Solved by this code -> https://gist.github.com/Sbreitzke/b26107798eee74e39ff85800abf71fb1
I searched the web for a CRC 4 implementation in C# because I have to calculate a checksum by
Changing the numbers of the barcode into Hex representation, then to bytes and then to bits and then calculate a CRC4 checksum on the bit stream.
I already found this question from 8 years ago without an answer CRC-4 implementation in C#.
I tried changing the CRC 8 and 16 implementations to CRC 4 but they don't quite get the result I require.
0130E0928270FFFFFFF
should evaluate to 7
.
I found two C implementation but was unable to convert them to C#. For example this one:
short[] crc4_tab = {
0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,
0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,
};
/**
* crc4 - calculate the 4-bit crc of a value.
* @crc: starting crc4
* @x: value to checksum
* @bits: number of bits in @x to checksum
*
* Returns the crc4 value of @x, using polynomial 0b10111.
*
* The @x value is treated as left-aligned, and bits above @bits are ignored
* in the crc calculations.
*/
short crc4(uint8_t c, uint64_t x, int bits)
{
int i;
/* mask off anything above the top bit */
x &= (1ull << bits) -1;
/* Align to 4-bits */
bits = (bits + 3) & ~0x3;
/* Calculate crc4 over four-bit nibbles, starting at the MSbit */
for (i = bits - 4; i >= 0; i -= 4)
c = crc4_tab[c ^ ((x >> i) & 0xf)];
return c;
}
My current generation code (unit test) looks like this:
[TestMethod]
public void x()
{
var ordnungskennzeichen = 01;
var kundennummer = 51251496;
var einlieferungsbel = 9999;
var sendungsnr = 16777215;
var hex_ordnungskennzeichen = ordnungskennzeichen.ToString("x2");
var hex_kundennummer = kundennummer.ToString("x2");
var hex_einlieferungsbel = einlieferungsbel.ToString("x2");
var hex_sendungsnr = sendungsnr.ToString("x2");
var complete = hex_ordnungskennzeichen + hex_kundennummer + hex_einlieferungsbel + hex_sendungsnr;
var bytes = Encoding.ASCII.GetBytes(complete);
//var computeChecksum = crc4(???);
// Console.WriteLine(computeChecksum);
}
short[] crc4_tab = {
0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,
0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,
};
/**
* crc4 - calculate the 4-bit crc of a value.
* @crc: starting crc4
* @x: value to checksum
* @bits: number of bits in @x to checksum
*
* Returns the crc4 value of @x, using polynomial 0b10111.
*
* The @x value is treated as left-aligned, and bits above @bits are ignored
* in the crc calculations.
*/
short crc4(byte c, ulong x, int bits)
{
int i;
/* mask off anything above the top bit */
x &= ((ulong)1 << bits) -1;
/* Align to 4-bits */
bits = (bits + 3) & ~0x3;
/* Calculate crc4 over four-bit nibbles, starting at the MSbit */
for (i = bits - 4; i >= 0; i -= 4)
c = (byte) crc4_tab[c ^ ((x >> i) & 0xf)];
return c;
}
After further testing and communication with the Deutsche Post AG we made a correct implementation (for the purpose of Deutsche Post at least):
https://gist.github.com/Sbreitzke/b26107798eee74e39ff85800abf71fb1