compcol 0.3.1

//! Google Snappy — raw block format.
//!
//! This module implements the **Snappy raw block format** described in
//! <https://github.com/google/snappy/blob/main/format_description.txt>.
//!
//! A raw Snappy block is a single self-delimiting unit:
//!
//! 1. The uncompressed length encoded as a Base-128 varint (1–5 bytes).
//! 2. A stream of tag bytes followed by their payloads:
//!    * `00` — literal. The high 6 bits hold `length-1` when in `0..=59`;
//!      values `60..=63` mean the length minus one is stored in 1, 2, 3 or 4
//!      little-endian bytes that follow the tag.
//!    * `01` — copy with a 1-byte offset. Bits `[2:4]` hold `length-4`
//!      (length is 4..=11), bits `[5:7]` hold the high three bits of the
//!      11-bit offset, followed by a 1-byte little-endian low offset.
//!    * `10` — copy with a 2-byte offset. Bits `[2:7]` hold `length-1`
//!      (length is 1..=64), followed by a 2-byte little-endian offset.
//!    * `11` — copy with a 4-byte offset. Bits `[2:7]` hold `length-1`
//!      (length is 1..=64), followed by a 4-byte little-endian offset.
//!
//! The framed `snzip` format (`framing_format.txt`) is *not* implemented —
//! the raw block format is what `Snappy_Compress` / `Snappy_Uncompress` in
//! the reference library produce and consume.
//!
//! ## Streaming model
//!
//! Snappy is a whole-block codec: the encoder must know the total
//! uncompressed length before it can emit the leading varint, and the
//! decoder may follow back-references that reach anywhere in the produced
//! output. Both sides therefore buffer their input on `encode` / `decode`
//! and do the actual work in `finish`, draining the compressed (or
//! decompressed) result across as many `finish` calls as the caller needs
//! to consume it.
//!
//! ## Encoder
//!
//! The encoder uses a small fixed-size hash table (14-bit index, 16 KiB
//! `u32` entries) to find back-references — the same general shape as the
//! reference `snappy::CompressFragment` implementation. Matches are
//! emitted greedily; runs that cannot find a back-reference are folded
//! into literal tags. The maximum match length per copy tag is 64 bytes,
//! and longer matches are split across multiple copy tags.

use alloc::vec::Vec;

use crate::error::Error;
use crate::traits::{Algorithm, RawDecoder, RawEncoder, RawProgress};

/// Zero-sized marker type implementing [`Algorithm`] for Snappy.
#[derive(Debug, Clone, Copy, Default)]
pub struct Snappy;

impl Algorithm for Snappy {
    const NAME: &'static str = "snappy";
    type Encoder = Encoder;
    type Decoder = Decoder;
    type EncoderConfig = ();
    type DecoderConfig = ();
    fn encoder_with(_: ()) -> Encoder {
        Encoder::new()
    }
    fn decoder_with(_: ()) -> Decoder {
        Decoder::new()
    }
}

// ─── varint helpers ───────────────────────────────────────────────────────

/// Append the Base-128 varint encoding of `value` to `out`. Up to 5 bytes
/// for a `u32`-sized length.
fn write_varint_u32(value: u32, out: &mut Vec<u8>) {
    let mut v = value;
    while v >= 0x80 {
        out.push(((v & 0x7F) as u8) | 0x80);
        v >>= 7;
    }
    out.push(v as u8);
}

/// Read a Base-128 varint from `buf`. Returns `(value, bytes_read)` on
/// success, or `Error::Corrupt` if the varint runs past 5 bytes or
/// `Error::UnexpectedEnd` if the buffer is exhausted mid-varint.
fn read_varint_u32(buf: &[u8]) -> Result<(u32, usize), Error> {
    let mut result: u64 = 0;
    let mut shift: u32 = 0;
    for (i, &b) in buf.iter().enumerate() {
        if i == 5 {
            // Snappy lengths are u32; a 6th continuation byte means overflow.
            return Err(Error::Corrupt);
        }
        result |= ((b & 0x7F) as u64) << shift;
        if b & 0x80 == 0 {
            if result > u32::MAX as u64 {
                return Err(Error::Corrupt);
            }
            return Ok((result as u32, i + 1));
        }
        shift += 7;
    }
    Err(Error::UnexpectedEnd)
}

// ─── encoder ──────────────────────────────────────────────────────────────

/// Streaming Snappy encoder.
///
/// Buffers the entire input on `encode`, then emits the compressed block
/// across one or more `finish` calls.
#[derive(Debug, Default)]
pub struct Encoder {
    /// Raw input buffered across `encode` calls.
    input: Vec<u8>,
    /// Compressed output produced once `finish` first runs.
    output: Vec<u8>,
    /// Bytes of `output` already handed back to the caller.
    out_pos: usize,
    /// Set the first time `finish` actually compresses the buffered input.
    compressed: bool,
}

impl Encoder {
    pub const fn new() -> Self {
        Self {
            input: Vec::new(),
            output: Vec::new(),
            out_pos: 0,
            compressed: false,
        }
    }
}

impl RawEncoder for Encoder {
    fn raw_encode(&mut self, input: &[u8], _output: &mut [u8]) -> Result<RawProgress, Error> {
        // Buffer the whole input — Snappy needs the total length up front,
        // and back-references can reach anywhere within the block, so we
        // can't emit anything until the caller signals end of input.
        self.input.extend_from_slice(input);
        Ok(RawProgress {
            consumed: input.len(),
            written: 0,
            done: false,
        })
    }

    fn raw_finish(&mut self, output: &mut [u8]) -> Result<RawProgress, Error> {
        if !self.compressed {
            compress_block(&self.input, &mut self.output);
            self.compressed = true;
        }
        let remaining = self.output.len() - self.out_pos;
        let n = remaining.min(output.len());
        output[..n].copy_from_slice(&self.output[self.out_pos..self.out_pos + n]);
        self.out_pos += n;
        let done = self.out_pos == self.output.len();
        Ok(RawProgress {
            consumed: 0,
            written: n,
            done,
        })
    }

    fn raw_reset(&mut self) {
        self.input.clear();
        self.output.clear();
        self.out_pos = 0;
        self.compressed = false;
    }
}

/// Compress `input` into `out` as a single Snappy raw block.
///
/// Layout:
/// * Varint-encoded `input.len()`.
/// * A literal+copy tag stream covering every byte of `input`.
fn compress_block(input: &[u8], out: &mut Vec<u8>) {
    out.clear();
    write_varint_u32(input.len() as u32, out);

    if input.is_empty() {
        return;
    }
    // For very short inputs there's no room for a 4-byte back-reference
    // anchor, so just emit one literal tag and return. This also keeps the
    // hash-matcher loop below from indexing past the end.
    if input.len() < 4 {
        emit_literal(input, out);
        return;
    }

    // Sized to roughly match the reference implementation's small-input
    // path. 14-bit index ⇒ 16384 slots, each a u32 byte position.
    const HASH_BITS: u32 = 14;
    const HASH_SIZE: usize = 1 << HASH_BITS;
    const NIL: u32 = u32::MAX;

    let mut table = alloc::vec![NIL; HASH_SIZE];

    let input_end = input.len();
    // The reference encoder reserves the last few bytes for a final
    // literal so that match probes never read past the buffer.
    let match_limit = input_end.saturating_sub(4);

    let mut next_emit = 0usize; // first byte not yet covered by a tag
    let mut ip = 0usize; // current position being hashed

    // Helper: produce a 32-bit hash of the 4-byte sequence at `pos`.
    let hash = |bytes: &[u8], pos: usize| -> usize {
        // Multiplicative hash with the canonical Snappy 0x1E35A7BD constant.
        let v = (bytes[pos] as u32)
            | ((bytes[pos + 1] as u32) << 8)
            | ((bytes[pos + 2] as u32) << 16)
            | ((bytes[pos + 3] as u32) << 24);
        ((v.wrapping_mul(0x1E35A7BD)) >> (32 - HASH_BITS)) as usize
    };

    // Match-or-literal main loop.
    while ip < match_limit {
        let h = hash(input, ip);
        let candidate = table[h] as usize;
        table[h] = ip as u32;

        // A candidate is valid if the 4 bytes at `candidate` equal the
        // 4 bytes at `ip` and the offset fits in u32. Offset 0 means
        // "no candidate yet" — guarded by the NIL sentinel above.
        let four_match = (table[h] != NIL)
            && (candidate < ip)
            && candidate + 3 < input_end
            && input[candidate] == input[ip]
            && input[candidate + 1] == input[ip + 1]
            && input[candidate + 2] == input[ip + 2]
            && input[candidate + 3] == input[ip + 3];

        if !four_match {
            ip += 1;
            continue;
        }

        // Found a 4-byte match. First, flush any pending literal.
        if next_emit < ip {
            emit_literal(&input[next_emit..ip], out);
        }

        // Extend the match as far as it'll go.
        let mut m_end = ip + 4;
        let mut c_end = candidate + 4;
        while m_end < input_end && input[m_end] == input[c_end] {
            m_end += 1;
            c_end += 1;
        }
        let mut match_len = m_end - ip;
        let offset = (ip - candidate) as u32;

        // Emit one or more copy tags (max 64 bytes per tag).
        let mut emitted = 0usize;
        while match_len > 0 {
            // Snappy copy tags need length >= 4. The remainder logic below
            // guarantees that: a length-extending loop pass either takes a
            // full 64-byte copy, or — for the final one — splits so that
            // both pieces are >= 4 bytes.
            let take = if match_len <= 64 {
                match_len
            } else if match_len < 68 {
                // Avoid leaving a < 4 byte remainder.
                match_len - 4
            } else {
                64
            };
            emit_copy(offset, take as u32, out);
            // Insert the position-after-emit into the hash table so that
            // subsequent passes can chain through the middle of long
            // matches (mirrors the reference encoder).
            let pos_after = ip + emitted + take;
            if pos_after + 3 < input_end {
                let h2 = hash(input, pos_after - 1);
                table[h2] = (pos_after - 1) as u32;
            }
            emitted += take;
            match_len -= take;
        }

        ip += emitted;
        next_emit = ip;
        // Seed the hash table with the position just before `ip` so the
        // next iteration can immediately find a match starting there.
        if ip + 3 < input_end {
            let h2 = hash(input, ip - 1);
            table[h2] = (ip - 1) as u32;
        }
    }

    // Flush trailing literal.
    if next_emit < input_end {
        emit_literal(&input[next_emit..], out);
    }
}

/// Emit a literal tag covering `data`.
fn emit_literal(data: &[u8], out: &mut Vec<u8>) {
    debug_assert!(!data.is_empty());
    let n = data.len();
    let n_minus_1 = (n - 1) as u32;
    // Tag byte's low 2 bits are 0b00 for a literal, so we just shift the
    // length-class bits into the upper 6 bits.
    if n_minus_1 < 60 {
        out.push((n_minus_1 as u8) << 2);
    } else if n_minus_1 < 1 << 8 {
        out.push(60 << 2);
        out.push(n_minus_1 as u8);
    } else if n_minus_1 < 1 << 16 {
        out.push(61 << 2);
        out.push(n_minus_1 as u8);
        out.push((n_minus_1 >> 8) as u8);
    } else if n_minus_1 < 1 << 24 {
        out.push(62 << 2);
        out.push(n_minus_1 as u8);
        out.push((n_minus_1 >> 8) as u8);
        out.push((n_minus_1 >> 16) as u8);
    } else {
        out.push(63 << 2);
        out.push(n_minus_1 as u8);
        out.push((n_minus_1 >> 8) as u8);
        out.push((n_minus_1 >> 16) as u8);
        out.push((n_minus_1 >> 24) as u8);
    }
    out.extend_from_slice(data);
}

/// Emit a single copy tag with `length` (4..=64) and `offset` (1..=u32::MAX).
fn emit_copy(offset: u32, length: u32, out: &mut Vec<u8>) {
    debug_assert!((4..=64).contains(&length));
    debug_assert!(offset >= 1);
    if length <= 11 && offset < (1 << 11) {
        // Tag 01: 1-byte offset.
        let len_bits = ((length - 4) as u8) << 2;
        let off_hi = ((offset >> 8) as u8) << 5;
        out.push(0b01 | len_bits | off_hi);
        out.push(offset as u8);
    } else if offset < (1 << 16) {
        // Tag 10: 2-byte offset.
        let len_bits = ((length - 1) as u8) << 2;
        out.push(0b10 | len_bits);
        out.push(offset as u8);
        out.push((offset >> 8) as u8);
    } else {
        // Tag 11: 4-byte offset.
        let len_bits = ((length - 1) as u8) << 2;
        out.push(0b11 | len_bits);
        out.push(offset as u8);
        out.push((offset >> 8) as u8);
        out.push((offset >> 16) as u8);
        out.push((offset >> 24) as u8);
    }
}

// ─── decoder ──────────────────────────────────────────────────────────────

/// Streaming Snappy decoder.
///
/// Buffers the entire compressed stream on `decode`, then emits the
/// decompressed bytes across one or more `finish` calls.
#[derive(Debug, Default)]
pub struct Decoder {
    /// Raw compressed bytes buffered across `decode` calls.
    input: Vec<u8>,
    /// Decompressed output produced once `finish` first runs.
    output: Vec<u8>,
    /// Bytes of `output` already handed back to the caller.
    out_pos: usize,
    /// Set the first time `finish` actually decompresses the buffered input.
    decompressed: bool,
}

impl Decoder {
    pub const fn new() -> Self {
        Self {
            input: Vec::new(),
            output: Vec::new(),
            out_pos: 0,
            decompressed: false,
        }
    }
}

impl RawDecoder for Decoder {
    fn raw_decode(&mut self, input: &[u8], _output: &mut [u8]) -> Result<RawProgress, Error> {
        self.input.extend_from_slice(input);
        Ok(RawProgress {
            consumed: input.len(),
            written: 0,
            done: false,
        })
    }

    fn raw_finish(&mut self, output: &mut [u8]) -> Result<RawProgress, Error> {
        if !self.decompressed {
            decompress_block(&self.input, &mut self.output)?;
            self.decompressed = true;
        }
        let remaining = self.output.len() - self.out_pos;
        let n = remaining.min(output.len());
        output[..n].copy_from_slice(&self.output[self.out_pos..self.out_pos + n]);
        self.out_pos += n;
        let done = self.out_pos == self.output.len();
        Ok(RawProgress {
            consumed: 0,
            written: n,
            done,
        })
    }

    fn raw_reset(&mut self) {
        self.input.clear();
        self.output.clear();
        self.out_pos = 0;
        self.decompressed = false;
    }
}

/// Decompress one Snappy raw block into `out`.
fn decompress_block(input: &[u8], out: &mut Vec<u8>) -> Result<(), Error> {
    out.clear();

    if input.is_empty() {
        return Err(Error::UnexpectedEnd);
    }

    let (uncompressed_len, vi_len) = read_varint_u32(input)?;
    let uncompressed_len = uncompressed_len as usize;
    out.reserve(uncompressed_len);

    let mut ip = vi_len;
    let input_end = input.len();

    while ip < input_end {
        let tag = input[ip];
        ip += 1;
        match tag & 0b11 {
            0b00 => {
                // Literal.
                let upper = (tag >> 2) as u32;
                let length = if upper < 60 {
                    upper + 1
                } else {
                    let extra = (upper - 59) as usize; // 1..=4
                    if ip + extra > input_end {
                        return Err(Error::UnexpectedEnd);
                    }
                    let mut len_minus_1: u32 = 0;
                    for i in 0..extra {
                        len_minus_1 |= (input[ip + i] as u32) << (8 * i);
                    }
                    ip += extra;
                    len_minus_1.wrapping_add(1)
                };
                let length = length as usize;
                if ip + length > input_end {
                    return Err(Error::UnexpectedEnd);
                }
                if out.len() + length > uncompressed_len {
                    return Err(Error::Corrupt);
                }
                out.extend_from_slice(&input[ip..ip + length]);
                ip += length;
            }
            0b01 => {
                // Copy with 1-byte offset.
                if ip >= input_end {
                    return Err(Error::UnexpectedEnd);
                }
                let length = (((tag >> 2) & 0x07) as usize) + 4;
                let off_hi = ((tag >> 5) & 0x07) as usize;
                let offset = (off_hi << 8) | (input[ip] as usize);
                ip += 1;
                copy_from_back(out, offset, length, uncompressed_len)?;
            }
            0b10 => {
                // Copy with 2-byte offset.
                if ip + 2 > input_end {
                    return Err(Error::UnexpectedEnd);
                }
                let length = (((tag >> 2) & 0x3F) as usize) + 1;
                let offset = (input[ip] as usize) | ((input[ip + 1] as usize) << 8);
                ip += 2;
                copy_from_back(out, offset, length, uncompressed_len)?;
            }
            0b11 => {
                // Copy with 4-byte offset.
                if ip + 4 > input_end {
                    return Err(Error::UnexpectedEnd);
                }
                let length = (((tag >> 2) & 0x3F) as usize) + 1;
                let offset = (input[ip] as usize)
                    | ((input[ip + 1] as usize) << 8)
                    | ((input[ip + 2] as usize) << 16)
                    | ((input[ip + 3] as usize) << 24);
                ip += 4;
                copy_from_back(out, offset, length, uncompressed_len)?;
            }
            _ => unreachable!(),
        }
    }

    if out.len() != uncompressed_len {
        return Err(Error::Corrupt);
    }
    Ok(())
}

/// Copy `length` bytes from `offset` bytes behind the current end of `out`
/// to the current end of `out`. Self-overlapping copies (offset < length)
/// are allowed and produce an LZ77-style RLE-style fill.
fn copy_from_back(
    out: &mut Vec<u8>,
    offset: usize,
    length: usize,
    uncompressed_len: usize,
) -> Result<(), Error> {
    if offset == 0 || offset > out.len() {
        return Err(Error::InvalidDistance);
    }
    if out.len() + length > uncompressed_len {
        return Err(Error::Corrupt);
    }
    let start = out.len() - offset;
    // Byte-by-byte so that self-overlapping copies replicate correctly.
    for i in 0..length {
        let b = out[start + i];
        out.push(b);
    }
    Ok(())
}