apps/codecs/libmusepack/crc32.c

   1 /*
   2 *  C Implementation: crc32
   3 *
   4 *  code from http://www.w3.org/TR/PNG/#D-CRCAppendix
   5 *
   6 */
   7 #include "internal.h"
   8
   9 /* Table of CRCs of all 8-bit messages. */
  10 static unsigned long crc_table[256];
  11
  12 /* Flag: has the table been computed? Initially false. */
  13 static int crc_table_computed = 0;
  14
  15 /* Make the table for a fast CRC. */
  16 static void make_crc_table(void)
  17 {
  18     unsigned long c;
  19     int n, k;
  20
  21     for (n = 0; n < 256; n++) {
  22         c = (unsigned long) n;
  23         for (k = 0; k < 8; k++) {
  24             if (c & 1)
  25                 c = 0xedb88320L ^ (c >> 1);
  26             else
  27                 c = c >> 1;
  28         }
  29         crc_table[n] = c;
  30     }
  31     crc_table_computed = 1;
  32 }
  33
  34
  35 /* Update a running CRC with the bytes buf[0..len-1]--the CRC
  36     should be initialized to all 1's, and the transmitted value
  37     is the 1's complement of the final running CRC (see the
  38     crc() routine below). */
  39
  40 static unsigned long update_crc(unsigned long crc, unsigned char *buf, int len)
  41 {
  42     unsigned long c = crc;
  43     int n;
  44
  45     if (!crc_table_computed)
  46         make_crc_table();
  47     for (n = 0; n < len; n++) {
  48         c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
  49     }
  50     return c;
  51 }
  52
  53 /* Return the CRC of the bytes buf[0..len-1]. */
  54 unsigned long mpc_crc32(unsigned char *buf, int len)
  55 {
  56     return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
  57 }