using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ArdupilotMega
{
    class MavlinkCRC
    {
        const int X25_INIT_CRC = 0xffff;
        const int X25_VALIDATE_CRC = 0xf0b8;

        public static ushort crc_accumulate(byte b, ushort crc)
        {
            unchecked
            {
                byte ch = (byte)(b ^ (byte)(crc & 0x00ff));
                ch = (byte)(ch ^ (ch << 4));
                return (ushort)((crc >> 8) ^ (ch << 8) ^ (ch << 3) ^ (ch >> 4));
            }
        }

        public static ushort crc_calculate(byte[] pBuffer, int length)
        {
            if (length < 1)
            {
                return 0xffff;
            }
            // For a "message" of length bytes contained in the unsigned char array
            // pointed to by pBuffer, calculate the CRC
            // crcCalculate(unsigned char* pBuffer, int length, unsigned short* checkConst) < not needed

            ushort crcTmp;
            int i;

            crcTmp = X25_INIT_CRC;

            for (i = 1; i < length; i++) // skips header U
            {
                crcTmp = crc_accumulate(pBuffer[i], crcTmp);
                //Console.WriteLine(crcTmp + " " + pBuffer[i] + " " + length);
            }

            return (crcTmp);
        }

    }
}
