PNG (Portable Network Graphics) Specification, Tenth Draft

Revision date: 5 May, 1995

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12. Appendix: Sample CRC Code

The following sample code represents a practical implementation of the CRC (Cyclic Redundancy Check) employed in PNG chunks.

The sample code provided is in the C programming language. (See also ISO 3309 and ITU-T V.42 for a formal specification.)

/* table of crc's of all 8-bit messages */
unsigned long crc_table[256];

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

/* make the table for a fast crc */
void make_crc_table(void)
{
  unsigned long c;
  int n, k;
 
  for (n = 0; n < 256; n++) {
    c = (unsigned long) n;
    for (k = 0; k < 8; k++)
      c = (c & 1) ? (0xedb88320L ^ (c >> 1)) : (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)). */

unsigned long update_crc(unsigned long crc, unsigned char *buf, int len)
{
  unsigned long c = crc;
  unsigned char *p = buf;
  int n = len;

  if (!crc_table_computed)
    make_crc_table();
  if (n > 0) do {
    c = crc_table[(c ^ (*p++)) & 0xff] ^ (c >> 8);
  } while (--n);
  return c;
}

/* return the crc of the bytes buf[0..len-1] */
unsigned long crc(unsigned char *buf, int len)
{
  if (!crc_table_computed)
    make_crc_table();
  return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
}

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