Function pack

nodedb-fts/src/codec/bitpack.rs:25–71

Pack a slice of u32 values into a compact byte buffer. Layout: `[num_values: u16 LE][bit_width: u8][packed bits...]` Total bytes = 3 + ceil(num_values * bit_width / 8).

(values: &[u32])

///
/// Total bytes = 3 + ceil(num_values * bit_width / 8).
pub fn pack(values: &[u32]) -> Vec<u8> {
    if values.is_empty() {
        let mut buf = Vec::with_capacity(3);
        buf.extend_from_slice(&0u16.to_le_bytes());
        buf.push(0);
        return buf;
    }

    let max_val = values.iter().copied().max().unwrap_or(0);
    let bit_width = bits_needed(max_val);

    let total_bits = values.len() as u64 * bit_width as u64;
    let data_bytes = total_bits.div_ceil(8) as usize;
    let mut buf = Vec::with_capacity(3 + data_bytes);

    // Header.
    buf.extend_from_slice(&(values.len() as u16).to_le_bytes());
    buf.push(bit_width);

    if bit_width == 0 {
        // All values are 0 — no data bytes needed.
        return buf;
    }

    // Pack bits.
    buf.resize(3 + data_bytes, 0);
    let data = &mut buf[3..];
    let mut bit_pos = 0u64;

    for &val in values {
        let byte_idx = (bit_pos / 8) as usize;
        let bit_offset = (bit_pos % 8) as u32;

        // Write value starting at bit_offset within data[byte_idx..].
        // A single value may span up to 5 bytes (32 bits + 7 bit offset).
        let wide = (val as u64) << bit_offset;
        let bytes = wide.to_le_bytes();
        let bytes_to_write = (bit_offset + bit_width as u32).div_ceil(8) as usize;
        for i in 0..bytes_to_write.min(data.len() - byte_idx) {
            data[byte_idx + i] |= bytes[i];
        }

        bit_pos += bit_width as u64;
    }

    buf
}

/// Unpack values from a bitpacked buffer.
///