I've been searching the net for a C# implementation of the 4-bit cyclic redundancy check (CRC-4-ITU) but so far I've been unsuccessful.
Is there anyone who's able to give me a reference implementation of CRC-4-ITU? Preferrably with the standard polynomial if there is a standard polynomial (I've read the spec pointed to by wikipedia as the CRC4 spec without finding a definition of the polynomial).
I'd also really appreciate some sort of test suite or test data to verify a CRC4 implementation.
Thanks!
The Cyclic Redundancy Check article at Wikipedia says the polynomial is x^4 + x + 1. There is also a pretty good description of how the checksum is computed.
Here is an algorithm for CRC16. I know it's not what you asked for, but it should be relatively straightforward to adapt it for 4 bits.
public ushort calculate(byte[] bytes) { int crc = 0xFFFF; // initial value // loop, calculating CRC for each byte of the string for (int byteIndex = 0; byteIndex < bytes.Length; byteIndex++) { ushort bit = 0x80; // initialize bit currently being tested for (int bitIndex = 0; bitIndex < 8; bitIndex++) { bool xorFlag = ((crc & 0x8000) == 0x8000); crc <<= 1; if (((bytes[byteIndex] & bit) ^ (ushort)0xff) != (ushort)0xff) { crc = crc + 1; } if (xorFlag) { crc = crc ^ 0x1021; } bit >>= 1; } } return (ushort)crc; }
http://www.experts-exchange.com/Programming/Languages/C_Sharp/Q_24775723.html
Also, there is this guide to computing checksums:
http://www.ross.net/crc/download/crc_v3.txt
"Everything you wanted to know about CRC algorithms, but were afraid to ask for fear that errors in your understanding might be detected."
