namespace StitchATon2.Infra.Encoders;

public static class Crc32
{
    private static readonly Lazy<uint[]> LazyTable = new(GenerateTable);
    private static uint[] Table => LazyTable.Value;

    public static uint Compute(Span<byte> buffer, uint initial = 0xFFFFFFFF)
    {
        uint crc = initial;
        foreach (var b in buffer)
        {
            crc = Table[(crc ^ b) & 0xFF] ^ (crc >> 8);
        }
        return ~crc;
    }

    public static uint Compute(Stream stream, int count, uint initial = 0xFFFFFFFF)
    {
        uint crc = initial;
        while (count-- > 0)
        {
            crc = Table[(crc ^ stream.ReadByte()) & 0xFF] ^ (crc >> 8);
        }
        
        return ~crc;
    }

    private static uint[] GenerateTable()
    {
        const uint poly = 0xEDB88320;
        var table = new uint[256];
        for (uint i = 0; i < 256; i++)
        {
            uint c = i;
            for (int j = 0; j < 8; j++)
            {
                c = (c & 1) != 0 ? (poly ^ (c >> 1)) : (c >> 1);
            }
            table[i] = c;
        }
        return table;
    }
}