//! Low-level building blocks for Reed-Solomon encoding/decoding.
//!
//! **This is an advanced module which is not needed for [simple usage] or [basic usage].**
//!
//! This module is relevant if you want to
//! - use [`rate`] module and need an [`Engine`] to use with it.
//! - create your own [`Engine`].
//! - understand/benchmark/test at low level.
//!
//! # Engines
//!
//! An [`Engine`] is an implementation of basic low-level algorithms
//! needed for Reed-Solomon encoding/decoding.
//!
//! - [`Naive`]
//!     - Simple reference implementation.
//! - [`NoSimd`]
//!     - Basic optimized engine without SIMD so that it works on all CPUs.
//! - `Avx2`
//!     - Optimized engine that takes advantage of the x86(-64) AVX2 SIMD instructions.
//! - `Ssse3`
//!     - Optimized engine that takes advantage of the x86(-64) SSSE3 SIMD instructions.
//! - `Neon`
//!     - Optimized engine that takes advantage of the `AArch64` Neon SIMD instructions.
//! - [`DefaultEngine`]
//!     - Default engine which is used when no specific engine is given.
//!     - Automatically selects best engine at runtime.
//!
//! [simple usage]: crate::reed_solomon#simple-usage
//! [basic usage]: crate::reed_solomon#basic-usage
//! [`Encoder`]: crate::reed_solomon::Encoder
//! [`Decoder`]: crate::reed_solomon::Decoder
//! [`rate`]: crate::reed_solomon::rate

#[cfg(target_arch = "aarch64")]
pub use self::engine_neon::Neon;
pub(crate) use self::shards::Shards;
#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
pub use self::{engine_avx2::Avx2, engine_ssse3::Ssse3};
pub use self::{
    engine_default::DefaultEngine, engine_naive::Naive, engine_nosimd::NoSimd, shards::ShardsRefMut,
};
pub(crate) use utils::{fft_skew_end, formal_derivative, ifft_skew_end, xor_within};

mod engine_default;
mod engine_naive;
mod engine_nosimd;

#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
mod engine_avx2;
#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
mod engine_ssse3;

#[cfg(target_arch = "aarch64")]
mod engine_neon;

mod fwht;
mod shards;

pub mod tables;
pub mod utils;

// ======================================================================
// CONST - PUBLIC

/// Size of Galois field element [`GfElement`] in bits.
pub const GF_BITS: usize = 16;

/// Galois field order, i.e. number of elements.
pub const GF_ORDER: usize = 65536;

/// `GF_ORDER - 1`
pub const GF_MODULUS: GfElement = 65535;

/// Galois field polynomial.
pub const GF_POLYNOMIAL: usize = 0x1002D;

/// Byte width of a shard chunk.
///
/// [`Engine`] methods process shard buffers as arrays of this size.
/// Input shards may span multiple chunks; any partial final chunk is padded
/// during processing and returned at the original shard length.
pub const SHARD_CHUNK_BYTES: usize = 64;

/// Cantor basis used by the additive FFT over GF(2^16).
pub const CANTOR_BASIS: [GfElement; GF_BITS] = [
    0x0001, 0xACCA, 0x3C0E, 0x163E, 0xC582, 0xED2E, 0x914C, 0x4012, 0x6C98, 0x10D8, 0x6A72, 0xB900,
    0xFDB8, 0xFB34, 0xFF38, 0x991E,
];

// ======================================================================
// TYPE ALIASES - PUBLIC

/// Galois field element.
pub type GfElement = u16;

// ======================================================================
// Engine - PUBLIC

/// Trait for compute-intensive low-level algorithms needed
/// for Reed-Solomon encoding/decoding.
///
/// This is the trait you would implement to provide SIMD support
/// for a CPU architecture not already provided.
///
/// [`Naive`] engine is provided for those who want to
/// study the source code to understand [`Engine`].
pub trait Engine {
    // ============================================================
    // REQUIRED

    /// In-place decimation-in-time FFT (fast Fourier transform).
    ///
    /// - FFT is done on chunk `data[pos .. pos + size]`
    /// - `size` must be `2^n`
    /// - Before function call `data[pos .. pos + size]` must be valid.
    /// - After function call
    ///     - `data[pos .. pos + truncated_size]`
    ///       contains valid FFT result.
    ///     - `data[pos + truncated_size .. pos + size]`
    ///       contains valid FFT result if this contained
    ///       only `0u8`:s and garbage otherwise.
    fn fft(
        &self,
        data: &mut ShardsRefMut<'_>,
        pos: usize,
        size: usize,
        truncated_size: usize,
        skew_delta: usize,
    );

    /// In-place decimation-in-time IFFT (inverse fast Fourier transform).
    ///
    /// - IFFT is done on chunk `data[pos .. pos + size]`
    /// - `size` must be `2^n`
    /// - Before function call `data[pos .. pos + size]` must be valid.
    /// - After function call
    ///     - `data[pos .. pos + truncated_size]`
    ///       contains valid IFFT result.
    ///     - `data[pos + truncated_size .. pos + size]`
    ///       contains valid IFFT result if this contained
    ///       only `0u8`:s and garbage otherwise.
    fn ifft(
        &self,
        data: &mut ShardsRefMut<'_>,
        pos: usize,
        size: usize,
        truncated_size: usize,
        skew_delta: usize,
    );

    /// `x[] *= log_m`
    fn mul(&self, x: &mut [[u8; SHARD_CHUNK_BYTES]], log_m: GfElement);

    // ============================================================
    // PROVIDED

    /// Evaluate polynomial.
    fn eval_poly(erasures: &mut [GfElement; GF_ORDER], truncated_size: usize)
    where
        Self: Sized,
    {
        utils::eval_poly(erasures, truncated_size);
    }
}

// ======================================================================
// TESTS

// Engines are tested indirectly via roundtrip tests of HighRate and LowRate.