structured-zstd 0.0.6

//! Structures and utilities used for compressing/encoding data into the Zstd format.

pub(crate) mod block_header;
pub(crate) mod blocks;
pub(crate) mod frame_header;
pub(crate) mod match_generator;
pub(crate) mod util;

mod frame_compressor;
mod levels;
mod streaming_encoder;
pub use frame_compressor::FrameCompressor;
pub use match_generator::MatchGeneratorDriver;
pub use streaming_encoder::StreamingEncoder;

use crate::io::{Read, Write};
use alloc::vec::Vec;

/// Convenience function to compress some source into a target without reusing any resources of the compressor
/// ```rust
/// use structured_zstd::encoding::{compress, CompressionLevel};
/// let data: &[u8] = &[0,0,0,0,0,0,0,0,0,0,0,0];
/// let mut target = Vec::new();
/// compress(data, &mut target, CompressionLevel::Fastest);
/// ```
pub fn compress<R: Read, W: Write>(source: R, target: W, level: CompressionLevel) {
    let mut frame_enc = FrameCompressor::new(level);
    frame_enc.set_source(source);
    frame_enc.set_drain(target);
    frame_enc.compress();
}

/// Convenience function to compress some source into a Vec without reusing any resources of the compressor
/// ```rust
/// use structured_zstd::encoding::{compress_to_vec, CompressionLevel};
/// let data: &[u8] = &[0,0,0,0,0,0,0,0,0,0,0,0];
/// let compressed = compress_to_vec(data, CompressionLevel::Fastest);
/// ```
pub fn compress_to_vec<R: Read>(source: R, level: CompressionLevel) -> Vec<u8> {
    let mut vec = Vec::new();
    compress(source, &mut vec, level);
    vec
}

/// The compression mode used impacts the speed of compression,
/// and resulting compression ratios. Faster compression will result
/// in worse compression ratios, and vice versa.
#[derive(Copy, Clone)]
pub enum CompressionLevel {
    /// This level does not compress the data at all, and simply wraps
    /// it in a Zstandard frame.
    Uncompressed,
    /// This level is roughly equivalent to Zstd compression level 1
    Fastest,
    /// This level uses the crate's dedicated `dfast`-style matcher to
    /// target a better speed/ratio tradeoff than [`CompressionLevel::Fastest`].
    ///
    /// It represents this crate's "default" compression setting and may
    /// evolve in future versions as the implementation moves closer to
    /// reference zstd level 3 behavior.
    Default,
    /// This level is roughly equivalent to Zstd level 7.
    ///
    /// Uses the hash-chain matcher with a lazy2 matching strategy: the encoder
    /// evaluates up to two positions ahead before committing to a match,
    /// trading speed for a better compression ratio than [`CompressionLevel::Default`].
    ///
    /// **Limitation:** hash-chain tables use 32-bit positions. For single-frame
    /// inputs exceeding ~4 GiB, matches can still be found for roughly one
    /// window past that point; once all in-window positions exceed `u32::MAX`
    /// (≈4 GiB + window size), matching becomes effectively repcode-only.
    /// Prefer [`CompressionLevel::Default`] for very large single-frame streams
    /// until table rebasing is implemented.
    Better,
    /// This level is roughly equivalent to Zstd level 11.
    ///
    /// Uses the hash-chain matcher with a deep lazy2 matching strategy and
    /// a 16 MiB window. Compared to [`CompressionLevel::Better`], this level
    /// uses larger hash and chain tables (2 M / 1 M entries vs 1 M / 512 K),
    /// a deeper search (32 candidates vs 16), and a higher target match
    /// length (128 vs 48), trading speed for the best compression ratio
    /// available in this crate.
    ///
    /// **Limitation:** hash-chain tables use 32-bit positions. For single-frame
    /// inputs exceeding ~4 GiB, matches can still be found for roughly one
    /// window past that point; once all in-window positions exceed `u32::MAX`
    /// (≈4 GiB + window size), matching becomes effectively repcode-only.
    /// Prefer [`CompressionLevel::Default`] for very large single-frame
    /// streams until table rebasing is implemented.
    Best,
}

/// Trait used by the encoder that users can use to extend the matching facilities with their own algorithm
/// making their own tradeoffs between runtime, memory usage and compression ratio
///
/// This trait operates on buffers that represent the chunks of data the matching algorithm wants to work on.
/// Each one of these buffers is referred to as a *space*. One or more of these buffers represent the window
/// the decoder will need to decode the data again.
///
/// This library asks the Matcher for a new buffer using `get_next_space` to allow reusing of allocated buffers when they are no longer part of the
/// window of data that is being used for matching.
///
/// The library fills the buffer with data that is to be compressed and commits them back to the matcher using `commit_space`.
///
/// Then it will either call `start_matching` or, if the space is deemed not worth compressing, `skip_matching` is called.
///
/// This is repeated until no more data is left to be compressed.
pub trait Matcher {
    /// Get a space where we can put data to be matched on. Will be encoded as one block. The maximum allowed size is 128 kB.
    fn get_next_space(&mut self) -> alloc::vec::Vec<u8>;
    /// Get a reference to the last commited space
    fn get_last_space(&mut self) -> &[u8];
    /// Commit a space to the matcher so it can be matched against
    fn commit_space(&mut self, space: alloc::vec::Vec<u8>);
    /// Just process the data in the last commited space for future matching
    fn skip_matching(&mut self);
    /// Process the data in the last commited space for future matching AND generate matches for the data
    fn start_matching(&mut self, handle_sequence: impl for<'a> FnMut(Sequence<'a>));
    /// Reset this matcher so it can be used for the next new frame
    fn reset(&mut self, level: CompressionLevel);
    /// Prime matcher state with dictionary history before compressing the next frame.
    /// Default implementation is a no-op for custom matchers that do not support this.
    fn prime_with_dictionary(&mut self, _dict_content: &[u8], _offset_hist: [u32; 3]) {}
    /// Returns whether this matcher can consume dictionary priming state and produce
    /// dictionary-dependent sequences. Defaults to `false` for custom matchers.
    fn supports_dictionary_priming(&self) -> bool {
        false
    }
    /// The size of the window the decoder will need to execute all sequences produced by this matcher.
    ///
    /// Must return a positive (non-zero) value; returning 0 causes
    /// [`StreamingEncoder`] to reject the first write with an invalid-input error
    /// (`InvalidInput` with `std`, `Other` with `no_std`).
    ///
    /// Must remain stable for the lifetime of a frame.
    /// It may change only after `reset()` is called for the next frame
    /// (for example because the compression level changed).
    fn window_size(&self) -> u64;
}

#[derive(PartialEq, Eq, Debug)]
/// Sequences that a [`Matcher`] can produce
pub enum Sequence<'data> {
    /// Is encoded as a sequence for the decoder sequence execution.
    ///
    /// First the literals will be copied to the decoded data,
    /// then `match_len` bytes are copied from `offset` bytes back in the decoded data
    Triple {
        literals: &'data [u8],
        offset: usize,
        match_len: usize,
    },
    /// This is returned as the last sequence in a block
    ///
    /// These literals will just be copied at the end of the sequence execution by the decoder
    Literals { literals: &'data [u8] },
}