betterMatchLen

fn betterMatchLen(old: u16, prev: []const u8, bytes: []const u8) u16

File

Code

fn betterMatchLen(old: u16, prev: []const u8, bytes: []const u8) u16 {
    assert(old < @min(bytes.len, token.max_length));
    assert(prev.len >= bytes.len);
    assert(bytes.len >= token.min_length);

    var i: u16 = 0;
    const Block = @Int(.unsigned, @min(math.divCeil(
        comptime_int,
        math.ceilPowerOfTwoAssert(usize, @bitSizeOf(usize)),
        8,
    ) catch unreachable, 256) * 8);

    if (bytes.len < token.max_length) {
        @branchHint(.unlikely); // Only end of stream

        while (bytes[i..].len >= @sizeOf(Block)) {
            const a = mem.readInt(Block, prev[i..][0..@sizeOf(Block)], .little);
            const b = mem.readInt(Block, bytes[i..][0..@sizeOf(Block)], .little);
            const diff = a ^ b;
            if (diff != 0) {
                @branchHint(.likely);
                i += @ctz(diff) / 8;
                return i;
            }
            i += @sizeOf(Block);
        }

        while (i != bytes.len and prev[i] == bytes[i]) {
            i += 1;
        }
        assert(i < token.max_length);
        return i;
    }

    if (old >= @sizeOf(Block)) {
        // Check that a longer end is present, otherwise the match is always worse
        const a = mem.readInt(Block, prev[old + 1 - @sizeOf(Block) ..][0..@sizeOf(Block)], .little);
        const b = mem.readInt(Block, bytes[old + 1 - @sizeOf(Block) ..][0..@sizeOf(Block)], .little);
        if (a != b) return i;
    }

    while (true) {
        const a = mem.readInt(Block, prev[i..][0..@sizeOf(Block)], .little);
        const b = mem.readInt(Block, bytes[i..][0..@sizeOf(Block)], .little);
        const diff = a ^ b;
        if (diff != 0) {
            i += @ctz(diff) / 8;
            return i;
        }
        i += @sizeOf(Block);
        if (i == 256) break;
    }

    const a = mem.readInt(u16, prev[i..][0..2], .little);
    const b = mem.readInt(u16, bytes[i..][0..2], .little);
    const diff = a ^ b;
    i += @ctz(diff) / 8;
    assert(i <= token.max_length);
    return i;
}