I have a problem with a certain CRC method when trying to combine CRC's.

I have been using the combine CRC method and even adapted it a while ago to work with CRC16, etc. and I (hope?) understand how the filling of 0's work according to the answer here https://stackoverflow.com/a/23126768/5495036 with zlib's code

Thing is, I've been rapping my brain on how to get an operator that I can use as I am also working with a non standard CRC calculation. The original CRC calculation is based on the CRC32 0xEDB88320 polynomial for the lookup table but the calculation itself was broken, so instead of 256 byte lookup table looking like 0x00000000,0x77073096,0xee0e612c,0x990951ba,0x076dc419,0x706af48f,0xe963a535,... it now looks like this 0x00000000,0x96000000,0x30960000,0x07309600,0x77073096,0x2c770730,0x612c7707,... and the combine CRC uses the operator to be able to zero out the rest of the bits.

I can't change the calculation unfortunately so that idea is out :P Any ideas?

**EDIT:**

The calculation is standard, it just uses a table that has been built using the standard polynomial of 0xEDB88320 but building it incorrectly. Table is still 256 ints. Starting CRC of 0xFFFFFFFF

Complete class code

```
public static byte[] ByteLookupTable =
{
0x00,0x00,0x00,0x00,0x96,0x30,0x07,0x77,0x2C,0x61,0x0E,0xEE,0xBA,0x51,
0x09,0x99,0x19,0xC4,0x6D,0x07,0x8F,0xF4,0x6A,0x70,0x35,0xA5,0x63,
0xE9,0xA3,0x95,0x64,0x9E,0x32,0x88,0xDB,0x0E,0xA4,0xB8,0xDC,0x79,0x1E,
0xE9,0xD5,0xE0,0x88,0xD9,0xD2,0x97,0x2B,0x4C,0xB6,0x09,0xBD,0x7C,
0xB1,0x7E,0x07,0x2D,0xB8,0xE7,0x91,0x1D,0xBF,0x90,0x64,0x10,0xB7,0x1D,
0xF2,0x20,0xB0,0x6A,0x48,0x71,0xB9,0xF3,0xDE,0x41,0xBE,0x84,0x7D,
0xD4,0xDA,0x1A,0xEB,0xE4,0xDD,0x6D,0x51,0xB5,0xD4,0xF4,0xC7,0x85,0xD3,
0x83,0x56,0x98,0x6C,0x13,0xC0,0xA8,0x6B,0x64,0x7A,0xF9,0x62,0xFD,
0xEC,0xC9,0x65,0x8A,0x4F,0x5C,0x01,0x14,0xD9,0x6C,0x06,0x63,0x63,0x3D,
0x0F,0xFA,0xF5,0x0D,0x08,0x8D,0xC8,0x20,0x6E,0x3B,0x5E,0x10,0x69,
0x4C,0xE4,0x41,0x60,0xD5,0x72,0x71,0x67,0xA2,0xD1,0xE4,0x03,0x3C,0x47,
0xD4,0x04,0x4B,0xFD,0x85,0x0D,0xD2,0x6B,0xB5,0x0A,0xA5,0xFA,0xA8,
0xB5,0x35,0x6C,0x98,0xB2,0x42,0xD6,0xC9,0xBB,0xDB,0x40,0xF9,0xBC,0xAC,
0xE3,0x6C,0xD8,0x32,0x75,0x5C,0xDF,0x45,0xCF,0x0D,0xD6,0xDC,0x59,
0x3D,0xD1,0xAB,0xAC,0x30,0xD9,0x26,0x3A,0x00,0xDE,0x51,0x80,0x51,0xD7,
0xC8,0x16,0x61,0xD0,0xBF,0xB5,0xF4,0xB4,0x21,0x23,0xC4,0xB3,0x56,
0x99,0x95,0xBA,0xCF,0x0F,0xA5,0xBD,0xB8,0x9E,0xB8,0x02,0x28,0x08,0x88,
0x05,0x5F,0xB2,0xD9,0x0C,0xC6,0x24,0xE9,0x0B,0xB1,0x87,0x7C,0x6F,
0x2F,0x11,0x4C,0x68,0x58,0xAB,0x1D,0x61,0xC1,0x3D,0x2D,0x66,0xB6,0x90,
0x41,0xDC,0x76,0x06,0x71,0xDB,0x01,0xBC,0x20,0xD2,0x98,0x2A,0x10,
0xD5,0xEF,0x89,0x85,0xB1,0x71,0x1F,0xB5,0xB6,0x06,0xA5,0xE4,0xBF,0x9F,
0x33,0xD4,0xB8,0xE8,0xA2,0xC9,0x07,0x78,0x34,0xF9,0x00,0x0F,0x8E,
0xA8,0x09,0x96,0x18,0x98,0x0E,0xE1,0xBB,0x0D,0x6A,0x7F,0x2D,0x3D,0x6D,
0x08,0x97,0x6C,0x64,0x91,0x01,0x5C,0x63,0xE6,0xF4,0x51,0x6B,0x6B,
0x62,0x61,0x6C,0x1C,0xD8,0x30,0x65,0x85,0x4E,0x00,0x62,0xF2,0xED,0x95,
0x06,0x6C,0x7B,0xA5,0x01,0x1B,0xC1,0xF4,0x08,0x82,0x57,0xC4,0x0F,
0xF5,0xC6,0xD9,0xB0,0x65,0x50,0xE9,0xB7,0x12,0xEA,0xB8,0xBE,0x8B,0x7C,
0x88,0xB9,0xFC,0xDF,0x1D,0xDD,0x62,0x49,0x2D,0xDA,0x15,0xF3,0x7C,
0xD3,0x8C,0x65,0x4C,0xD4,0xFB,0x58,0x61,0xB2,0x4D,0xCE,0x51,0xB5,0x3A,
0x74,0x00,0xBC,0xA3,0xE2,0x30,0xBB,0xD4,0x41,0xA5,0xDF,0x4A,0xD7,
0x95,0xD8,0x3D,0x6D,0xC4,0xD1,0xA4,0xFB,0xF4,0xD6,0xD3,0x6A,0xE9,0x69,
0x43,0xFC,0xD9,0x6E,0x34,0x46,0x88,0x67,0xAD,0xD0,0xB8,0x60,0xDA,
0x73,0x2D,0x04,0x44,0xE5,0x1D,0x03,0x33,0x5F,0x4C,0x0A,0xAA,0xC9,0x7C,
0x0D,0xDD,0x3C,0x71,0x05,0x50,0xAA,0x41,0x02,0x27,0x10,0x10,0x0B,
0xBE,0x86,0x20,0x0C,0xC9,0x25,0xB5,0x68,0x57,0xB3,0x85,0x6F,0x20,0x09,
0xD4,0x66,0xB9,0x9F,0xE4,0x61,0xCE,0x0E,0xF9,0xDE,0x5E,0x98,0xC9,
0xD9,0x29,0x22,0x98,0xD0,0xB0,0xB4,0xA8,0xD7,0xC7,0x17,0x3D,0xB3,0x59,
0x81,0x0D,0xB4,0x2E,0x3B,0x5C,0xBD,0xB7,0xAD,0x6C,0xBA,0xC0,0x20,
0x83,0xB8,0xED,0xB6,0xB3,0xBF,0x9A,0x0C,0xE2,0xB6,0x03,0x9A,0xD2,0xB1,
0x74,0x39,0x47,0xD5,0xEA,0xAF,0x77,0xD2,0x9D,0x15,0x26,0xDB,0x04,
0x83,0x16,0xDC,0x73,0x12,0x0B,0x63,0xE3,0x84,0x3B,0x64,0x94,0x3E,0x6A,
0x6D,0x0D,0xA8,0x5A,0x6A,0x7A,0x0B,0xCF,0x0E,0xE4,0x9D,0xFF,0x09,
0x93,0x27,0xAE,0x00,0x0A,0xB1,0x9E,0x07,0x7D,0x44,0x93,0x0F,0xF0,0xD2,
0xA3,0x08,0x87,0x68,0xF2,0x01,0x1E,0xFE,0xC2,0x06,0x69,0x5D,0x57,
0x62,0xF7,0xCB,0x67,0x65,0x80,0x71,0x36,0x6C,0x19,0xE7,0x06,0x6B,0x6E,
0x76,0x1B,0xD4,0xFE,0xE0,0x2B,0xD3,0x89,0x5A,0x7A,0xDA,0x10,0xCC,
0x4A,0xDD,0x67,0x6F,0xDF,0xB9,0xF9,0xF9,0xEF,0xBE,0x8E,0x43,0xBE,0xB7,
0x17,0xD5,0x8E,0xB0,0x60,0xE8,0xA3,0xD6,0xD6,0x7E,0x93,0xD1,0xA1,
0xC4,0xC2,0xD8,0x38,0x52,0xF2,0xDF,0x4F,0xF1,0x67,0xBB,0xD1,0x67,0x57,
0xBC,0xA6,0xDD,0x06,0xB5,0x3F,0x4B,0x36,0xB2,0x48,0xDA,0x2B,0x0D,
0xD8,0x4C,0x1B,0x0A,0xAF,0xF6,0x4A,0x03,0x36,0x60,0x7A,0x04,0x41,0xC3,
0xEF,0x60,0xDF,0x55,0xDF,0x67,0xA8,0xEF,0x8E,0x6E,0x31,0x79,0xBE,
0x69,0x46,0x8C,0xB3,0x61,0xCB,0x1A,0x83,0x66,0xBC,0xA0,0xD2,0x6F,0x25,
0x36,0xE2,0x68,0x52,0x95,0x77,0x0C,0xCC,0x03,0x47,0x0B,0xBB,0xB9,
0x16,0x02,0x22,0x2F,0x26,0x05,0x55,0xBE,0x3B,0xBA,0xC5,0x28,0x0B,0xBD,
0xB2,0x92,0x5A,0xB4,0x2B,0x04,0x6A,0xB3,0x5C,0xA7,0xFF,0xD7,0xC2,
0x31,0xCF,0xD0,0xB5,0x8B,0x9E,0xD9,0x2C,0x1D,0xAE,0xDE,0x5B,0xB0,0xC2,
0x64,0x9B,0x26,0xF2,0x63,0xEC,0x9C,0xA3,0x6A,0x75,0x0A,0x93,0x6D,
0x02,0xA9,0x06,0x09,0x9C,0x3F,0x36,0x0E,0xEB,0x85,0x67,0x07,0x72,0x13,
0x57,0x00,0x05,0x82,0x4A,0xBF,0x95,0x14,0x7A,0xB8,0xE2,0xAE,0x2B,
0xB1,0x7B,0x38,0x1B,0xB6,0x0C,0x9B,0x8E,0xD2,0x92,0x0D,0xBE,0xD5,0xE5,
0xB7,0xEF,0xDC,0x7C,0x21,0xDF,0xDB,0x0B,0xD4,0xD2,0xD3,0x86,0x42,
0xE2,0xD4,0xF1,0xF8,0xB3,0xDD,0x68,0x6E,0x83,0xDA,0x1F,0xCD,0x16,0xBE,
0x81,0x5B,0x26,0xB9,0xF6,0xE1,0x77,0xB0,0x6F,0x77,0x47,0xB7,0x18,
0xE6,0x5A,0x08,0x88,0x70,0x6A,0x0F,0xFF,0xCA,0x3B,0x06,0x66,0x5C,0x0B,
0x01,0x11,0xFF,0x9E,0x65,0x8F,0x69,0xAE,0x62,0xF8,0xD3,0xFF,0x6B,
0x61,0x45,0xCF,0x6C,0x16,0x78,0xE2,0x0A,0xA0,0xEE,0xD2,0x0D,0xD7,0x54,
0x83,0x04,0x4E,0xC2,0xB3,0x03,0x39,0x61,0x26,0x67,0xA7,0xF7,0x16,
0x60,0xD0,0x4D,0x47,0x69,0x49,0xDB,0x77,0x6E,0x3E,0x4A,0x6A,0xD1,0xAE,
0xDC,0x5A,0xD6,0xD9,0x66,0x0B,0xDF,0x40,0xF0,0x3B,0xD8,0x37,0x53,
0xAE,0xBC,0xA9,0xC5,0x9E,0xBB,0xDE,0x7F,0xCF,0xB2,0x47,0xE9,0xFF,0xB5,
0x30,0x1C,0xF2,0xBD,0xBD,0x8A,0xC2,0xBA,0xCA,0x30,0x93,0xB3,0x53,
0xA6,0xA3,0xB4,0x24,0x05,0x36,0xD0,0xBA,0x93,0x06,0xD7,0xCD,0x29,0x57,
0xDE,0x54,0xBF,0x67,0xD9,0x23,0x2E,0x7A,0x66,0xB3,0xB8,0x4A,0x61,
0xC4,0x02,0x1B,0x68,0x5D,0x94,0x2B,0x6F,0x2A,0x37,0xBE,0x0B,0xB4,0xA1,
0x8E,0x0C,0xC3,0x1B,0xDF,0x05,0x5A,0x8D,0xEF,0x02,0x2D,0xC3,0x8D,0x40,0x00
};
protected void BuildLookupTable()
{
if (LookupTable == null)
{
LookupTable = new uint[256];
for (int i = 0; i < LookupTable.Length; i++)
{
LookupTable[i] = BitConverter.ToUInt32(ByteLookupTable, i);
}
}
}
protected override uint CalculateBuffer(byte[] buffer, uint crc, int startPos, int endPos)
{
for (int i = 0; i < endPos; i++)
{
crc = LookupTable[(crc ^ buffer[i]) & 0xff] ^ (crc >> 8);
}
return crc;
}
```

The lookup table constants are correct for that polynomial, but apparently the conversion done by `BuildLookupTable()`

is totally messed up. It would have been easy to rewrite the table to avoid needing `BuildLookupTable()`

.

What follows is from the original answer, which assumed that the table was converted correctly. Which it isn't.

As it is, this isn't a CRC, so the combination approach for a CRC does not apply here.

The one thing missing from your definition is what the initial value for the CRC is, and possibly if there is an exclusive-or done on the final CRC. That polynomial is the same as used by zlib, PKZIP, etc., where the initial CRC value is `0xffffffff`

and the final exclusive-or is with `0xffffffff`

. That CRC is referred to as CRC-32/ISO-HDLC.

Whatever your initial and the final exclusive-or values are, if they are equal, then you can use the `crc32_combine()`

function from zlib as is. If they are not, you can still use `crc32_combine()`

, but you need to exclusive-or the input and output CRC values of that function with the exclusive-or of the initial and final exclusive-or values.

User contributions licensed under CC BY-SA 3.0