path: root/templates/image_manip/png_crc.c

/* PNG Chunk layout:
   4-byte: length (of data) // Excluded from CRC
   4-byte: type (ASCII alphabetic)
   ?-byte: chunk data
   4-byte: crc (see below...)

   References:
   http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html
   http://www.libpng.org/pub/png/spec/1.2/PNG-CRCAppendix.html */


/* Table of CRCs of all 8-bit messages. */
unsigned long crc_table[256];
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++) {
      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). */
unsigned long update_crc(unsigned long crc, const unsigned char *buf, int len)
{
  unsigned long c = crc;
  int n;

  if (!crc_table_computed)
    make_crc_table();
  for (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 crc(const unsigned char *buf, int len)
{
  return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
}