debug/programs/checksum.c

   1 #include <stdint.h>
   2
   3 // CRC code from http://www.hackersdelight.org/hdcodetxt/crc.c.txt
   4
   5 // Reverses (reflects) bits in a 32-bit word.
   6 unsigned reverse(unsigned x) {
   7    x = ((x & 0x55555555) <<  1) | ((x >>  1) & 0x55555555);
   8    x = ((x & 0x33333333) <<  2) | ((x >>  2) & 0x33333333);
   9    x = ((x & 0x0F0F0F0F) <<  4) | ((x >>  4) & 0x0F0F0F0F);
  10    x = (x << 24) | ((x & 0xFF00) << 8) |
  11        ((x >> 8) & 0xFF00) | (x >> 24);
  12    return x;
  13 }
  14
  15 // ----------------------------- crc32a --------------------------------
  16
  17 /* This is the basic CRC algorithm with no optimizations. It follows the
  18 logic circuit as closely as possible. */
  19
  20 unsigned int crc32a(uint8_t *message, unsigned int size) {
  21    int i, j;
  22    unsigned int byte, crc;
  23
  24    i = 0;
  25    crc = 0xFFFFFFFF;
  26    while (i < size) {
  27       byte = message[i];            // Get next byte.
  28       byte = reverse(byte);         // 32-bit reversal.
  29       for (j = 0; j <= 7; j++) {    // Do eight times.
  30          if ((int)(crc ^ byte) < 0)
  31               crc = (crc << 1) ^ 0x04C11DB7;
  32          else crc = crc << 1;
  33          byte = byte << 1;          // Ready next msg bit.
  34       }
  35       i = i + 1;
  36    }
  37    return reverse(~crc);
  38 }