Contents

// from http://www.w3.org/TR/PNG/#D-CRCAppendix

#include "crc32.h"

/* Table of CRCs of all 8-bit messages. */
static unsigned long crc_table[256];

/* Flag: has the table been computed? Initially false. */
static int crc_table_computed = 0;

/* Make the table for a fast CRC. */
static void make_crc_table(void) {
  unsigned long c;
  for (int n = 0; n < 256; n++) {
    c = (unsigned long) n;
    for (int k = 0; k < 8; k++) {
      if (c & 1) c = 0xedb88320L ^ (c >> 1);
      else c = c >> 1;
    }
    crc_table[n] = c;
  }
  crc_table_computed = 1;
}

/* Update a running CRC with the bytes buf[0..len-1]--the CRC
 should be initialized to all 1's, and the transmitted value
 is the 1's complement of the final running CRC (see the
 crc() routine below). */

static unsigned long update_crc(unsigned long crc, unsigned char *buf, int len) {
  unsigned long c = crc;
  if (! crc_table_computed) make_crc_table();
  for (int n = 0; n < len; n++) c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
  return c;
}

/* Return the CRC of the bytes buf[0..len-1]. */
unsigned long crc32(unsigned char *buf, int len) {
  return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
}

Version data entries

2 entries across 2 versions & 1 rubygems

Version	Path
national_grid-0.2.0	ext/ostn02c/OSTN02/crc32.c
national_grid-0.1.2	ext/ostn02c/OSTN02/crc32.c