Isaac Rebirth Save Checksum

private uint CalcChecksum(byte[] data)
        {
            byte[] save = new byte[0x4EF];
            Array.Copy(data, save, save.Length); //steam cloud saves are bigger than I care about
            uint seed = BitConverter.ToUInt32(save, 0x4E7);
            uint desired = BitConverter.ToUInt32(save, 0x4EB);
            uint chksum = 0;
            int ofs = 0x14;
            chksum = seed + 2137939259;
            uint magic = BitConverter.ToUInt32(save, 0x10);
            uint[] SectData = new uint[3];
            int[] entrylens = new int[] { 1, 4, 4, 1, 1, 1, 1, 4, 4 };
            for (int i = 0; i < 9; i++)
            {
                for (int j = 0; j < 3; j++)
                {
                    SectData[j] = BitConverter.ToUInt32(save, ofs);
                    chksum = Update(SectData[j], chksum);
                }
                for (int j = 0; j < SectData[2]; j++)
                {
                    uint val = 0;
                    if (entrylens[i] == 1)
                    {
                        val = save[ofs];
                    }
                    else if (entrylens[i] == 4)
                    {
                        val = BitConverter.ToUInt32(save, ofs);
                    }
                    chksum = Update(val, chksum);
                    ofs += entrylens[i];
                }

            }
            chksum = Update(magic, chksum);
            chksum ^= 0x96696996;
            return chksum;
        }

        private uint Update(uint val, uint chksum)
        {
            return (val + ((chksum << 31) + (chksum >> 1)));
        }