//! Utility functions for interacting with any runtime.

#[cfg(test)]
use crate::Runner;
use crate::{Metrics, Spawner};
#[cfg(test)]
use futures::stream::{FuturesUnordered, StreamExt};
use futures::task::ArcWake;
use rayon::{ThreadPool as RThreadPool, ThreadPoolBuildError, ThreadPoolBuilder};
use std::{
    any::Any,
    future::Future,
    pin::Pin,
    sync::{Arc, Condvar, Mutex},
    task::{Context, Poll},
};

pub mod buffer;
pub mod signal;

mod handle;
pub use handle::Handle;
pub(crate) use handle::{Aborter, MetricHandle, Panicked, Panicker};

mod cell;
pub use cell::Cell as ContextCell;

pub(crate) mod supervision;

/// The execution mode of a task.
#[derive(Copy, Clone, Debug)]
pub enum Execution {
    /// Task runs on a dedicated thread.
    Dedicated,
    /// Task runs on the shared executor. `true` marks short blocking work that should
    /// use the runtime's blocking-friendly pool.
    Shared(bool),
}

impl Default for Execution {
    fn default() -> Self {
        Self::Shared(false)
    }
}

/// Yield control back to the runtime.
pub async fn reschedule() {
    struct Reschedule {
        yielded: bool,
    }

    impl Future for Reschedule {
        type Output = ();

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
            if self.yielded {
                Poll::Ready(())
            } else {
                self.yielded = true;
                cx.waker().wake_by_ref();
                Poll::Pending
            }
        }
    }

    Reschedule { yielded: false }.await
}

fn extract_panic_message(err: &(dyn Any + Send)) -> String {
    err.downcast_ref::<&str>().map_or_else(
        || {
            err.downcast_ref::<String>()
                .map_or_else(|| format!("{err:?}"), |s| s.clone())
        },
        |s| s.to_string(),
    )
}

/// A clone-able wrapper around a [rayon]-compatible thread pool.
pub type ThreadPool = Arc<RThreadPool>;

/// Creates a clone-able [rayon]-compatible thread pool with [Spawner::spawn].
///
/// # Arguments
/// - `context`: The runtime context implementing the [Spawner] trait.
/// - `concurrency`: The number of tasks to execute concurrently in the pool.
///
/// # Returns
/// A `Result` containing the configured [rayon::ThreadPool] or a [rayon::ThreadPoolBuildError] if the pool cannot be built.
pub fn create_pool<S: Spawner + Metrics>(
    context: S,
    concurrency: usize,
) -> Result<ThreadPool, ThreadPoolBuildError> {
    let pool = ThreadPoolBuilder::new()
        .num_threads(concurrency)
        .spawn_handler(move |thread| {
            // Tasks spawned in a thread pool are expected to run longer than any single
            // task and thus should be provisioned as a dedicated thread.
            context
                .with_label("rayon_thread")
                .dedicated()
                .spawn(move |_| async move { thread.run() });
            Ok(())
        })
        .build()?;

    Ok(Arc::new(pool))
}

/// Async reader–writer lock.
///
/// Powered by [async_lock::RwLock], `RwLock` provides both fair writer acquisition
/// and `try_read` / `try_write` without waiting (without any runtime-specific dependencies).
///
/// Usage:
/// ```rust
/// use commonware_runtime::{Spawner, Runner, deterministic, RwLock};
///
/// let executor = deterministic::Runner::default();
/// executor.start(|context| async move {
///     // Create a new RwLock
///     let lock = RwLock::new(2);
///
///     // many concurrent readers
///     let r1 = lock.read().await;
///     let r2 = lock.read().await;
///     assert_eq!(*r1 + *r2, 4);
///
///     // exclusive writer
///     drop((r1, r2));
///     let mut w = lock.write().await;
///     *w += 1;
/// });
/// ```
pub struct RwLock<T>(async_lock::RwLock<T>);

/// Shared guard returned by [RwLock::read].
pub type RwLockReadGuard<'a, T> = async_lock::RwLockReadGuard<'a, T>;

/// Exclusive guard returned by [RwLock::write].
pub type RwLockWriteGuard<'a, T> = async_lock::RwLockWriteGuard<'a, T>;

impl<T> RwLock<T> {
    /// Create a new lock.
    #[inline]
    pub const fn new(value: T) -> Self {
        Self(async_lock::RwLock::new(value))
    }

    /// Acquire a shared read guard.
    #[inline]
    pub async fn read(&self) -> RwLockReadGuard<'_, T> {
        self.0.read().await
    }

    /// Acquire an exclusive write guard.
    #[inline]
    pub async fn write(&self) -> RwLockWriteGuard<'_, T> {
        self.0.write().await
    }

    /// Try to get a read guard without waiting.
    #[inline]
    pub fn try_read(&self) -> Option<RwLockReadGuard<'_, T>> {
        self.0.try_read()
    }

    /// Try to get a write guard without waiting.
    #[inline]
    pub fn try_write(&self) -> Option<RwLockWriteGuard<'_, T>> {
        self.0.try_write()
    }

    /// Get mutable access without locking (requires `&mut self`).
    #[inline]
    pub fn get_mut(&mut self) -> &mut T {
        self.0.get_mut()
    }

    /// Consume the lock, returning the inner value.
    #[inline]
    pub fn into_inner(self) -> T {
        self.0.into_inner()
    }
}

/// Synchronization primitive that enables a thread to block until a waker delivers a signal.
pub struct Blocker {
    /// Tracks whether a wake-up signal has been delivered (even if wait has not started yet).
    state: Mutex<bool>,
    /// Condvar used to park and resume the thread when the signal flips to true.
    cv: Condvar,
}

impl Blocker {
    /// Create a new [Blocker].
    pub fn new() -> Arc<Self> {
        Arc::new(Self {
            state: Mutex::new(false),
            cv: Condvar::new(),
        })
    }

    /// Block the current thread until a waker delivers a signal.
    pub fn wait(&self) {
        // Use a loop to tolerate spurious wake-ups and only proceed once a real signal arrives.
        let mut signaled = self.state.lock().unwrap();
        while !*signaled {
            signaled = self.cv.wait(signaled).unwrap();
        }

        // Reset the flag so subsequent waits park again until the next wake signal.
        *signaled = false;
    }
}

impl ArcWake for Blocker {
    fn wake_by_ref(arc_self: &Arc<Self>) {
        let mut signaled = arc_self.state.lock().unwrap();
        *signaled = true;

        // Notify a single waiter so the blocked thread re-checks the flag.
        arc_self.cv.notify_one();
    }
}

/// Validates that a label matches Prometheus metric name format: `[a-zA-Z][a-zA-Z0-9_]*`.
///
/// # Panics
///
/// Panics if the label is empty, starts with a non-alphabetic character,
/// or contains characters other than `[a-zA-Z0-9_]`.
pub fn validate_label(label: &str) {
    let mut chars = label.chars();
    assert!(
        chars.next().is_some_and(|c| c.is_ascii_alphabetic()),
        "label must start with [a-zA-Z]: {label}"
    );
    assert!(
        chars.all(|c| c.is_ascii_alphanumeric() || c == '_'),
        "label must only contain [a-zA-Z0-9_]: {label}"
    );
}

#[cfg(test)]
async fn task(i: usize) -> usize {
    for _ in 0..5 {
        reschedule().await;
    }
    i
}

#[cfg(test)]
pub fn run_tasks(tasks: usize, runner: crate::deterministic::Runner) -> (String, Vec<usize>) {
    runner.start(|context| async move {
        // Randomly schedule tasks
        let mut handles = FuturesUnordered::new();
        for i in 0..=tasks - 1 {
            handles.push(context.clone().spawn(move |_| task(i)));
        }

        // Collect output order
        let mut outputs = Vec::new();
        while let Some(result) = handles.next().await {
            outputs.push(result.unwrap());
        }
        assert_eq!(outputs.len(), tasks);
        (context.auditor().state(), outputs)
    })
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{deterministic, tokio, Metrics};
    use commonware_macros::test_traced;
    use futures::task::waker;
    use rayon::iter::{IntoParallelRefIterator, ParallelIterator};
    use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};

    #[test_traced]
    fn test_create_pool() {
        let executor = tokio::Runner::default();
        executor.start(|context| async move {
            // Create a thread pool with 4 threads
            let pool = create_pool(context.with_label("pool"), 4).unwrap();

            // Create a vector of numbers
            let v: Vec<_> = (0..10000).collect();

            // Use the thread pool to sum the numbers
            pool.install(|| {
                assert_eq!(v.par_iter().sum::<i32>(), 10000 * 9999 / 2);
            });
        });
    }

    #[test_traced]
    fn test_rwlock() {
        let executor = deterministic::Runner::default();
        executor.start(|_| async move {
            // Create a new RwLock
            let lock = RwLock::new(100);

            // many concurrent readers
            let r1 = lock.read().await;
            let r2 = lock.read().await;
            assert_eq!(*r1 + *r2, 200);

            // exclusive writer
            drop((r1, r2)); // all readers must go away
            let mut w = lock.write().await;
            *w += 1;

            // Check the value
            assert_eq!(*w, 101);
        });
    }

    #[test]
    fn test_blocker_waits_until_wake() {
        let blocker = Blocker::new();
        let started = Arc::new(AtomicBool::new(false));
        let completed = Arc::new(AtomicBool::new(false));

        let thread_blocker = blocker.clone();
        let thread_started = started.clone();
        let thread_completed = completed.clone();
        let handle = std::thread::spawn(move || {
            thread_started.store(true, Ordering::SeqCst);
            thread_blocker.wait();
            thread_completed.store(true, Ordering::SeqCst);
        });

        while !started.load(Ordering::SeqCst) {
            std::thread::yield_now();
        }

        assert!(!completed.load(Ordering::SeqCst));
        waker(blocker).wake();
        handle.join().unwrap();
        assert!(completed.load(Ordering::SeqCst));
    }

    #[test]
    fn test_blocker_handles_pre_wake() {
        let blocker = Blocker::new();
        waker(blocker.clone()).wake();

        let completed = Arc::new(AtomicBool::new(false));
        let thread_blocker = blocker;
        let thread_completed = completed.clone();
        std::thread::spawn(move || {
            thread_blocker.wait();
            thread_completed.store(true, Ordering::SeqCst);
        })
        .join()
        .unwrap();

        assert!(completed.load(Ordering::SeqCst));
    }

    #[test]
    fn test_blocker_reusable_across_signals() {
        let blocker = Blocker::new();
        let completed = Arc::new(AtomicUsize::new(0));

        let thread_blocker = blocker.clone();
        let thread_completed = completed.clone();
        let handle = std::thread::spawn(move || {
            for _ in 0..2 {
                thread_blocker.wait();
                thread_completed.fetch_add(1, Ordering::SeqCst);
            }
        });

        for expected in 1..=2 {
            waker(blocker.clone()).wake();
            while completed.load(Ordering::SeqCst) < expected {
                std::thread::yield_now();
            }
        }

        handle.join().unwrap();
        assert_eq!(completed.load(Ordering::SeqCst), 2);
    }
}