namespace MoShiro.Protocol;

/// <summary>CRC-32C（Castagnoli），RawChunk 的完整性校验。</summary>
public static class Crc32c
{
    private static readonly uint[] s_table = BuildTable();

    public static uint Compute(ReadOnlySpan<byte> data)
    {
        uint crc = uint.MaxValue;
        foreach (byte b in data)
        {
            crc = s_table[(crc ^ b) & 0xFF] ^ (crc >> 8);
        }

        return ~crc;
    }

    private static uint[] BuildTable()
    {
        var table = new uint[256];
        for (uint i = 0; i < 256; i++)
        {
            uint c = i;
            for (int k = 0; k < 8; k++)
            {
                c = (c & 1) != 0 ? (c >> 1) ^ 0x82F63B78u : c >> 1;
            }

            table[i] = c;
        }

        return table;
    }
}