crcCode.h

/**
 * CRC functions from PNG Spec:
 * 		http://www.w3.org/TR/PNG-CRCAppendix.html
*/

#ifndef CRCCODE_H
#define CRCCODE_H

/* Table of CRCs 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;

	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)). */
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 crc(unsigned char *buf, int len) {
	return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
}

#endif