Function decode

nodedb-codec/src/rans.rs:127–219

Decompress interleaved rANS data.

(data: &[u8])

/// Decompress interleaved rANS data.
pub fn decode(data: &[u8]) -> Result<Vec<u8>, CodecError> {
    if data.len() < HEADER_SIZE {
        return Err(CodecError::Truncated {
            expected: HEADER_SIZE,
            actual: data.len(),
        });
    }

    let uncompressed_size = u32::from_le_bytes([data[0], data[1], data[2], data[3]]) as usize;
    if uncompressed_size == 0 {
        return Ok(Vec::new());
    }

    // Read frequency table.
    let mut norm_freqs = [0u32; 256];
    for (i, freq) in norm_freqs.iter_mut().enumerate() {
        let pos = 4 + i * 4;
        *freq = u32::from_le_bytes([data[pos], data[pos + 1], data[pos + 2], data[pos + 3]]);
    }

    let (cum_freqs, sym_freqs) = build_cum_table(&norm_freqs);

    // Build reverse lookup table for decoding.
    let lookup = build_decode_table(&cum_freqs, &sym_freqs);

    let _comp_size = u32::from_le_bytes([
        data[HEADER_SIZE - 4],
        data[HEADER_SIZE - 3],
        data[HEADER_SIZE - 2],
        data[HEADER_SIZE - 1],
    ]) as usize;

    // Read per-stream sizes.
    let mut pos = HEADER_SIZE;
    if pos + NUM_STREAMS * 4 > data.len() {
        return Err(CodecError::Truncated {
            expected: pos + NUM_STREAMS * 4,
            actual: data.len(),
        });
    }

    let mut stream_sizes = [0usize; NUM_STREAMS];
    for size in stream_sizes.iter_mut() {
        *size =
            u32::from_le_bytes([data[pos], data[pos + 1], data[pos + 2], data[pos + 3]]) as usize;
        pos += 4;
    }

    // Read streams.
    let mut stream_data: [Vec<u8>; NUM_STREAMS] = std::array::from_fn(|_| Vec::new());
    for i in 0..NUM_STREAMS {
        let end = pos + stream_sizes[i];
        if end > data.len() {
            return Err(CodecError::Truncated {
                expected: end,
                actual: data.len(),
            });
        }