert_state(&prefix, &data[..end]);
            assert_state(&suffix, &data[end..]);

            for step in [1, 2, 3, data.len()] {
                let mut advanced = prefix.clone();
                let mut copied = prefix.clone();
                let mut offset = 0;
                while offset < end {
                    let count = step.min(end - offset);
                    advanced.advance(count);
                    assert_eq!(copied.copy_to_bytes(count), &data[offset..offset + count]);
                    offset += count;
                    assert_state(&advanced, &data[offset..end]);
                    assert_state(&copied, &data[offset..end]);
                }

                // Empty inputs preserve emptiness, and new bytes make a drained buffer readable.
                advanced.append(IoBuf::default());
                advanced.prepend(IoBuf::default());
                assert_state(&advanced, b"");
                advanced.append(IoBuf::from(b"z"));
                advanced.prepend(IoBuf::from(b"y"));
                assert_state(&advanced, b"yz");
            }

            let mut input = prefix;
            let mut output = IoBufMut::with_capacity(data.len());
            output.put(&mut input);
            assert_eq!(output.as_ref(), &data[..end]);
            assert_state(&input, b"");
        }
    }

    #[test]
    fn test_iobufs_for_each_chunk_single_and_empty() {
        // The Single arm is bypassed by coalesce (which returns the buffer
        // directly), so exercise the public method on both Single shapes.
        let mut seen = Vec::new();
        IoBufs::from(b"hello").for_each_chunk(|chunk| seen.push(chunk.to_vec()));
        assert_eq!(seen, vec![b"hello".to_vec()]);

        let mut count = 0;
        IoBufs::default().for_each_chunk(|_| count += 1);
        assert_eq!(count, 0);
    }

    #[test]
    fn test_iobufs_shapes_and_read_paths() {
        // Empty construction normalizes to an empty single chunk.
        let empty = IoBufs::from(Vec::<u8>::new());
        assert!(empty.is_empty());
        assert!(empty.is_single());
        assert!(empty.as_single().is_some());

        // Single-buffer read path.
        let mut single = IoBufs::from(b"hello world");
        assert!(single.is_single());
        assert_eq!(single.chunk(), b"hello world");
        single.advance(6);
        assert_eq!(single.chunk(), b"world");
        assert_eq!(single.copy_to_bytes(5).as_ref(), b"world");
        assert_eq!(single.remaining(), 0);

        // Fast-path shapes (Pair/Triple/Chunked).
        let mut pair = IoBufs::from(IoBuf::from(b"a"));
        pair.append(IoBuf::from(b"b"));
        assert!(matches!(pair.inner, IoBufsInner::Pair(_)));
        assert!(pair.as_single().is_none());

        let mut triple = IoBufs::from(IoBuf::from(b"a"));
        triple.append(IoBuf::from(b"b"));
        triple.append(IoBuf::from(b"c"));
        assert!(matches!(triple.inner, IoBufsInner::Triple(_)));

        let mut chunked = IoBufs::from(IoBuf::from(b"a"));
        chunked.append(IoBuf::from(b"b"));
        chunked.append(IoBuf::from(b"c"));
        chunked.append(IoBuf::from(b"d"));
        assert!(matches!(chunked.inner, IoBufsInner::Chunked(_)));

        // prepend + append preserve ordering.
        let mut joined = IoBufs::from(b"middle");
        joined.prepend(IoBuf::from(b"start "));
        joined.append(IoBuf::from(b" end"));
        assert_eq!(joined.coalesce(), b"start middle end");

        // prepending empty is a no-op, and prepending into pair upgrades to triple.
        let mut prepend_noop = IoBufs::from(b"x");
        prepend_noop.prepend(IoBuf::default());
        assert_eq!(prepend_noop.coalesce(), b"x");

        // Prepending into an empty aggregate should stay on the single-buffer fast path.
        let mut prepend_into_empty = IoBufs::default();
        prepend_into_empty.prepend(IoBuf::from(b"z"));
        assert!(prepend_into_empty.is_single());
        assert_eq!(prepend_into_empty.coalesce(), b"z");

        let mut prepend_pair = IoBufs::from(vec![IoBuf::from(b"b"), IoBuf::from(b"c")]);
        prepend_pair.prepend(IoBuf::from(b"a"));
        assert!(matches!(prepend_pair.inner, IoBufsInner::Triple(_)));
        assert_eq!(prepend_pair.coalesce(), b"abc");

        // canonicalizing a non-empty single should keep the same representation.
        let mut canonical_single = IoBufs::from(b"q");
        canonical_single.canonicalize();
        assert!(canonical_single.is_single());
        assert_eq!(canonical_single.coalesce(), b"q");
    }

    #[test]
    fn test_iobufs_split_to_cases() {
        // Zero and full split on a single chunk.
        let mut bufs = IoBufs::from(b"hello");

        let empty = bufs.split_to(0);
        assert!(empty.is_empty());
        assert_eq!(bufs.coalesce(), b"hello");

        let mut bufs = IoBufs::from(b"hello");
        let all = bufs.split_to(5);
        assert_eq!(all.coalesce(), b"hello");
        assert!(bufs.is_single());
        assert!(bufs.is_empty());

        // Single split in the middle.
        let mut single_mid = IoBufs::from(b"hello");
        let single_prefix = single_mid.split_to(2);
        assert!(single_prefix.is_single());
        assert_eq!(single_prefix.coalesce(), b"he");
        assert_eq!(single_mid.coalesce(), b"llo");

        // Pair split paths: in-first, boundary-after-first, crossing-into-second.
        let mut pair = IoBufs::from(vec![IoBuf::from(b"ab"), IoBuf::from(b"cd")]);
        let pair_prefix = pair.split_to(1);
        assert!(pair_prefix.is_single());
        assert_eq!(pair_prefix.coalesce(), b"a");
        assert!(matches!(pair.inner, IoBufsInner::Pair(_)));
        assert_eq!(pair.coalesce(), b"bcd");

        let mut pair = IoBufs::from(vec![IoBuf::from(b"ab"), IoBuf::from(b"cd")]);
        let pair_prefix = pair.split_to(2);
        assert!(pair_prefix.is_single());
        assert_eq!(pair_prefix.coalesce(), b"ab");
        assert!(pair.is_single());
        assert_eq!(pair.coalesce(), b"cd");

        let mut pair = IoBufs::from(vec![IoBuf::from(b"ab"), IoBuf::from(b"cd")]);
        let pair_prefix = pair.split_to(3);
        assert!(matches!(pair_prefix.inner, IoBufsInner::Pair(_)));
        assert_eq!(pair_prefix.coalesce(), b"abc");
        assert!(pair.is_single());
        assert_eq!(pair.coalesce(), b"d");

        // Triple split paths: in-first, boundary-after-first, in-second, boundary-after-second,
        // and reaching into third.
        let mut triple = IoBufs::from(vec![
            IoBuf::from(b"ab"),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
        ]);
        let triple_prefix = triple.split_to(1);
        assert!(triple_prefix.is_single());
        assert_eq!(triple_prefix.coalesce(), b"a");
        assert!(matches!(triple.inner, IoBufsInner::Triple(_)));
        assert_eq!(triple.coalesce(), b"bcdef");

        let mut triple = IoBufs::from(vec![
            IoBuf::from(b"ab"),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
        ]);
        let triple_prefix = triple.split_to(2);
        assert!(triple_prefix.is_single());
        assert_eq!(triple_prefix.coalesce(), b"ab");
        assert!(matches!(triple.inner, IoBufsInner::Pair(_)));
        assert_eq!(triple.coalesce(), b"cdef");

        let mut triple = IoBufs::from(vec![
            IoBuf::from(b"ab"),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
        ]);
        let triple_prefix = triple.split_to(3);
        assert!(matches!(triple_prefix.inner, IoBufsInner::Pair(_)));
        assert_eq!(triple_prefix.coalesce(), b"abc");
        assert!(matches!(triple.inner, IoBufsInner::Pair(_)));
        assert_eq!(triple.coalesce(), b"def");

        let mut triple = IoBufs::from(vec![
            IoBuf::from(b"ab"),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
        ]);
        let triple_prefix = triple.split_to(4);
        assert!(matches!(triple_prefix.inner, IoBufsInner::Pair(_)));
        assert_eq!(triple_prefix.coalesce(), b"abcd");
        assert!(triple.is_single());
        assert_eq!(triple.coalesce(), b"ef");

        let mut triple = IoBufs::from(vec![
            IoBuf::from(b"ab"),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
        ]);
        let triple_prefix = triple.split_to(5);
        assert!(matches!(triple_prefix.inner, IoBufsInner::Triple(_)));
        assert_eq!(triple_prefix.coalesce(), b"abcde");
        assert!(triple.is_single());
        assert_eq!(triple.coalesce(), b"f");

        // Chunked split can canonicalize remainder/prefix shapes.
        let mut bufs = IoBufs::from(vec![
            IoBuf::from(b"ab"),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
            IoBuf::from(b"gh"),
        ]);
        let prefix = bufs.split_to(4);
        assert!(matches!(prefix.inner, IoBufsInner::Pair(_)));
        assert_eq!(prefix.coalesce(), b"abcd");
        assert!(matches!(bufs.inner, IoBufsInner::Pair(_)));
        assert_eq!(bufs.coalesce(), b"efgh");

        // Chunked split inside a chunk.
        let mut bufs = IoBufs::from(vec![
            IoBuf::from(b"ab"),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
            IoBuf::from(b"gh"),
        ]);
        let prefix = bufs.split_to(5);
        assert!(matches!(prefix.inner, IoBufsInner::Triple(_)));
        assert_eq!(prefix.coalesce(), b"abcde");
        assert!(matches!(bufs.inner, IoBufsInner::Pair(_)));
        assert_eq!(bufs.coalesce(), b"fgh");

        // Chunked split can remain chunked on both sides when both have >= 4 chunks.
        let mut bufs = IoBufs::from(vec![
            IoBuf::from(b"a"),
            IoBuf::from(b"b"),
            IoBuf::from(b"c"),
            IoBuf::from(b"d"),
            IoBuf::from(b"e"),
            IoBuf::from(b"f"),
            IoBuf::from(b"g"),
            IoBuf::from(b"h"),
        ]);
        let prefix = bufs.split_to(4);
        assert!(matches!(prefix.inner, IoBufsInner::Chunked(_)));
        assert_eq!(prefix.coalesce(), b"abcd");
        assert!(matches!(bufs.inner, IoBufsInner::Chunked(_)));
        assert_eq!(bufs.coalesce(), b"efgh");

        // Chunked split inside a chunk that leaves both sides chunked.
        let mut bufs: IoBufs = (b'a'..b'i')
            .map(|byte| IoBuf::from(vec![byte; 2]))
            .collect();
        let prefix = bufs.split_to(9);
        assert!(matches!(prefix.inner, IoBufsInner::Chunked(_)));
        assert_eq!(prefix.chunk_count(), 5);
        assert_eq!(prefix.coalesce(), b"aabbccdde");
        assert!(matches!(bufs.inner, IoBufsInner::Chunked(_)));
        assert_eq!(bufs.chunk_count(), 4);
        assert_eq!(bufs.coalesce(), b"effgghh");

        // Defensive path: tolerate accidental empty chunks in non-canonical chunked input.
        let mut bufs = IoBufs {
            inner: IoBufsInner::Chunked(VecDeque::from([
                IoBuf::default(),
                IoBuf::from(b"ab"),
                IoBuf::from(b"cd"),
                IoBuf::from(b"ef"),
                IoBuf::from(b"gh"),
            ])),
        };
        let prefix = bufs.split_to(3);
        assert_eq!(prefix.coalesce(), b"abc");
        assert_eq!(bufs.coalesce(), b"defgh");
    }

    #[test]
    #[should_panic(expected = "split_to out of bounds")]
    fn test_iobufs_split_to_out_of_bounds() {
        let mut bufs = IoBufs::from(b"abc");
        let _ = bufs.split_to(4);
    }

    /// Splits at every position of every representation keep the bytes in order and leave both
    /// sides canonical. The positions cover prefixes within the front chunk and across chunks,
    /// and chunked splits that move out either side.
    #[test]
    fn test_iobufs_split_positions() {
        for count in 1..10 {
            for rotation in 0..count {
                for at in 0..=2 * count {
                    // Rotation wraps a chunked deque around the end of its buffer, so the drains
                    // also run over a deque stored in two slices. Four or more chunks keep that
                    // deque as their chunked representation.
                    let mut chunks: VecDeque<IoBuf> = (0..count as u8)
                        .map(|byte| IoBuf::copy_from_slice(&[byte; 2]))
                        .collect();
                    chunks.rotate_left(rotation);
                    let expected: Vec<u8> = chunks
                        .iter()
                        .flat_map(|buf| buf.as_ref().to_vec())
                        .collect();
                    let mut bufs = if count >= 4 {
                        IoBufs {
                            inner: IoBufsInner::Chunked(chunks),
                        }
                    } else {
                        IoBufs::from_chunks_iter(chunks)
                    };

                    let prefix = bufs.split_to(at);

                    // Each side stores only non-empty chunks, in the representation its chunk
                    // count calls for, and the two sides hold the original bytes in order.
                    for side in [&prefix, &bufs] {
                        let chunks = side.chunk_count();
                        assert_eq!(side.iter().filter(|buf| !buf.is_empty()).count(), chunks);
                        assert_eq!(side.is_single(), chunks <= 1);
                        assert_eq!(matches!(side.inner, IoBufsInner::Chunked(_)), chunks >= 4);
                    }
                    assert_eq!(prefix.coalesce().as_ref(), &expected[..at]);
                    assert_eq!(bufs.coalesce().as_ref(), &expected[at..]);
                }
            }
        }
    }

    /// A split of a chunked container moves out the side with fewer chunks, counting the piece
    /// toward the prefix, and keeps the deque for the other, so taking a small prefix or leaving a
    /// small suffix allocates nothing however many chunks remain. The chunks are static, so
    /// splitting one in place allocates nothing either.
    #[test]
    fn test_iobufs_chunked_split_allocations() {
        // Every split folds over the remaining chunks, so the count stays small enough for Miri
        // while keeping the container chunked throughout.
        const CHUNKS: usize = 256;

        // Splitting inside the first of many chunks.
        let mut bufs: IoBufs = (0..CHUNKS).map(|_| IoBuf::from(b"ab")).collect();
        let (prefix, calls) = allocation_calls_during(|| bufs.split_to(1));
        assert_eq!(calls, 0);
        assert_eq!(prefix.coalesce(), b"a");
        assert_eq!(bufs.len(), 2 * CHUNKS - 1);

        // A split across the first two chunks moves out only those two.
        let (prefix, calls) = allocation_calls_during(|| bufs.split_to(2));
        assert_eq!(calls, 0);
        assert_eq!(prefix.coalesce(), b"ba");

        // A split that leaves only the last two chunks moves out only those two.
        let at = bufs.len() - 3;
        let (_, calls) = allocation_calls_during(|| bufs.split_to(at));
        assert_eq!(calls, 0);
        assert_eq!(bufs.coalesce(), b"bab");

        // A split inside the fourth of six chunks puts four chunks in the prefix and three in the
        // suffix, so the suffix moves out and the deque stays.
        let mut bufs: IoBufs = (0..6).map(|_| IoBuf::from(b"ab")).collect();
        let (prefix, calls) = allocation_calls_during(|| bufs.split_to(7));
        assert_eq!(calls, 0);
        assert_eq!(prefix.coalesce(), b"abababa");
        assert_eq!(bufs.coalesce(), b"babab");

        // Extracting the chunks one at a time.
        let mut bufs: IoBufs = (0..CHUNKS).map(|_| IoBuf::from(b"ab")).collect();
        let ((), calls) = allocation_calls_during(|| {
            while bufs.chunk_count() >= 4 {
                assert_eq!(bufs.split_to(2).coalesce(), b"ab");
            }
        });
        assert_eq!(calls, 0);
    }

    #[test]
    fn test_iobufs_chunk_count() {
        assert_eq!(IoBufs::default().chunk_count(), 0);
        assert_eq!(IoBufs::from(IoBuf::from(b"a")).chunk_count(), 1);
        assert_eq!(
            IoBufs::from(vec![IoBuf::from(b"b"), IoBuf::from(b"c")]).chunk_count(),
            2
        );
        assert_eq!(
            IoBufs::from(vec![
                IoBuf::from(b"a"),
                IoBuf::from(b"b"),
                IoBuf::from(b"c")
            ])
            .chunk_count(),
            3
        );
        assert_eq!(
            IoBufs::from(vec![
                IoBuf::from(b"a"),
                IoBuf::from(b"b"),
                IoBuf::from(b"c"),
                IoBuf::from(b"d")
            ])
            .chunk_count(),
            4
        );
    }

    #[test]
    fn test_iobufs_chunk_at() {
        let chunks = [b"ab".as_slice(), b"cd", b"ef", b"gh", b"ij"];
        for count in 0..=chunks.len() {
            let mut bufs = IoBufs::from(
                chunks[..count]
                    .iter()
                    .map(|chunk| IoBuf::from(*chunk))
                    .collect::<Vec<_>>(),
            );
            for (index, chunk) in chunks[..count].iter().enumerate() {
                assert_eq!(bufs.chunk_at(index), Some(*chunk));
            }
            assert_eq!(bufs.chunk_at(count), None);
            assert_eq!(bufs.chunk_at(usize::MAX), None);
            assert_eq!(bufs.len(), count * 2);

            // Indexing follows the readable chunks after partial and whole
            // chunks are consumed, including representation transitions.
            if count > 0 {
                bufs.advance(1);
                assert_eq!(bufs.chunk_at(0), Some(b"b".as_slice()));
                bufs.advance(1);
                assert_eq!(bufs.chunk_at(0), chunks[1..count].first().copied());
            }
        }
    }

    #[test]
    fn test_iobufs_iter_borrows_every_owner() {
        let chunks = [b"ab".as_slice(), b"cd", b"ef", b"gh", b"ij"];
        for count in 0..=chunks.len() {
            let bufs = IoBufs::from(
                chunks[..count]
                    .iter()
                    .map(|chunk| IoBuf::from(*chunk))
                    .collect::<Vec<_>>(),
            );
            let iter = bufs.iter();
            assert_eq!(iter.len(), count);
            assert_eq!(
                iter.clone().map(AsRef::as_ref).collect::<Vec<_>>(),
                chunks[..count]
            );
            assert_eq!(iter.count(), count);
        }
    }

    #[test]
    fn test_iobufs_coalesce_after_advance() {
        let mut bufs = IoBufs::from(IoBuf::from(b"hello"));
        bufs.append(IoBuf::from(b" world"));

        assert_eq!(bufs.len(), 11);

        bufs.advance(3);
        assert_eq!(bufs.len(), 8);

        assert_eq!(bufs.coalesce(), b"lo world");
    }

    #[test]
    fn test_iobufs_coalesce_shapes() {
        // Single buffer: zero-copy (same pointer)
        let buf = IoBuf::from(vec![1u8, 2, 3, 4, 5]);
        let original_ptr = buf.as_ptr();
        let bufs = IoBufs::from(buf);
        let coalesced = bufs.coalesce();
        assert_eq!(coalesced, [1, 2, 3, 4, 5]);
        assert_eq!(coalesced.as_ptr(), original_ptr);

        // Multiple buffers: merged into one allocation
        let mut bufs = IoBufs::from(IoBuf::from(b"hello"));
        bufs.append(IoBuf::from(b" world"));
        let coalesced = bufs.coalesce();
        assert_eq!(coalesced, b"hello world");

        // Multiple buffers after advance: only remaining data coalesced
        let mut bufs = IoBufs::from(IoBuf::from(b"hello"));
        bufs.append(IoBuf::from(b" world"));
        bufs.advance(3);
        let coalesced = bufs.coalesce();
        assert_eq!(coalesced, b"lo world");

        // Empty buffers in the middle
        let mut bufs = IoBufs::from(IoBuf::from(b"hello"));
        bufs.append(IoBuf::default());
        bufs.append(IoBuf::from(b" world"));
        let coalesced = bufs.coalesce();
        assert_eq!(coalesced, b"hello world");

        // Empty IoBufs
        let bufs = IoBufs::default();
        let coalesced = bufs.coalesce();
        assert!(coalesced.is_empty());

        // 4+ buffers: exercise the chunked coalesce path.
        let bufs = IoBufs::from(vec![
            IoBuf::from(b"ab"),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
            IoBuf::from(b"gh"),
        ]);
        let coalesced = bufs.coalesce();
        assert_eq!(coalesced, b"abcdefgh");
    }

    #[test]
    fn test_iobufsmut_decode_preserves_byte_fields() {
        let first = IoBufMut::from(&b"\x02\x03abc"[..]);
        let second = IoBufMut::from(&b"\x03def"[..]);
        let ranges = [
            first.as_ref().as_ptr_range(),
            second.as_ref().as_ptr_range(),
        ];
        let source = IoBufsMut::from(vec![first, second]);
        let decoded = Vec::<Bytes>::decode_cfg(source, &((..).into(), (..).into())).unwrap();
        assert_eq!(
            decoded,
            [Bytes::from_static(b"abc"), Bytes::from_static(b"def")]
        );
        for (field, range) in decoded.iter().zip(ranges) {
            assert!(range.contains(&field.as_ptr()));
        }
    }

    #[test]
    fn test_iobufs_empty_chunks_and_copy_to_bytes_paths() {
        // Empty chunks are skipped while reading across multiple chunks.
        let mut bufs = IoBufs::default();
        bufs.append(IoBuf::from(b"hello"));
        bufs.append(IoBuf::default());
        bufs.append(IoBuf::from(b" "));
        bufs.append(IoBuf::default());
        bufs.append(IoBuf::from(b"world"));
        assert_eq!(bufs.len(), 11);
        assert_eq!(bufs.chunk(), b"hello");
        bufs.advance(5);
        assert_eq!(bufs.chunk(), b" ");
        bufs.advance(1);
        assert_eq!(bufs.chunk(), b"world");

        // Single-buffer copy_to_bytes path.
        let mut single = IoBufs::from(b"hello world");
        assert_eq!(single.copy_to_bytes(5).as_ref(), b"hello");
        assert_eq!(single.remaining(), 6);

        // Multi-buffer copy_to_bytes path across boundaries.
        let mut multi = IoBufs::from(b"hello");
        multi.prepend(IoBuf::from(b"say "));
        assert_eq!(multi.copy_to_bytes(7).as_ref(), b"say hel");
        assert_eq!(multi.copy_to_bytes(2).as_ref(), b"lo");
    }

    #[test]
    fn test_iobufs_copy_to_bytes_pair_and_triple() {
        // Pair: crossing one boundary should collapse to the trailing single chunk.
        let mut pair = IoBufs::from(IoBuf::from(b"ab"));
        pair.append(IoBuf::from(b"cd"));
        let first = pair.copy_to_bytes(3);
        assert_eq!(&first[..], b"abc");
        assert!(pair.is_single());
        assert_eq!(pair.chunk(), b"d");

        // Triple: draining across two chunks leaves the final chunk readable.
        let mut triple = IoBufs::from(IoBuf::from(b"ab"));
        triple.append(IoBuf::from(b"cd"));
        triple.append(IoBuf::from(b"ef"));
        let first = triple.copy_to_bytes(5);
        assert_eq!(&first[..], b"abcde");
        assert!(triple.is_single());
        assert_eq!(triple.chunk(), b"f");
    }

    #[test]
    fn test_iobufs_copy_to_bytes_chunked_four_plus() {
        let mut bufs = IoBufs::from(vec![
            IoBuf::from(b"ab"),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
            IoBuf::from(b"gh"),
        ]);

        // Chunked fast-path: first chunk alone satisfies request.
        let first = bufs.copy_to_bytes(1);
        assert_eq!(&first[..], b"a");

        // Chunked slow-path: request crosses chunk boundaries.
        let second = bufs.copy_to_bytes(4);
        assert_eq!(&second[..], b"bcde");

        let rest = bufs.copy_to_bytes(3);
        assert_eq!(&rest[..], b"fgh");
        assert_eq!(bufs.remaining(), 0);
    }

    #[test]
    fn test_iobufs_copy_to_bytes_edge_cases() {
        // A non-canonical leading empty chunk in the deque path is popped
        // without affecting the copied payload. Canonical construction never
        // stores empties, so the state is built directly.
        let mut iobufs = IoBufs {
            inner: IoBufsInner::Chunked(VecDeque::from([
                IoBuf::default(),
                IoBuf::from(b"hel"),
                IoBuf::from(b"lo"),
                IoBuf::from(b" world"),
            ])),
        };
        assert_eq!(iobufs.copy_to_bytes(5).as_ref(), b"hello");
        assert_eq!(iobufs.remaining(), 6);

        // Boundary-aligned reads should return exact chunk payloads in-order.
        let mut boundary = IoBufs::from(IoBuf::from(b"hello"));
        boundary.append(IoBuf::from(b"world"));
        assert_eq!(boundary.copy_to_bytes(5).as_ref(), b"hello");
        assert_eq!(boundary.copy_to_bytes(5).as_ref(), b"world");
        assert_eq!(boundary.remaining(), 0);
    }

    #[test]
    #[should_panic(expected = "cannot advance past end of buffer")]
    fn test_iobufs_advance_past_end() {
        let mut bufs = IoBufs::from(b"hel");
        bufs.append(IoBuf::from(b"lo"));
        bufs.advance(10);
    }

    #[test]
    #[should_panic(expected = "not enough data")]
    fn test_iobufs_copy_to_bytes_past_end() {
        let mut bufs = IoBufs::from(b"hel");
        bufs.append(IoBuf::from(b"lo"));
        bufs.copy_to_bytes(10);
    }

    #[test]
    fn test_iobufs_matches_bytes_chain() {
        let b1 = Bytes::from_static(b"hello");
        let b2 = Bytes::from_static(b" ");
        let b3 = Bytes::from_static(b"world");

        // Buf parity for remaining/chunk/advance should match `Bytes::chain`.
        let mut chain = b1.clone().chain(b2.clone()).chain(b3.clone());
        let mut iobufs = IoBufs::from(IoBuf::from(b1.clone()));
        iobufs.append(IoBuf::from(b2.clone()));
        iobufs.append(IoBuf::from(b3.clone()));

        assert_eq!(chain.remaining(), iobufs.remaining());
        assert_eq!(chain.chunk(), iobufs.chunk());

        chain.advance(3);
        iobufs.advance(3);
        assert_eq!(chain.remaining(), iobufs.remaining());
        assert_eq!(chain.chunk(), iobufs.chunk());

        chain.advance(3);
        iobufs.advance(3);
        assert_eq!(chain.remaining(), iobufs.remaining());
        assert_eq!(chain.chunk(), iobufs.chunk());

        // Test copy_to_bytes
        let mut chain = b1.clone().chain(b2.clone()).chain(b3.clone());
        let mut iobufs = IoBufs::from(IoBuf::from(b1));
        iobufs.append(IoBuf::from(b2));
        iobufs.append(IoBuf::from(b3));

        assert_eq!(chain.copy_to_bytes(3), iobufs.copy_to_bytes(3));
        assert_eq!(chain.copy_to_bytes(4), iobufs.copy_to_bytes(4));
        assert_eq!(
            chain.copy_to_bytes(chain.remaining()),
            iobufs.copy_to_bytes(iobufs.remaining())
        );
        assert_eq!(chain.remaining(), 0);
        assert_eq!(iobufs.remaining(), 0);
    }

    #[test]
    fn test_iobufs_try_into_single() {
        let single = IoBufs::from(IoBuf::from(b"hello"));
        let single = single.try_into_single().expect("single expected");
        assert_eq!(single, b"hello");

        let multi = IoBufs::from(vec![IoBuf::from(b"ab"), IoBuf::from(b"cd")]);
        let multi = multi.try_into_single().expect_err("multi expected");
        assert_eq!(multi.coalesce(), b"abcd");
    }

    #[test]
    fn test_iobufs_chunks_vectored_multiple_slices() {
        // Single non-empty buffers should export exactly one slice.
        let single = IoBufs::from(IoBuf::from(b"xy"));
        let mut single_dst = [IoSlice::new(&[]); 2];
        let count = single.chunks_vectored(&mut single_dst);
        assert_eq!(count, 1);
        assert_eq!(&single_dst[0][..], b"xy");

        // Single empty buffers should export no slices.
        let empty_single = IoBufs::default();
        let mut empty_single_dst = [IoSlice::new(&[]); 1];
        assert_eq!(empty_single.chunks_vectored(&mut empty_single_dst), 0);

        let bufs = IoBufs::from(vec![
            IoBuf::from(b"ab"),
            IoBuf::from(b"cd"),
            IoBuf::from(b"ef"),
            IoBuf::from(b"gh"),
        ]);

        // Destination capacity should cap how many chunks we export.
        let mut small = [IoSlice::new(&[]); 2];
        let count = bufs.chunks_vectored(&mut small);
        assert_eq!(count, 2);
        assert_eq!(&small[0][..], b"ab");
        assert_eq!(&small[1][..], b"cd");

        // Larger destination should include every readable chunk.
        let mut large = [IoSlice::new(&[]); 8];
        let count = bufs.chunks_vectored(&mut large);
        assert_eq!(count, 4);
        assert_eq!(&large[0][..], b"ab");
        assert_eq!(&large[1][..], b"cd");
        assert_eq!(&large[2][..], b"ef");
        assert_eq!(&large[3][..], b"gh");

        // Empty destination cannot accept any slices.
        let mut empty_dst: [IoSlice<'_>; 0] = [];
        assert_eq!(bufs.chunks_vectored(&mut empty_dst), 0);

        // Non-canonical shapes should skip empty leading chunks.
        let sparse = IoBufs {
            inner: IoBufsInner::Pair([IoBuf::default(), IoBuf::from(b"x")]),
        };
        let mut dst = [IoSlice::new(&[]); 2];
        let count = sparse.chunks_vectored(&mut dst);
        assert_eq!(count, 1);
        assert_eq!(&dst[0][..], b"x");

        // Triple should skip empty chunks and preserve readable order.
        let sparse_triple = IoBufs {
            inner: IoBufsInner::Triple([IoBuf::default(), IoBuf::from(b"y"), IoBuf::from(b"z")]),
        };
        let mut dst = [IoSlice::new(&[]); 3];
        let count = sparse_triple.chunks_vectored(&mut dst);
        assert_eq!(count, 2);
        assert_eq!(&dst[0][..], b"y");
        assert_eq!(&dst[1][..], b"z");

        // Chunked shapes with only empty buffers should export no slices.
        let empty_chunked = IoBufs {
            inner: IoBufsInner::Chunked(VecDeque::from([IoBuf::default(), IoBuf::default()])),
        };
        let mut dst = [IoSlice::new(&[]); 2];
        assert_eq!(empty_chunked.chunks_vectored(&mut dst), 0);
    }

    #[test]
    fn test_iobufsmut_freeze_chunked() {
        // Multiple non-empty buffers stay multi-chunk.
        let buf1 = IoBufMut::from(b"hello".as_ref());
        let buf2 = IoBufMut::from(b" world".as_ref());
        let bufs = IoBufsMut::from(vec![buf1, buf2]);
        let mut frozen = bufs.freeze();
        assert!(!frozen.is_single());
        assert_eq!(frozen.chunk(), b"hello");
        frozen.advance(5);
        assert_eq!(frozen.chunk(), b" world");
        frozen.advance(6);
        assert_eq!(frozen.remaining(), 0);

        // Empty buffers are filtered out.
        let buf1 = IoBufMut::from(b"hello".as_ref());
        let empty = IoBufMut::default();
        let buf2 = IoBufMut::from(b" world".as_ref());
        let bufs = IoBufsMut::from(vec![buf1, empty, buf2]);
        let mut frozen = bufs.freeze();
        assert!(!frozen.is_single());
        assert_eq!(frozen.chunk(), b"hello");
        frozen.advance(5);
        assert_eq!(frozen.chunk(), b" world");
        frozen.advance(6);
        assert_eq!(frozen.remaining(), 0);

        // Collapses to Single when one non-empty buffer remains
        let empty1 = IoBufMut::default();
        let buf = IoBufMut::from(b"only one".as_ref());
        let empty2 = IoBufMut::default();
        let bufs = IoBufsMut::from(vec![empty1, buf, empty2]);
        let frozen = bufs.freeze();
        assert!(frozen.is_single());
        assert_eq!(frozen.coalesce(), b"only one");

        // All empty buffers -> Single with empty buffer
        let empty1 = IoBufMut::default();
        let empty2 = IoBufMut::default();
        let bufs = IoBufsMut::from(vec![empty1, empty2]);
        let frozen = bufs.freeze();
        assert!(frozen.is_single());
        assert!(frozen.is_empty());
    }

    #[test]
    fn test_iobufsmut_from_vec() {
        // Empty Vec becomes Single with empty buffer
        let bufs = IoBufsMut::from(Vec::<IoBufMut>::new());
        assert!(bufs.is_single());
        assert!(bufs.is_empty());

        // Vec with one element becomes Single
        let buf = IoBufMut::from(b"test");
        let bufs = IoBufsMut::from(vec![buf]);
        assert!(bufs.is_single());
        assert_eq!(bufs.chunk(), b"test");

        // Vec with multiple elements becomes multi-chunk.
        let buf1 = IoBufMut::from(b"hello");
        let buf2 = IoBufMut::from(b" world");
        let bufs = IoBufsMut::from(vec![buf1, buf2]);
        assert!(!bufs.is_single());
    }

    #[test]
    fn test_iobufsmut_fast_path_shapes() {
        let pair = IoBufsMut::from(vec![IoBufMut::from(b"a"), IoBufMut::from(b"b")]);
        assert!(matches!(pair.inner, IoBufsMutInner::Pair(_)));

        let triple = IoBufsMut::from(vec![
            IoBufMut::from(b"a"),
            IoBufMut::from(b"b"),
            IoBufMut::from(b"c"),
        ]);
        assert!(matches!(triple.inner, IoBufsMutInner::Triple(_)));

        let chunked = IoBufsMut::from(vec![
            IoBufMut::from(b"a"),
            IoBufMut::from(b"b"),
            IoBufMut::from(b"c"),
            IoBufMut::from(b"d"),
        ]);
        assert!(matches!(chunked.inner, IoBufsMutInner::Chunked(_)));
    }

    #[test]
    fn test_iobufsmut_default() {
        // Default IoBufsMut should be a single empty chunk.
        let bufs = IoBufsMut::default();
        assert!(bufs.is_single());
        assert!(bufs.is_empty());
        assert_eq!(bufs.len(), 0);
    }

    #[test]
    fn test_iobufsmut_from_array() {
        // From<[u8; N]> should create a single-chunk container with the array data.
        let bufs = IoBufsMut::from([1u8, 2, 3, 4, 5]);
        assert!(bufs.is_single());
        assert_eq!(bufs.len(), 5);
        assert_eq!(bufs.chunk(), &[1, 2, 3, 4, 5]);
    }

    #[test]
    fn test_iobufsmut_buf_trait_chunked() {
        let buf1 = IoBufMut::from(b"hello");
        let buf2 = IoBufMut::from(b" ");
        let buf3 = IoBufMut::from(b"world");
        let mut bufs = IoBufsMut::from(vec![buf1, buf2, buf3]);

        assert_eq!(bufs.remaining(), 11);
        assert_eq!(bufs.chunk(), b"hello");

        // Advance within first buffer
        bufs.advance(3);
        assert_eq!(bufs.remaining(), 8);
        assert_eq!(bufs.chunk(), b"lo");

        // Advance past first buffer (should pop_front)
        bufs.advance(2);
        assert_eq!(bufs.remaining(), 6);
        assert_eq!(bufs.chunk(), b" ");

        // Advance exactly one buffer
        bufs.advance(1);
        assert_eq!(bufs.remaining(), 5);
        assert_eq!(bufs.chunk(), b"world");

        // Advance to end
        bufs.advance(5);
        assert_eq!(bufs.remaining(), 0);
    }

    #[test]
    #[should_panic(expected = "cannot advance past end of buffer")]
    fn test_iobufsmut_advance_past_end() {
        let buf1 = IoBufMut::from(b"hello");
        let buf2 = IoBufMut::from(b" world");
        let mut bufs = IoBufsMut::from(vec![buf1, buf2]);
        bufs.advance(20);
    }

    #[test]
    fn test_iobufsmut_bufmut_trait_single() {
        let mut bufs = IoBufsMut::from(IoBufMut::with_capacity(20));
        assert_eq!(bufs.remaining_mut(), 20);

        bufs.put_slice(b"hello");
        assert_eq!(bufs.chunk(), b"hello");
        assert_eq!(bufs.len(), 5);
        assert_eq!(bufs.remaining_mut(), 15);

        bufs.put_slice(b" world");
        assert_eq!(bufs.coalesce(), b"hello world");
    }

    #[test]
    fn test_iobufsmut_zeroed_write() {
        // Use zeroed buffers which have a fixed length
        let bufs = IoBufsMut::from(IoBufMut::zeroed(20));
        assert_eq!(bufs.len(), 20);

        // Can write using as_mut on coalesced buffer
        let mut coalesced = bufs.coalesce();
        coalesced.as_mut()[..5].copy_from_slice(b"hello");
        assert_eq!(&coalesced.as_ref()[..5], b"hello");
    }

    #[test]
    fn test_iobufsmut_bufmut_put_slice() {
        // Test writing across multiple buffers
        let buf1 = IoBufMut::with_capacity(5);
        let buf2 = IoBufMut::with_capacity(6);
        let mut bufs = IoBufsMut::from(vec![buf1, buf2]);

        // Write data
        bufs.put_slice(b"hello");
        bufs.put_slice(b" world");
        assert_eq!(bufs.coalesce(), b"hello world");
    }

    #[test]
    fn test_iobufs_advance_drains_buffers() {
        let mut bufs = IoBufs::from(IoBuf::from(b"hello"));
        bufs.append(IoBuf::from(b" "));
        bufs.append(IoBuf::from(b"world"));

        // Advance exactly past first buffer
        bufs.advance(5);
        assert_eq!(bufs.remaining(), 6);
        assert_eq!(bufs.chunk(), b" ");

        // Advance across multiple buffers
        bufs.advance(4);
        assert_eq!(bufs.remaining(), 2);
        assert_eq!(bufs.chunk(), b"ld");
    }

    #[test]
    fn test_iobufs_advance_exactly_to_boundary() {
        let mut bufs = IoBufs::from(IoBuf::from(b"abc"));
        bufs.append(IoBuf::from(b"def"));

        // Advance exactly to first buffer boundary
        bufs.advance(3);
        assert_eq!(bufs.remaining(), 3);
        assert_eq!(bufs.chunk(), b"def");

        // Advance exactly to end
        bufs.advance(3);
        assert_eq!(bufs.remaining(), 0);
    }

    #[test]
    fn test_iobufs_advance_canonicalizes_pair_to_single() {
        let mut bufs = IoBufs::from(IoBuf::from(b"ab"));
        bufs.append(IoBuf::from(b"cd"));
        bufs.advance(2);
        assert!(bufs.is_single());
        assert_eq!(bufs.chunk(), b"cd");
    }

    #[test]
    fn test_iobufsmut_emptiness_with_reserved_capacity() {
        for count in 0..=8 {
            for readable in 0..=count {
                let mut chunks = Vec::new();
                for index in 0..count {
                    let mut chunk = IoBufMut::with_capacity(4);
                    if index == readable {
                        chunk.put_u8(7);
                    }
                    chunks.push(chunk);
                }
                let mut bufs = IoBufsMut::from(chunks);
                assert_eq!(bufs.is_empty(), readable == count);
                assert_eq!(bufs.has_remaining(), readable != count);
                if readable != count {
                    bufs.advance(1);
                    assert!(bufs.is_empty());
                    assert!(!bufs.has_remaining());
                }
                if bufs.remaining_mut() > 0 {
                    bufs.put_slice(&[9]);
                    assert!(!bufs.is_empty());
                    assert!(bufs.has_remaining());
                }
            }
        }
    }

    #[test]
    fn test_iobufsmut_with_empty_buffers() {
        let buf1 = IoBufMut::from(b"hello");
        let buf2 = IoBufMut::default();
        let buf3 = IoBufMut::from(b" world");
        let mut bufs = IoBufsMut::from(vec![buf1, buf2, buf3]);

        assert_eq!(bufs.remaining(), 11);
        assert_eq!(bufs.chunk(), b"hello");

        // Advance past first buffer
        bufs.advance(5);
        // Empty buffer should be skipped
        assert_eq!(bufs.chunk(), b" world");
        assert_eq!(bufs.remaining(), 6);
    }

    #[test]
    fn test_iobufsmut_advance_skips_leading_writable_empty_chunk() {
        // A leading chunk with capacity but no readable bytes (len == 0) should
        // be skipped during advance, reaching the next readable chunk.
        let empty_writable = IoBufMut::with_capacity(4);
        let payload = IoBufMut::from(b"xy");
        let mut bufs = IoBufsMut::from(vec![empty_writable, payload]);

        bufs.advance(1);
        assert_eq!(bufs.chunk(), b"y");
        assert_eq!(bufs.remaining(), 1);
    }

    #[test]
    fn test_iobufsmut_read_ops_preserve_writable_capacity() {
        // Draining the filled first chunk must keep the never-filled second
        // chunk's reserved capacity, so reads leave remaining_mut unchanged.
        let mut a = IoBufMut::with_capacity(8);
        a.put_slice(&[1u8; 8]);
        let b = IoBufMut::with_capacity(8);
        let mut bufs = IoBufsMut::from(vec![a, b]);
        assert_eq!(bufs.remaining(), 8);
        assert_eq!(bufs.remaining_mut(), 8);
        bufs.advance(8);
        assert_eq!(bufs.remaining(), 0);
        assert_eq!(bufs.remaining_mut(), 8);
        bufs.put_slice(&[2u8; 8]);
        assert_eq!(bufs.copy_to_bytes(8).as_ref(), &[2u8; 8]);

        // copy_to_bytes drains must preserve capacity the same way.
        let mut a = IoBufMut::with_capacity(8);
        a.put_slice(&[3u8; 8]);
        let b = IoBufMut::with_capacity(8);
        let mut bufs = IoBufsMut::from(vec![a, b]);
        assert_eq!(bufs.copy_to_bytes(8).as_ref(), &[3u8; 8]);
        assert_eq!(bufs.remaining_mut(), 8);

        // Zero-length drains do not disturb chunk state at all.
        assert!(bufs.copy_to_bytes(0).is_empty());
        assert_eq!(bufs.remaining_mut(), 8);
    }

    #[test]
    fn test_iobufsmut_from_vec_u8_preserves_capacity() {
        // The vec's allocation is adopted and its reserved capacity is
        // preserved exactly.
        let mut vec = Vec::with_capacity(128);
        vec.extend_from_slice(b"abc");
        let cap = vec.capacity();
        let mut bufs = IoBufsMut::from(vec);
        assert_eq!(bufs.remaining(), 3);
        assert_eq!(bufs.capacity(), cap);
        bufs.put_slice(b"d");
        assert_eq!(bufs.copy_to_bytes(4).as_ref(), b"abcd");

        // An empty vec keeps its full reservation, so `read_at_buf` can fill a
        // `Vec::with_capacity(len)` in place through `set_len(len)`.
        let vec = Vec::<u8>::with_capacity(64);
        let len = vec.capacity();
        let mut bufs = IoBufsMut::from(vec);
        assert!(bufs.is_empty());
        assert_eq!(bufs.capacity(), len);
        // SAFETY: all `len` bytes are initialized by copy_from_slice below.
        unsafe { bufs.set_len(len) };
        bufs.copy_from_slice(&vec![9u8; len]);
        assert_eq!(bufs.copy_to_bytes(len).as_ref(), vec![9u8; len].as_slice());

        // Exactly-sized vec: adopted with contents intact.
        let bufs = IoBufsMut::from(vec![1u8, 2, 3]);
        assert_eq!(bufs.remaining(), 3);
        assert_eq!(bufs.capacity(), 3);
        assert_eq!(bufs.chunk(), &[1, 2, 3]);
    }

    #[test]
    fn test_iobufsmut_coalesce_after_advance() {
        // Advance mid-chunk: advance 3 of 11 bytes
        let buf1 = IoBufMut::from(b"hello");
        let buf2 = IoBufMut::from(b" world");
        let mut bufs = IoBufsMut::from(vec![buf1, buf2]);

        bufs.advance(3);
        assert_eq!(bufs.coalesce(), b"lo world");

        // Advance to exact chunk boundary: advance 5 of 11 bytes
        let buf1 = IoBufMut::from(b"hello");
        let buf2 = IoBufMut::from(b" world");
        let mut bufs = IoBufsMut::from(vec![buf1, buf2]);

        bufs.advance(5);
        assert_eq!(bufs.coalesce(), b" world");
    }

    #[test]
    fn test_iobufsmut_copy_to_bytes() {
        let buf1 = IoBufMut::from(b"hello");
        let buf2 = IoBufMut::from(b" world");
        let mut bufs = IoBufsMut::from(vec![buf1, buf2]);

        // First read spans chunks and leaves unread suffix.
        let first = bufs.copy_to_bytes(7);
        assert_eq!(&first[..], b"hello w");
        assert_eq!(bufs.remaining(), 4);

        // Second read drains the remainder.
        let rest = bufs.copy_to_bytes(4);
        assert_eq!(&rest[..], b"orld");
        assert_eq!(bufs.remaining(), 0);
    }

    #[test]
    fn test_iobufsmut_copy_to_bytes_chunked_four_plus() {
        let mut bufs = IoBufsMut::from(vec![
            IoBufMut::from(b"ab"),
            IoBufMut::from(b"cd"),
            IoBufMut::from(b"ef"),
            IoBufMut::from(b"gh"),
        ]);

        // Exercise chunked advance path before copy_to_bytes.
        bufs.advance(1);
        assert_eq!(bufs.chunk(), b"b");
        bufs.advance(1);
        assert_eq!(bufs.chunk(), b"cd");

        // Chunked fast-path: first chunk alone satisfies request.
        let first = bufs.copy_to_bytes(1);
        assert_eq!(&first[..], b"c");

        // Chunked slow-path: request crosses chunk boundaries.
        let second = bufs.copy_to_bytes(4);
        assert_eq!(&second[..], b"defg");

        let rest = bufs.copy_to_bytes(1);
        assert_eq!(&rest[..], b"h");
        assert_eq!(bufs.remaining(), 0);

        // Enter copy_to_bytes while still in chunked representation.
        let mut bufs = IoBufsMut::from(vec![
            IoBufMut::from(b"a"),
            IoBufMut::from(b"b"),
            IoBufMut::from(b"c"),
            IoBufMut::from(b"d"),
            IoBufMut::from(b"e"),
        ]);
        assert!(matches!(bufs.inner, IoBufsMutInner::Chunked(_)));
        let first = bufs.copy_to_bytes(1);
        assert_eq!(&first[..], b"a");
        // Stay chunked while consuming across multiple tiny chunks.
        let next = bufs.copy_to_bytes(3);
        assert_eq!(&next[..], b"bcd");
        assert_eq!(bufs.chunk(), b"e");
        assert_eq!(bufs.remaining(), 1);
    }

    #[test]
    fn test_iobufsmut_copy_to_bytes_canonicalizes_pair() {
        let mut bufs = IoBufsMut::from(vec![IoBufMut::from(b"ab"), IoBufMut::from(b"cd")]);
        assert!(matches!(bufs.inner, IoBufsMutInner::Pair(_)));

        let first = bufs.copy_to_bytes(2);
        assert_eq!(&first[..], b"ab");

        assert!(bufs.is_single());
        assert_eq!(bufs.chunk(), b"cd");
        assert_eq!(bufs.remaining(), 2);
    }

    #[test]
    fn test_iobufsmut_copy_from_slice_single() {
        let mut bufs = IoBufsMut::from(IoBufMut::zeroed(11));
        bufs.copy_from_slice(b"hello world");
        assert_eq!(bufs.coalesce(), b"hello world");
    }

    #[test]
    fn test_iobufsmut_copy_from_slice_chunked() {
        let buf1 = IoBufMut::zeroed(5);
        let buf2 = IoBufMut::zeroed(6);
        let mut bufs = IoBufsMut::from(vec![buf1, buf2]);

        bufs.copy_from_slice(b"hello world");

        // Verify each chunk was filled correctly.
        assert_eq!(bufs.chunk(), b"hello");
        bufs.advance(5);
        assert_eq!(bufs.chunk(), b" world");
        bufs.advance(6);
        assert_eq!(bufs.remaining(), 0);
    }

    #[test]
    #[should_panic(expected = "source slice length must match buffer length")]
    fn test_iobufsmut_copy_from_slice_wrong_length() {
        let mut bufs = IoBufsMut::from(IoBufMut::zeroed(5));
        bufs.copy_from_slice(b"hello world"); // 11 bytes into 5-byte buffer
    }

    #[test]
    fn test_iobufsmut_matches_bytesmut_chain() {
        // Create three BytesMut with capacity for final content comparison.
        let mut bm1 = BytesMut::with_capacity(5);
        let mut bm2 = BytesMut::with_capacity(6);
        let mut bm3 = BytesMut::with_capacity(7);

        // Create matching IoBufsMut
        let mut iobufs = IoBufsMut::from(vec![
            IoBufMut::with_capacity(5),
            IoBufMut::with_capacity(6),
            IoBufMut::with_capacity(7),
        ]);

        // Fixed-capacity IoBufsMut exposes the current writable chunk.
        assert_eq!(iobufs.chunk_mut().len(), 5);

        // Write some data
        (&mut bm1)
            .chain_mut(&mut bm2)
            .chain_mut(&mut bm3)
            .put_slice(b"hel");
        iobufs.put_slice(b"hel");

        assert_eq!(iobufs.chunk_mut().len(), 2);

        // Write more data
        (&mut bm1)
            .chain_mut(&mut bm2)
            .chain_mut(&mut bm3)
            .put_slice(b"lo world!");
        iobufs.put_slice(b"lo world!");

        assert_eq!(iobufs.chunk_mut().len(), 6);

        // Verify final content matches
        let frozen = iobufs.freeze().coalesce();
        let mut chain_content = bm1.to_vec();
        chain_content.extend_from_slice(&bm2);
        chain_content.extend_from_slice(&bm3);
        assert_eq!(frozen, chain_content.as_slice());
        assert_eq!(frozen, b"hello world!");
    }

    #[test]
    fn test_iobufsmut_buf_matches_bytes_chain() {
        // Create pre-filled Bytes buffers
        let mut b1 = Bytes::from_static(b"hello");
        let mut b2 = Bytes::from_static(b" world");
        let b3 = Bytes::from_static(b"!");

        // Create matching IoBufsMut
        let mut iobufs = IoBufsMut::from(vec![
            IoBufMut::from(b"hello"),
            IoBufMut::from(b" world"),
            IoBufMut::from(b"!"),
        ]);

        // Test Buf::remaining matches
        let chain_remaining = b1.clone().chain(b2.clone()).chain(b3.clone()).remaining();
        assert_eq!(chain_remaining, iobufs.remaining());

        // Test Buf::chunk matches
        let chain_chunk = b1
            .clone()
            .chain(b2.clone())
            .chain(b3.clone())
            .chunk()
            .to_vec();
        assert_eq!(chain_chunk, iobufs.chunk().to_vec());

        // Advance and test again
        b1.advance(3);
        iobufs.advance(3);

        let chain_remaining = b1.clone().chain(b2.clone()).chain(b3.clone()).remaining();
        assert_eq!(chain_remaining, iobufs.remaining());

        let chain_chunk = b1
            .clone()
            .chain(b2.clone())
            .chain(b3.clone())
            .chunk()
            .to_vec();
        assert_eq!(chain_chunk, iobufs.chunk().to_vec());

        // Advance past first buffer boundary into second
        b1.advance(2);
        iobufs.advance(2);

        let chain_remaining = b1.clone().chain(b2.clone()).chain(b3.clone()).remaining();
        assert_eq!(chain_remaining, iobufs.remaining());

        // Now we should be in the second buffer
        let chain_chunk = b1
            .clone()
            .chain(b2.clone())
            .chain(b3.clone())
            .chunk()
            .to_vec();
        assert_eq!(chain_chunk, iobufs.chunk().to_vec());

        // Advance past second buffer boundary into third
        b2.advance(6);
        iobufs.advance(6);

        let chain_remaining = b1.clone().chain(b2.clone()).chain(b3.clone()).remaining();
        assert_eq!(chain_remaining, iobufs.remaining());

        // Now we should be in the third buffer
        let chain_chunk = b1.chain(b2).chain(b3).chunk().to_vec();
        assert_eq!(chain_chunk, iobufs.chunk().to_vec());

        // Test copy_to_bytes
        let b1 = Bytes::from_static(b"hello");
        let b2 = Bytes::from_static(b" world");
        let b3 = Bytes::from_static(b"!");
        let mut iobufs = IoBufsMut::from(vec![
            IoBufMut::from(b"hello"),
            IoBufMut::from(b" world"),
            IoBufMut::from(b"!"),
        ]);

        let chain_bytes = b1.chain(b2).chain(b3).copy_to_bytes(8);
        let iobufs_bytes = iobufs.copy_to_bytes(8);
        assert_eq!(chain_bytes, iobufs_bytes);
        assert_eq!(chain_bytes.as_ref(), b"hello wo");
    }

    #[test]
    fn test_iobufsmut_chunks_vectored_multiple_slices() {
        // Single non-empty buffers should export exactly one slice.
        let single = IoBufsMut::from(IoBufMut::from(b"xy"));
        let mut single_dst = [IoSlice::new(&[]); 2];
        let count = single.chunks_vectored(&mut single_dst);
        assert_eq!(count, 1);
        assert_eq!(&single_dst[0][..], b"xy");

        // Single empty buffers should export no slices.
        let empty_single = IoBufsMut::default();
        let mut empty_single_dst = [IoSlice::new(&[]); 1];
        assert_eq!(empty_single.chunks_vectored(&mut empty_single_dst), 0);

        let bufs = IoBufsMut::from(vec![
            IoBufMut::from(b"ab"),
            IoBufMut::from(b"cd"),
            IoBufMut::from(b"ef"),
            IoBufMut::from(b"gh"),
        ]);

        // Destination capacity should cap how many chunks we export.
        let mut small = [IoSlice::new(&[]); 2];
        let count = bufs.chunks_vectored(&mut small);
        assert_eq!(count, 2);
        assert_eq!(&small[0][..], b"ab");
        assert_eq!(&small[1][..], b"cd");

        // Larger destination should include every readable chunk.
        let mut large = [IoSlice::new(&[]); 8];
        let count = bufs.chunks_vectored(&mut large);
        assert_eq!(count, 4);
        assert_eq!(&large[0][..], b"ab");
        assert_eq!(&large[1][..], b"cd");
        assert_eq!(&large[2][..], b"ef");
        assert_eq!(&large[3][..], b"gh");

        // Empty destination cannot accept any slices.
        let mut empty_dst: [IoSlice<'_>; 0] = [];
        assert_eq!(bufs.chunks_vectored(&mut empty_dst), 0);

        // Non-canonical shapes should skip empty leading chunks.
        let sparse = IoBufsMut {
            inner: IoBufsMutInner::Pair([IoBufMut::default(), IoBufMut::from(b"y")]),
        };
        let mut dst = [IoSlice::new(&[]); 2];
        let count = sparse.chunks_vectored(&mut dst);
        assert_eq!(count, 1);
        assert_eq!(&dst[0][..], b"y");

        // Triple should skip empty chunks and preserve readable order.
        let sparse_triple = IoBufsMut {
            inner: IoBufsMutInner::Triple([
                IoBufMut::default(),
                IoBufMut::from(b"z"),
                IoBufMut::from(b"w"),
            ]),
        };
        let mut dst = [IoSlice::new(&[]); 3];
        let count = sparse_triple.chunks_vectored(&mut dst);
        assert_eq!(count, 2);
        assert_eq!(&dst[0][..], b"z");
        assert_eq!(&dst[1][..], b"w");

        // Chunked shapes with only empty buffers should export no slices.
        let empty_chunked = IoBufsMut {
            inner: IoBufsMutInner::Chunked(VecDeque::from([
                IoBufMut::default(),
                IoBufMut::default(),
            ])),
        };
        let mut dst = [IoSlice::new(&[]); 2];
        assert_eq!(empty_chunked.chunks_vectored(&mut dst), 0);
    }

    #[test]
    fn test_iobufsmut_try_into_single() {
        let single = IoBufsMut::from(IoBufMut::from(b"hello"));
        let single = single.try_into_single().expect("single expected");
        assert_eq!(single, b"hello");

        let multi = IoBufsMut::from(vec![IoBufMut::from(b"ab"), IoBufMut::from(b"cd")]);
        let multi = multi.try_into_single().expect_err("multi expected");
        assert_eq!(multi.coalesce(), b"abcd");
    }

    #[test]
    fn test_iobufsmut_freeze_after_advance() {
        // Partial advance: advance 3 of 11 bytes
        let buf1 = IoBufMut::from(b"hello");
        let buf2 = IoBufMut::from(b" world");
        let mut bufs = IoBufsMut::from(vec![buf1, buf2]);

        bufs.advance(3);
        assert_eq!(bufs.len(), 8);

        let frozen = bufs.freeze();
        assert_eq!(frozen.len(), 8);
        assert_eq!(frozen.coalesce(), b"lo world");

        // Exact boundary advance: advance 5 of 11 bytes (first buf is 5 bytes)
        let buf1 = IoBufMut::from(b"hello");
        let buf2 = IoBufMut::from(b" world");
        let mut bufs = IoBufsMut::from(vec![buf1, buf2]);

        bufs.advance(5);
        assert_eq!(bufs.len(), 6);

        // First buffer should be fully consumed (empty after advance)
        // freeze() filters empty buffers, so result should be Single
        let frozen = bufs.freeze();
        assert!(frozen.is_single());
        assert_eq!(frozen.coalesce(), b" world");
    }

    #[test]
    fn test_iobufsmut_coalesce_shapes() {
        // A single buffer is returned as is.
        let mut buf = IoBufMut::from(b"hello");
        let original_ptr = buf.as_mut_ptr();
        let bufs = IoBufsMut::from(buf);
        let coalesced = bufs.coalesce();
        assert_eq!(coalesced, b"hello");
        assert_eq!(coalesced.as_ref().as_ptr(), original_ptr);

        // Pair, triple, and chunked (4+) shapes copy into one allocation of
        // exactly the readable length.
        let pair = IoBufsMut::from(vec![IoBufMut::from(b"ab"), IoBufMut::from(b"cd")]);
        let pair = pair.coalesce();
        assert_eq!(pair, b"abcd");
        assert_eq!(pair.capacity(), 4);

        let triple = IoBufsMut::from(vec![
            IoBufMut::from(b"a"),
            IoBufMut::from(b"b"),
            IoBufMut::from(b"c"),
        ]);
        let triple = triple.coalesce();
        assert_eq!(triple, b"abc");
        assert_eq!(triple.capacity(), 3);

        let chunked = IoBufsMut::from(vec![
            IoBufMut::from(b"1"),
            IoBufMut::from(b"2"),
            IoBufMut::from(b"3"),
            IoBufMut::from(b"4"),
        ]);
        let chunked = chunked.coalesce();
        assert_eq!(chunked, b"1234");
        assert_eq!(chunked.capacity(), 4);
    }

    #[test]
    fn test_iobufs_additional_shape_and_conversion_paths() {
        // Constructor coverage for mutable/immutable/slice-backed inputs.
        let from_mut = IoBufs::from(IoBufMut::from(b"m"));
        assert_eq!(from_mut.chunk(), b"m");
        let from_bytes = IoBufs::from(Bytes::from_static(b"b"));
        assert_eq!(from_bytes.chunk(), b"b");
        let from_bytesmut = IoBufs::from(BytesMut::from(&b"bm"[..]));
        assert_eq!(from_bytesmut.chunk(), b"bm");
        let from_vec = IoBufs::from(vec![1u8, 2u8]);
        assert_eq!(from_vec.chunk(), &[1u8, 2]);
        let static_slice: &'static [u8] = b"slice";
        let from_static = IoBufs::from(static_slice);
        assert_eq!(from_static.chunk(), b"slice");

        // Canonicalizing an already-empty buffer remains a single empty chunk.
        let mut single_empty = IoBufs::default();
        single_empty.canonicalize();
        assert!(single_empty.is_single());

        // Triple path: prepend/append can promote into chunked while preserving order.
        let mut triple = IoBufs::from(vec![
            IoBuf::from(b"a".to_vec()),
            IoBuf::from(b"b".to_vec()),
            IoBuf::from(b"c".to_vec()),
        ]);
        assert!(triple.as_single().is_none());
        triple.prepend(IoBuf::from(vec![b'0']));
        triple.prepend(IoBuf::from(vec![b'1']));
        triple.append(IoBuf::from(vec![b'2']));
        assert_eq!(triple.copy_to_bytes(triple.remaining()).as_ref(), b"10abc2");

        // Appending to an existing triple keeps byte order stable.
        let mut triple_append = IoBufs::from(vec![
            IoBuf::from(b"x".to_vec()),
            IoBuf::from(b"y".to_vec()),
            IoBuf::from(b"z".to_vec()),
        ]);
        triple_append.append(IoBuf::from(vec![b'w']));
        assert_eq!(triple_append.coalesce(), b"xyzw");

        // coalesce on a triple should preserve contents.
        let triple = IoBufs::from(vec![
            IoBuf::from(b"a".to_vec()),
            IoBuf::from(b"b".to_vec()),
            IoBuf::from(b"c".to_vec()),
        ]);
        assert_eq!(triple.coalesce(), b"abc");

        // Coalescing after advancing a four-chunk input preserves the remaining bytes.
        let mut chunked_advanced = IoBufs::from(vec![
            IoBuf::from(b"a".to_vec()),
            IoBuf::from(b"b".to_vec()),
            IoBuf::from(b"c".to_vec()),
            IoBuf::from(b"d".to_vec()),
        ]);
        assert_eq!(chunked_advanced.remaining(), 4);
        chunked_advanced.advance(1);
        assert_eq!(chunked_advanced.coalesce(), b"bcd");

        // Non-canonical Pair/Triple/Chunked shapes should still expose the first readable chunk.
        let pair_second = IoBufs {
            inner: IoBufsInner::Pair([IoBuf::default(), IoBuf::from(vec![1u8])]),
        };
        assert_eq!(pair_second.chunk(), &[1u8]);
        let pair_empty = IoBufs {
            inner: IoBufsInner::Pair([IoBuf::default(), IoBuf::default()]),
        };
        assert_eq!(pair_empty.chunk(), b"");

        let triple_third = IoBufs {
            inner: IoBufsInner::Triple([
                IoBuf::default(),
                IoBuf::default(),
                IoBuf::from(vec![3u8]),
            ]),
        };
        assert_eq!(triple_third.chunk(), &[3u8]);
        let triple_second = IoBufs {
            inner: IoBufsInner::Triple([
                IoBuf::default(),
                IoBuf::from(vec![2u8]),
                IoBuf::default(),
            ]),
        };
        assert_eq!(triple_second.chunk(), &[2u8]);
        let triple_empty = IoBufs {
            inner: IoBufsInner::Triple([IoBuf::default(), IoBuf::default(), IoBuf::default()]),
        };
        assert_eq!(triple_empty.chunk(), b"");

        let chunked_second = IoBufs {
            inner: IoBufsInner::Chunked(VecDeque::from([IoBuf::default(), IoBuf::from(vec![9u8])])),
        };
        assert_eq!(chunked_second.chunk(), &[9u8]);
        let chunked_empty = IoBufs {
            inner: IoBufsInner::Chunked(VecDeque::from([IoBuf::default()])),
        };
        assert_eq!(chunked_empty.chunk(), b"");
    }

    #[test]
    fn test_iobufsmut_additional_shape_and_conversion_paths() {
        // `as_single` accessors should work only for single-shape containers.
        let mut single = IoBufsMut::from(IoBufMut::from(b"x"));
        assert!(single.as_single().is_some());
        assert!(single.as_single_mut().is_some());
        single.canonicalize();
        assert!(single.is_single());

        let mut pair = IoBufsMut::from(vec![IoBufMut::from(b"a"), IoBufMut::from(b"b")]);
        assert!(pair.as_single().is_none());
        assert!(pair.as_single_mut().is_none());

        // Constructor coverage for raw vec and BytesMut sources.
        let from_vec = IoBufsMut::from(vec![1u8, 2u8]);
        assert_eq!(from_vec.chunk(), &[1u8, 2]);
        let from_bytesmut = IoBufsMut::from(BytesMut::from(&b"cd"[..]));
        assert_eq!(from_bytesmut.chunk(), b"cd");

        // Chunked write path: set_len + copy_from_slice + freeze round-trip.
        let mut chunked = IoBufsMut::from(vec![
            IoBufMut::with_capacity(1),
            IoBufMut::with_capacity(1),
            IoBufMut::with_capacity(1),
            IoBufMut::with_capacity(1),
        ]);
        // SAFETY: We only write/read initialized bytes after `copy_from_slice`.
        unsafe { chunked.set_len(4) };
        chunked.copy_from_slice(b"wxyz");
        assert_eq!(chunked.capacity(), 4);
        assert_eq!(chunked.remaining(), 4);
        let frozen = chunked.freeze();
        assert_eq!(frozen.coalesce(), b"wxyz");
    }

    #[test]
    fn test_iobufsmut_noncanonical_chunk_and_chunk_mut_paths() {
        fn no_spare_capacity_buf() -> IoBufMut {
            let mut buf = IoBufMut::with_capacity(1);
            let cap = buf.capacity();
            // SAFETY: We never read from this buffer in this helper.
            unsafe { buf.set_len(cap) };
            buf
        }
        // `chunk()` should skip empty front buffers across all shapes.
        let pair_second = IoBufsMut {
            inner: IoBufsMutInner::Pair([IoBufMut::default(), IoBufMut::from(b"b")]),
        };
        assert_eq!(pair_second.chunk(), b"b");
        let pair_empty = IoBufsMut {
            inner: IoBufsMutInner::Pair([IoBufMut::default(), IoBufMut::default()]),
        };
        assert_eq!(pair_empty.chunk(), b"");

        let triple_third = IoBufsMut {
            inner: IoBufsMutInner::Triple([
                IoBufMut::default(),
                IoBufMut::default(),
                IoBufMut::from(b"c"),
            ]),
        };
        assert_eq!(triple_third.chunk(), b"c");
        let triple_second = IoBufsMut {
            inner: IoBufsMutInner::Triple([
                IoBufMut::default(),
                IoBufMut::from(b"b"),
                IoBufMut::default(),
            ]),
        };
        assert_eq!(triple_second.chunk(), b"b");
        let triple_empty = IoBufsMut {
            inner: IoBufsMutInner::Triple([
                IoBufMut::default(),
                IoBufMut::default(),
                IoBufMut::default(),
            ]),
        };
        assert_eq!(triple_empty.chunk(), b"");

        let chunked_second = IoBufsMut {
            inner: IoBufsMutInner::Chunked(VecDeque::from([
                IoBufMut::default(),
                IoBufMut::from(b"d"),
            ])),
        };
        assert_eq!(chunked_second.chunk(), b"d");
        let chunked_empty = IoBufsMut {
            inner: IoBufsMutInner::Chunked(VecDeque::from([IoBufMut::default()])),
        };
        assert_eq!(chunked_empty.chunk(), b"");

        // `chunk_mut()` should skip non-writable fronts and return first writable chunk.
        let mut pair_chunk_mut = IoBufsMut {
            inner: IoBufsMutInner::Pair([no_spare_capacity_buf(), IoBufMut::with_capacity(2)]),
        };
        assert!(pair_chunk_mut.chunk_mut().len() >= 2);

        let mut pair_chunk_mut_empty = IoBufsMut {
            inner: IoBufsMutInner::Pair([no_spare_capacity_buf(), no_spare_capacity_buf()]),
        };
        assert_eq!(pair_chunk_mut_empty.chunk_mut().len(), 0);

        let mut triple_chunk_mut = IoBufsMut {
            inner: IoBufsMutInner::Triple([
                no_spare_capacity_buf(),
                no_spare_capacity_buf(),
                IoBufMut::with_capacity(3),
            ]),
        };
        assert!(triple_chunk_mut.chunk_mut().len() >= 3);
        let mut triple_chunk_mut_second = IoBufsMut {
            inner: IoBufsMutInner::Triple([
                no_spare_capacity_buf(),
                IoBufMut::with_capacity(2),
                no_spare_capacity_buf(),
            ]),
        };
        assert!(triple_chunk_mut_second.chunk_mut().len() >= 2);

        let mut triple_chunk_mut_empty = IoBufsMut {
            inner: IoBufsMutInner::Triple([
                no_spare_capacity_buf(),
                no_spare_capacity_buf(),
                no_spare_capacity_buf(),
            ]),
        };
        assert_eq!(triple_chunk_mut_empty.chunk_mut().len(), 0);

        let mut chunked_chunk_mut = IoBufsMut {
            inner: IoBufsMutInner::Chunked(VecDeque::from([
                IoBufMut::default(),
                IoBufMut::with_capacity(4),
            ])),
        };
        assert!(chunked_chunk_mut.chunk_mut().len() >= 4);

        let mut chunked_chunk_mut_empty = IoBufsMut {
            inner: IoBufsMutInner::Chunked(VecDeque::from([no_spare_capacity_buf()])),
        };
        assert_eq!(chunked_chunk_mut_empty.chunk_mut().len(), 0);
    }

    #[test]
    fn test_iobuf_internal_chunk_helpers() {
        // `copy_to_bytes_chunked` drops leading empties on zero-length reads
        // and asks for canonicalization so the emptied deque collapses back
        // to the Single representation.
        let mut empty_with_leading = VecDeque::from([IoBuf::default()]);
        let (bytes, needs_canonicalize) = copy_to_bytes_chunked(&mut empty_with_leading, 0, "x");
        assert!(bytes.is_empty());
        assert!(needs_canonicalize);
        assert!(empty_with_leading.is_empty());

        // Fast path: front chunk can fully satisfy the request.
        let mut fast = VecDeque::from([
            IoBuf::from(b"ab".to_vec()),
            IoBuf::from(b"cd".to_vec()),
            IoBuf::from(b"ef".to_vec()),
            IoBuf::from(b"gh".to_vec()),
        ]);
        let (bytes, needs_canonicalize) = copy_to_bytes_chunked(&mut fast, 2, "x");
        assert_eq!(bytes.as_ref(), b"ab");
        assert!(needs_canonicalize);
        assert_eq!(fast.front().expect("front exists").as_ref(), b"cd");

        // Slow path: request spans multiple chunks.
        let mut slow = VecDeque::from([
            IoBuf::from(b"a".to_vec()),
            IoBuf::from(b"bc".to_vec()),
            IoBuf::from(b"d".to_vec()),
            IoBuf::from(b"e".to_vec()),
        ]);
        let (bytes, needs_canonicalize) = copy_to_bytes_chunked(&mut slow, 3, "x");
        assert_eq!(bytes.as_ref(), b"abc");
        assert!(needs_canonicalize);

        let mut empty_with_leading_mut = VecDeque::from([IoBufMut::default()]);
        let (bytes, needs_canonicalize) =
            copy_to_bytes_chunked(&mut empty_with_leading_mut, 0, "x");
        assert!(bytes.is_empty());
        assert!(needs_canonicalize);
        assert!(empty_with_leading_mut.is_empty());

        // Mirror the fast/slow chunked helper paths for mutable chunks too.
        let mut fast_mut = VecDeque::from([
            IoBufMut::from(b"ab"),
            IoBufMut::from(b"cd"),
            IoBufMut::from(b"ef"),
            IoBufMut::from(b"gh"),
        ]);
        let (bytes, needs_canonicalize) = copy_to_bytes_chunked(&mut fast_mut, 2, "x");
        assert_eq!(bytes.as_ref(), b"ab");
        assert!(needs_canonicalize);
        assert_eq!(fast_mut.front().expect("front exists").as_ref(), b"cd");

        let mut slow_mut = VecDeque::from([
            IoBufMut::from(b"a"),
            IoBufMut::from(b"bc"),
            IoBufMut::from(b"de"),
            IoBufMut::from(b"f"),
        ]);
        let (bytes, needs_canonicalize) = copy_to_bytes_chunked(&mut slow_mut, 4, "x");
        assert_eq!(bytes.as_ref(), b"abcd");
        assert!(needs_canonicalize);
        assert_eq!(slow_mut.front().expect("front exists").as_ref(), b"e");

        // `advance_chunked_front` should skip empties and drain in linear order.
        let mut advance_chunked = VecDeque::from([
            IoBuf::default(),
            IoBuf::from(b"abc".to_vec()),
            IoBuf::from(b"d".to_vec()),
        ]);
        advance_chunked_front(&mut advance_chunked, 2);
        assert_eq!(
            advance_chunked.front().expect("front exists").as_ref(),
            b"c"
        );
        advance_chunked_front(&mut advance_chunked, 2);
        assert!(advance_chunked.is_empty());

        // The front-advance helper also has a separate mutable monomorphization.
        let mut advance_chunked_mut = VecDeque::from([
            IoBufMut::default(),
            IoBufMut::from(b"abc"),
            IoBufMut::from(b"d"),
        ]);
        advance_chunked_front(&mut advance_chunked_mut, 2);
        assert_eq!(
            advance_chunked_mut.front().expect("front exists").as_ref(),
            b"c"
        );
        advance_chunked_front(&mut advance_chunked_mut, 2);
        assert!(advance_chunked_mut.is_empty());

        // `advance_small_chunks` signals canonicalization when front chunks are exhausted.
        let mut small = [IoBuf::default(), IoBuf::from(b"abc".to_vec())];
        let needs_canonicalize = advance_small_chunks(&mut small, 2);
        assert!(needs_canonicalize);
        assert_eq!(small[1].as_ref(), b"c");

        let mut small_exact = [
            IoBuf::from(b"a".to_vec()),
            IoBuf::from(b"b".to_vec()),
            IoBuf::from(b"c".to_vec()),
        ];
        let needs_canonicalize = advance_small_chunks(&mut small_exact, 3);
        assert!(needs_canonicalize);
        assert_eq!(small_exact[0].remaining(), 0);
        assert_eq!(small_exact[1].remaining(), 0);
        assert_eq!(small_exact[2].remaining(), 0);

        // Small-chunk copy canonicalization is also instantiated for mutable chunks.
        let mut small_mut = [
            IoBufMut::from(b"a"),
            IoBufMut::from(b"bc"),
            IoBufMut::from(b"d"),
        ];
        let (bytes, needs_canonicalize) = copy_to_bytes_small_chunks(&mut small_mut, 3, "x");
        assert_eq!(bytes.as_ref(), b"abc");
        assert!(needs_canonicalize);
        assert_eq!(small_mut[2].as_ref(), b"d");

        // `advance_mut_in_chunks` returns whether the request fully fit in writable chunks.
        let mut writable = [IoBufMut::with_capacity(2), IoBufMut::with_capacity(1)];
        let mut remaining = 3usize;
        // SAFETY: We do not read from advanced bytes in this test.
        let all_advanced = unsafe { advance_mut_in_chunks(&mut writable, &mut remaining) };
        assert!(all_advanced);
        assert_eq!(remaining, 0);

        // `advance_mut_in_chunks` should skip non-writable chunks.
        let mut full = IoBufMut::with_capacity(1);
        // SAFETY: We only mark initialized capacity and never read bytes.
        unsafe { full.set_len(full.capacity()) };
        let mut writable_after_full = [full, IoBufMut::with_capacity(2)];
        let mut remaining = 2usize;
        // SAFETY: We do not read from advanced bytes in this test.
        let all_advanced =
            unsafe { advance_mut_in_chunks(&mut writable_after_full, &mut remaining) };
        assert!(all_advanced);
        assert_eq!(remaining, 0);

        let mut writable_short = [IoBufMut::with_capacity(1), IoBufMut::with_capacity(1)];
        let mut remaining = 3usize;
        // SAFETY: We do not read from advanced bytes in this test.
        let all_advanced = unsafe { advance_mut_in_chunks(&mut writable_short, &mut remaining) };
        assert!(!all_advanced);
        assert_eq!(remaining, 1);
    }

    #[test]
    fn test_iobufsmut_advance_mut_success_paths() {
        // Pair path.
        let mut pair = IoBufsMut {
            inner: IoBufsMutInner::Pair([IoBufMut::with_capacity(2), IoBufMut::with_capacity(2)]),
        };
        // SAFETY: We only verify cursor movement (`remaining`) and do not read bytes.
        unsafe { pair.advance_mut(3) };
        assert_eq!(pair.remaining(), 3);

        // Triple path.
        let mut triple = IoBufsMut {
            inner: IoBufsMutInner::Triple([
                IoBufMut::with_capacity(1),
                IoBufMut::with_capacity(1),
                IoBufMut::with_capacity(1),
            ]),
        };
        // SAFETY: We only verify cursor movement (`remaining`) and do not read bytes.
        unsafe { triple.advance_mut(2) };
        assert_eq!(triple.remaining(), 2);

        // Chunked wrapped-VecDeque path.
        let mut wrapped = VecDeque::with_capacity(5);
        wrapped.push_back(IoBufMut::with_capacity(1));
        wrapped.push_back(IoBufMut::with_capacity(1));
        wrapped.push_back(IoBufMut::with_capacity(1));
        wrapped.push_back(IoBufMut::with_capacity(1));
        wrapped.push_back(IoBufMut::with_capacity(1));
        let _ = wrapped.pop_front();
        wrapped.push_back(IoBufMut::with_capacity(1));
        let (first, second) = wrapped.as_slices();
        assert!(!first.is_empty());
        assert!(!second.is_empty());

        // Force `advance_mut` to consume across the wrapped second slice as well.
        let to_advance = first.len() + 1;
        let mut chunked = IoBufsMut {
            inner: IoBufsMutInner::Chunked(wrapped),
        };
        let before = chunked.remaining_mut();
        // SAFETY: We only verify cursor movement (`remaining`) and do not read bytes.
        unsafe { chunked.advance_mut(to_advance) };
        assert_eq!(chunked.remaining(), to_advance);
        assert_eq!(chunked.remaining_mut(), before - to_advance);
    }

    #[test]
    fn test_iobufsmut_advance_mut_zero_noop_when_full() {
        fn full_chunk() -> IoBufMut {
            // IoBufMut capacity is fixed, so setting len to capacity leaves
            // remaining_mut() at zero.
            let mut buf = IoBufMut::with_capacity(1);
            let cap = buf.capacity();
            // SAFETY: We never read from this buffer in this test.
            unsafe { buf.set_len(cap) };
            buf
        }

        // Pair path: fully-written chunks should allow advance_mut(0) as a no-op.
        let mut pair = IoBufsMut::from(vec![full_chunk(), full_chunk()]);
        assert!(matches!(pair.inner, IoBufsMutInner::Pair(_)));
        assert_eq!(pair.remaining_mut(), 0);
        let before = pair.remaining();
        // SAFETY: Advancing by 0 does not expose uninitialized bytes.
        unsafe { pair.advance_mut(0) };
        assert_eq!(pair.remaining(), before);

        // Triple path: same no-op behavior.
        let mut triple = IoBufsMut::from(vec![full_chunk(), full_chunk(), full_chunk()]);
        assert!(matches!(triple.inner, IoBufsMutInner::Triple(_)));
        assert_eq!(triple.remaining_mut(), 0);
        let before = triple.remaining();
        // SAFETY: Advancing by 0 does not expose uninitialized bytes.
        unsafe { triple.advance_mut(0) };
        assert_eq!(triple.remaining(), before);

        // Chunked path: 4+ fully-written chunks should also no-op.
        let mut chunked =
            IoBufsMut::from(vec![full_chunk(), full_chunk(), full_chunk(), full_chunk()]);
        assert!(matches!(chunked.inner, IoBufsMutInner::Chunked(_)));
        assert_eq!(chunked.remaining_mut(), 0);
        let before = chunked.remaining();
        // SAFETY: Advancing by 0 does not expose uninitialized bytes.
        unsafe { chunked.advance_mut(0) };
        assert_eq!(chunked.remaining(), before);
    }

    #[test]
    #[should_panic(expected = "cannot advance past end of buffer")]
    fn test_iobufsmut_advance_mut_past_end_pair() {
        let mut pair = IoBufsMut {
            inner: IoBufsMutInner::Pair([IoBufMut::with_capacity(1), IoBufMut::with_capacity(1)]),
        };
        // SAFETY: Intentional panic path coverage.
        unsafe { pair.advance_mut(3) };
    }

    #[test]
    #[should_panic(expected = "cannot advance past end of buffer")]
    fn test_iobufsmut_advance_mut_past_end_triple() {
        let mut triple = IoBufsMut {
            inner: IoBufsMutInner::Triple([
                IoBufMut::with_capacity(1),
                IoBufMut::with_capacity(1),
                IoBufMut::with_capacity(1),
            ]),
        };
        // SAFETY: Intentional panic path coverage.
        unsafe { triple.advance_mut(4) };
    }

    #[test]
    #[should_panic(expected = "cannot advance past end of buffer")]
    fn test_iobufsmut_advance_mut_past_end_chunked() {
        let mut chunked = IoBufsMut {
            inner: IoBufsMutInner::Chunked(VecDeque::from([
                IoBufMut::with_capacity(1),
                IoBufMut::with_capacity(1),
                IoBufMut::with_capacity(1),
                IoBufMut::with_capacity(1),
            ])),
        };
        // SAFETY: Intentional panic path coverage.
        unsafe { chunked.advance_mut(5) };
    }

    #[test]
    fn test_iobufsmut_set_len() {
        // SAFETY: we don't read the uninitialized bytes.
        unsafe {
            // Single buffer
            let mut bufs = IoBufsMut::from(IoBufMut::with_capacity(16));
            bufs.set_len(10);
            assert_eq!(bufs.len(), 10);

            // Chunked: distributes across chunks [cap 5, cap 10], set 12 -> [5, 7]
            let mut bufs = IoBufsMut::from(vec![
                IoBufMut::with_capacity(5),
                IoBufMut::with_capacity(10),
            ]);
            bufs.set_len(12);
            assert_eq!(bufs.len(), 12);
            assert_eq!(bufs.chunk().len(), 5);
            bufs.advance(5);
            assert_eq!(bufs.chunk().len(), 7);
            bufs.advance(7);
            assert_eq!(bufs.remaining(), 0);

            // Uneven capacities [3, 20, 2], set 18 -> [3, 15, 0].
            let mut bufs = IoBufsMut::from(vec![
                IoBufMut::with_capacity(3),
                IoBufMut::with_capacity(20),
                IoBufMut::with_capacity(2),
            ]);
            bufs.set_len(18);
            assert_eq!(bufs.chunk().len(), 3);
            bufs.advance(3);
            assert_eq!(bufs.chunk().len(), 15);
            bufs.advance(15);
            assert_eq!(bufs.remaining(), 0);

            // Exact total capacity [4, 4], set 8 -> [4, 4]
            let mut bufs =
                IoBufsMut::from(vec![IoBufMut::with_capacity(4), IoBufMut::with_capacity(4)]);
            bufs.set_len(8);
            assert_eq!(bufs.chunk().len(), 4);
            bufs.advance(4);
            assert_eq!(bufs.chunk().len(), 4);
            bufs.advance(4);
            assert_eq!(bufs.remaining(), 0);

            // Zero length preserves caller-provided layout.
            let mut bufs =
                IoBufsMut::from(vec![IoBufMut::with_capacity(4), IoBufMut::with_capacity(4)]);
            bufs.set_len(0);
            assert_eq!(bufs.len(), 0);
            assert_eq!(bufs.chunk(), b"");
        }
    }

    #[test]
    #[should_panic(expected = "set_len(9) exceeds capacity(8)")]
    fn test_iobufsmut_set_len_overflow() {
        let mut bufs =
            IoBufsMut::from(vec![IoBufMut::with_capacity(4), IoBufMut::with_capacity(4)]);
        // SAFETY: this will panic before any read.
        unsafe { bufs.set_len(9) };
    }

    #[test]
    fn test_encode_iobufs_matches_encode() {
        let value = vec![1u8, 2, 3, 4, 5, 6];
        assert_encode_iobufs_matches_encode(&value);
    }

    /// Reports a larger encoding size than `write` produces, so
    /// [`EncodeExt::encode_iobufs`] fails its size assert.
    struct UnderWriter;

    impl Write for UnderWriter {
        fn write(&self, buf: &mut impl BufMut) {
            buf.put_slice(b"ab");
        }
    }

    impl EncodeSize for UnderWriter {
        fn encode_size(&self) -> usize {
            4
        }
    }

    #[test]
    #[should_panic(expected = "write_bufs() must write exactly encode_size() bytes")]
    fn test_encode_iobufs_rejects_short_write() {
        let _ = UnderWriter.encode_iobufs();
    }

    #[test]
    fn test_iobuf_encode_iobufs_matches_encode() {
        let value = IoBuf::from(vec![0xAB; 512]);
        assert_encode_iobufs_matches_encode(&value);
    }

    /// A pushed native [`IoBuf`] becomes its own chunk as is: once the caller's
    /// handle is gone, the chunk recovers the original allocation as an [`IoBufMut`].
    #[test]
    fn test_encode_iobufs_keeps_pushed_iobuf() {
        let value = IoBuf::from(vec![0xAB; 512]);
        let ptr = value.as_ptr();
        let mut encoded = value.encode_iobufs();
        drop(value);

        let prefix = 512usize.encode_size();
        drop(encoded.split_to(prefix));
        let chunk = encoded
            .try_into_single()
            .expect("the payload must be one chunk");
        let recovered = chunk
            .try_into_mut()
            .expect("the chunk must be the pushed native buffer");
        assert_eq!(recovered.as_ref().as_ptr(), ptr);
    }

    #[test]
    fn test_nested_container_encode_iobufs_matches_encode() {
        let value = (
            Some(Bytes::from(vec![0xAA; 256])),
            vec![Bytes::from(vec![0xBB; 128]), Bytes::from(vec![0xCC; 64])],
        );
        assert_encode_iobufs_matches_encode(&value);
    }

    #[test]
    fn test_map_encode_iobufs_matches_encode() {
        let mut btree = BTreeMap::new();
        btree.insert(2u8, Bytes::from(vec![0xDD; 96]));
        btree.insert(1u8, Bytes::from(vec![0xEE; 48]));
        assert_encode_iobufs_matches_encode(&btree);

        let mut hash = HashMap::new();
        hash.insert(2u8, Bytes::from(vec![0x11; 96]));
        hash.insert(1u8, Bytes::from(vec![0x22; 48]));
        assert_encode_iobufs_matches_encode(&hash);
    }

    #[test]
    fn test_lazy_encode_iobufs_matches_encode() {
        let value = Lazy::new(Bytes::from(vec![0x44; 200]));
        assert_encode_iobufs_matches_encode(&value);

        // A deferred value pushes its pending encoding as the only chunk and
        // writes no inline bytes.
        let mut pending = 42u64.encode();
        let deferred = Lazy::<u64>::deferred(&mut pending, ());
        assert_eq!(deferred.encode_inline_size(), 0);
        assert_encode_iobufs_matches_encode(&deferred);
        assert_eq!(deferred.encode_iobufs().chunk_count(), 1);
    }

    /// A value with neither inline bytes nor pushed chunks encodes without
    /// allocating.
    #[test]
    fn test_encode_iobufs_without_inline_bytes_does_not_allocate() {
        let (bufs, calls) = allocation_calls_during(|| [0u8; 0].encode_iobufs());
        assert_eq!(calls, 0);
        assert_eq!(bufs.remaining(), 0);
        assert_eq!(bufs.chunk_count(), 0);
    }

    #[test]
    fn test_non_empty_range_encode_iobufs_matches_encode() {
        let range =
            NonEmptyRange::new(Bytes::from(vec![0x10; 32])..Bytes::from(vec![0x20; 48])).unwrap();
        assert_encode_iobufs_matches_encode(&range);
    }

    mod builder_tests {
        use super::*;
        use commonware_codec::{BufsMut, Encode, Write};

        // Only inline writes, no pushes.
        #[test]
        fn test_inline_only() {
            let mut b = Builder::new(64);
            b.put_u32(42);
            b.put_u8(7);
            let mut r = b.finish();
            assert_eq!(r.remaining(), 5);
            assert_eq!(r.get_u32(), 42);
            assert_eq!(r.get_u8(), 7);
        }

        // Only zero-copy pushes, no inline writes.
        #[test]
        fn test_push_only() {
            let mut b = Builder::new(64);
            let data = Bytes::from(vec![0xAA; 1024]);
            b.push(data.clone());
            let mut r = b.finish();
            assert_eq!(r.remaining(), 1024);
            assert_eq!(r.copy_to_bytes(1024), data);
        }

        // Pushed Bytes appear in the output without a payload copy.
        #[test]
        fn test_push_is_zero_copy() {
            let mut b = Builder::new(64);
            b.put_u16(99);
            let payload = Bytes::from(vec![0xDD; 1024]);
            b.push(payload.clone());
            let mut r = b.finish();
            r.advance(2);
            assert_eq!(r.chunk().as_ptr(), payload.as_ptr());
        }

        // Interleaved: inline header, zero-copy push, inline trailer.
        #[test]
        fn test_inline_push_inline() {
            let mut b = Builder::new(64);
            b.put_u16(99);
            let payload = Bytes::from(vec![0xBB; 512]);
            b.push(payload.clone());
            b.put_u8(1);
            let mut r = b.finish();
            assert_eq!(r.remaining(), 2 + 512 + 1);
            assert_eq!(r.get_u16(), 99);
            assert_eq!(r.copy_to_bytes(512), payload);
            assert_eq!(r.get_u8(), 1);
        }

        // Bytes::write_bufs produces identical wire format to Bytes::write.
        #[test]
        fn test_write_bufs_matches_write() {
            let data = Bytes::from(vec![0xCC; 256]);
            let mut b = Builder::new(64);
            data.write_bufs(&mut b);
            let mut bufs = b.finish();

            let mut out = vec![0u8; bufs.remaining()];
            bufs.copy_to_slice(&mut out);
            assert_eq!(out, data.encode().as_ref());
        }

        // Finishing an unused builder produces empty IoBufs.
        #[test]
        fn test_empty() {
            let bufs = Builder::new(64).finish();
            assert_eq!(bufs.remaining(), 0);
        }

        // Inline writes exceeding capacity panic. `Builder` does not override
        // the `BufMut` write methods, so the panic (and message) comes from
        // bytes' trait defaults checking `remaining_mut`.
        #[test]
        #[should_panic(expected = "advance out of bounds")]
        fn test_inline_overflow_panics() {
            let mut b = Builder::new(1);
            let cap = b.remaining_mut();
            b.put_slice(&vec![0xFF; cap]);
            b.put_u8(1); // exceeds capacity
        }

        // Pushing empty Bytes is a no-op.
        #[test]
        fn test_empty_push_ignored() {
            let mut b = Builder::new(64);
            b.push(Bytes::new());
            b.put_u8(1);
            let bufs = b.finish();
            assert_eq!(bufs.remaining(), 1);
        }

        // Consecutive pushes without inline writes between them.
        #[test]
        fn test_multiple_pushes() {
            let mut b = Builder::new(64);
            let a = Bytes::from(vec![0xAA; 100]);
            let c = Bytes::from(vec![0xCC; 200]);
            b.push(a.clone());
            b.push(c.clone());
            let mut r = b.finish();
            assert_eq!(r.remaining(), 300);
            assert_eq!(r.copy_to_bytes(100), a);
            assert_eq!(r.copy_to_bytes(200), c);
        }

        // A zero-capacity builder allocates no inline buffer and emits the
        // pushed chunks alone, with no empty inline slices between them.
        #[test]
        fn test_zero_capacity_push_only() {
            let (bufs, calls) = allocation_calls_during(|| Builder::new(0).finish());
            assert_eq!(calls, 0);
            assert!(bufs.is_empty());

            let mut b = Builder::new(0);
            assert_eq!(b.remaining_mut(), 0);
            let a = Bytes::from(vec![0xAA; 100]);
            let c = Bytes::from(vec![0xCC; 200]);
            b.push(a.clone());
            b.push(Bytes::new());
            b.push(c.clone());
            let mut r = b.finish();
            assert_eq!(r.chunk_count(), 2);
            assert_eq!(r.chunk().as_ptr(), a.as_ptr());
            assert_eq!(r.copy_to_bytes(100), a);
            assert_eq!(r.chunk().as_ptr(), c.as_ptr());
            assert_eq!(r.copy_to_bytes(200), c);
        }

        // The inline segments around a push share the inline buffer, so
        // finishing allocates only that buffer's owner record.
        #[test]
        fn test_inline_segments_share_one_record() {
            let mut b = Builder::new(64);
            b.put_slice(b"head");
            let pushed = IoBuf::from(b"pushed");
            b.push(pushed.clone());
            b.put_slice(b"tail");
            let (mut r, calls) = allocation_calls_during(|| b.finish());
            assert_eq!(calls, 1);
            assert_eq!(r.chunk_count(), 3);
            let head = r.chunk().as_ptr();
            assert_eq!(r.copy_to_bytes(4), &b"head"[..]);
            assert_eq!(r.chunk().as_ptr(), pushed.as_ptr());
            assert_eq!(r.copy_to_bytes(6), &b"pushed"[..]);

            // The tail segment starts right after the head in the same buffer.
            assert_eq!(r.chunk().as_ptr(), head.wrapping_add(4));
            assert_eq!(r.copy_to_bytes(4), &b"tail"[..]);
        }

        // put() exceeding capacity panics (in bytes' trait default).
        #[test]
        #[should_panic(expected = "advance out of bounds")]
        fn test_put_exceeding_capacity_panics() {
            let mut b = Builder::new(1);
            let cap = b.remaining_mut();
            let src = Bytes::from(vec![0xAB; cap + 1]);
            b.put(src);
        }

        // put_slice() exceeding capacity panics (in bytes' trait default).
        #[test]
        #[should_panic(expected = "advance out of bounds")]
        fn test_put_slice_exceeding_capacity_panics() {
            let mut b = Builder::new(1);
            let cap = b.remaining_mut();
            b.put_slice(&vec![0xFE; cap + 1]);
        }

        // Simulates a multi-field struct: [u16 | Bytes (via push) | u32].
        // Verifies write_bufs produces identical wire format to write.
        #[test]
        fn test_multi_field_struct_equivalence() {
            let header: u16 = 0xCAFE;
            let payload = Bytes::from(vec![0xDD; 1024]);
            let trailer: u32 = 0xDEADBEEF;

            // Flat encoding via write.
            let size = header.encode_size() + payload.encode_size() + trailer.encode_size();
            let mut flat = BytesMut::with_capacity(size);
            header.write(&mut flat);
            payload.write(&mut flat);
            trailer.write(&mut flat);

            // Multi-buffer encoding via write_bufs.
            let mut b = Builder::new(64);
            header.write(&mut b);
            payload.write_bufs(&mut b);
            trailer.write(&mut b);
            let mut bufs = b.finish();

            let mut out = vec![0u8; bufs.remaining()];
            bufs.copy_to_slice(&mut out);
            assert_eq!(out, flat.as_ref());
        }

        // encode_iobufs (Builder path) matches encode (flat BytesMut path).
        #[test]
        fn test_encode_iobufs_matches_encode() {
            let data = Bytes::from(vec![0xEE; 500]);
            let mut encoded = data.encode_iobufs();
            let baseline = data.encode();
            let mut out = vec![0u8; encoded.remaining()];
            encoded.copy_to_slice(&mut out);
            assert_eq!(out, baseline.as_ref());
        }

        // Exercise remaining_mut, chunk_mut, and advance_mut directly.
        #[test]
        fn test_chunk_mut_and_advance_mut() {
            let mut b = Builder::new(64);
            let initial = b.remaining_mut();
            assert!(initial >= 64);
            let chunk = b.chunk_mut();
            chunk[0..1].copy_from_slice(&[0xAB]);
            // SAFETY: We just wrote 1 byte into chunk_mut above.
            unsafe { b.advance_mut(1) };
            assert_eq!(b.remaining_mut(), initial - 1);
            let mut r = b.finish();
            assert_eq!(r.remaining(), 1);
            assert_eq!(r.get_u8(), 0xAB);
        }

        // Writing past a full buffer panics (fixed capacity, with the panic coming
        // from bytes' trait default).
        #[test]
        #[should_panic(expected = "advance out of bounds")]
        fn test_write_past_full_panics() {
            let mut b = Builder::new(1);
            let cap = b.remaining_mut();
            b.put_slice(&vec![0xFF; cap]); // fill the buffer completely
            assert_eq!(b.remaining_mut(), 0);
            b.put_u8(0x42); // panics
        }

        // Push at offset 0 with inline trailer exercises finish branch
        // where offset == pos (no inline prefix before push).
        #[test]
        fn test_push_at_start_with_trailer() {
            let mut b = Builder::new(64);
            let payload = Bytes::from(vec![0xCC; 32]);
            b.push(payload.clone());
            b.put_u8(0x01);
            let mut r = b.finish();
            assert_eq!(r.remaining(), 33);
            assert_eq!(r.copy_to_bytes(32), payload);
            assert_eq!(r.get_u8(), 0x01);
        }
    }
}