busybox/libbb/crc32.c

/* vi: set sw=4 ts=4: */
/*
 * CRC32 table fill function
 * Copyright (C) 2006 by Rob Sullivan <cogito.ergo.cogito@gmail.com>
 * (I can't really claim much credit however, as the algorithm is
 * very well-known)
 *
 * The following function creates a CRC32 table depending on whether
 * a big-endian (0x04c11db7) or little-endian (0xedb88320) CRC32 is
 * required. Admittedly, there are other CRC32 polynomials floating
 * around, but Busybox doesn't use them.
 *
 * endian = 1: big-endian
 * endian = 0: little-endian
 */

#include "libbb.h"

uint32_t *crc32_filltable(int endian)
{

	uint32_t *crc_table = xmalloc(256 * sizeof(uint32_t));
	uint32_t polynomial = endian ? 0x04c11db7 : 0xedb88320;
	uint32_t c;
	int i, j;

	for (i = 0; i < 256; i++) {
		c = endian ? (i << 24) : i;
		for (j = 8; j; j--) {
			if (endian)
				c = (c&0x80000000) ? ((c << 1) ^ polynomial) : (c << 1);
			else
				c = (c&1) ? ((c >> 1) ^ polynomial) : (c >> 1);
		}
		*crc_table++ = c;
	}

	return crc_table - 256;
}