libarchive/archive_crc32.h

/*-
 * Copyright (c) 2009 Joerg  Sonnenberger
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR(S) ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR(S) BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * $FreeBSD: head/lib/libarchive/archive_crc32.h 201102 2009-12-28 03:11:36Z kientzle $
 */

#ifndef __LIBARCHIVE_BUILD
#error This header is only to be used internally to libarchive.
#endif

/*
 * When zlib is unavailable, we should still be able to validate
 * uncompressed zip archives.  That requires us to be able to compute
 * the CRC32 check value.  This is a drop-in compatible replacement
 * for crc32() from zlib.  It's slower than the zlib implementation,
 * but still pretty fast: This runs about 300MB/s on my 3GHz P4
 * compared to about 800MB/s for the zlib implementation.
 */
static unsigned long
crc32(unsigned long crc, const void *_p, size_t len)
{
	unsigned long crc2, b, i;
	const unsigned char *p = _p;
	static volatile int crc_tbl_inited = 0;
	static unsigned long crc_tbl[256];

	if (!crc_tbl_inited) {
		for (b = 0; b < 256; ++b) {
			crc2 = b;
			for (i = 8; i > 0; --i) {
				if (crc2 & 1)
					crc2 = (crc2 >> 1) ^ 0xedb88320UL;
				else    
					crc2 = (crc2 >> 1);
			}
			crc_tbl[b] = crc2;
		}
		crc_tbl_inited = 1;
	}

	crc = crc ^ 0xffffffffUL;
	while (len--)
		crc = crc_tbl[(crc ^ *p++) & 0xff] ^ (crc >> 8);
	return (crc ^ 0xffffffffUL);
}