// Common/CRC.cs

namespace NewLife.Compression
{
    class CRC
    {
        public static readonly uint[] Table;

        static CRC()
        {
            Table = new uint[256];
            const uint kPoly = 0xEDB88320;
            for (uint i = 0; i < 256; i++)
            {
                uint r = i;
                for (int j = 0; j < 8; j++)
                    if ((r & 1) != 0)
                        r = (r >> 1) ^ kPoly;
                    else
                        r >>= 1;
                Table[i] = r;
            }
        }

        uint _value = 0xFFFFFFFF;

        public void Init() { _value = 0xFFFFFFFF; }

        public void UpdateByte(byte b)
        {
            _value = Table[(((byte)(_value)) ^ b)] ^ (_value >> 8);
        }

        public void Update(byte[] data, uint offset, uint size)
        {
            for (uint i = 0; i < size; i++)
                _value = Table[(((byte)(_value)) ^ data[offset + i])] ^ (_value >> 8);
        }

        public uint GetDigest() { return _value ^ 0xFFFFFFFF; }

        static uint CalculateDigest(byte[] data, uint offset, uint size)
        {
            var crc = new CRC();
            // crc.Init();
            crc.Update(data, offset, size);
            return crc.GetDigest();
        }

        static bool VerifyDigest(uint digest, byte[] data, uint offset, uint size)
        {
            return (CalculateDigest(data, offset, size) == digest);
        }
    }
}