checksum code in c#

Question

I need to rewrite the code below in c#. I think the code is written in c but I only understand a few lines. Can some help me explain the code? byte crc8(byte *str int len) { int i f; byte data; byte crc; crc = 0 ; while (len--) { data = *str++; for = (i = 0; i { f = 1 & (data ^ crc); crc >>= 1; data >>= 1 ; if (f) { crc ^= 0x8c; } } } return crc; }

Vulpes · Answer

Yes it looks like C code with a typedef for byte as unsigned char would normally be used for that particular type. Anyway this C program works fine and outputs 22: #include #include typedef unsigned char byte; byte crc8(byte *str int len) { int i f; byte data; byte crc; crc = 0 ; while (len--) { data = *str++; for(i = 0; i { f = 1 & (data ^ crc); crc >>= 1; data >>= 1; if (f) { crc ^= 0x8c; } } } return crc; } int main() { byte *str = The quick brown fox jumps over the lazy dog; size_t len = strlen(str); byte crc = crc8(str len); printf(%d crc); return 0; } You can actually convert this to C# with very little change if you use 'unsafe' code and compile with the /unsafe switch: using System; using System.Text; unsafe class Program { static byte crc8(byte *str int len) { int i f; byte data; byte crc; crc = 0 ; while (len-- != 0) { data = *str++; for(i = 0; i { f = 1 & (data ^ crc); crc >>= 1; data >>= 1; if (f != 0) { crc ^= 0x8c; } } } return crc; } static void Main() { string str = The quick brown fox jumps over the lazy dog; byte[] bytes = Encoding.ASCII.GetBytes(str); int len = bytes.Length; fixed(byte* pb = bytes) { byte crc = crc8(pb len); Console.WriteLine(crc); } } } This again outputs 22. However if you'd prefer to use 'safe' code you can use this instead - execution speed will probably be a little slower: using System; using System.Text; class Program { static byte crc8(byte[] str) // no need to pass length in 'safe' code { int len = str.Length; int i f; byte data; byte crc; crc = 0 ; int j = 0; while (len-- != 0) { data = str[j++]; for(i = 0; i { f = 1 & (data ^ crc); crc >>= 1; data >>= 1; if (f != 0) { crc ^= 0x8c; } } } return crc; } static void Main() { string s = The quick brown fox jumps over the lazy dog; byte[] str = Encoding.ASCII.GetBytes(s); byte crc = crc8(str); Console.WriteLine(crc); } } The programs are of course computing a 'cyclic redundancy check' which is often used for checksums and the like. If you need to understand the mathematics of how this works I'd check out this Wikipedia article and follow the associated links: http://en.wikipedia.org/wiki/Cyclic_redundancy_check

Tamas Szigeti · Answer

Thanks a lot! I really appreciate your help