goodmask.c

This a good VPN source

/*
 * minor utilities for subnet-mask manipulation
 * Copyright (C) 1998, 1999  Henry Spencer.
 * 
 * This library is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Library General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or (at your
 * option) any later version.  See <http://www.fsf.org/copyleft/lgpl.txt>.
 * 
 * This library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
 * License for more details.
 *
 * RCSID $Id: goodmask.c,v 1.12 2004/07/10 07:43:47 mcr Exp $
 */
#include "openswan.h"

#ifndef ABITS
#define	ABITS	32	/* bits in an IPv4 address */
#endif

/*
 - goodmask - is this a good (^1*0*$) subnet mask?
 * You are not expected to understand this.  See Henry S. Warren Jr, 
 * "Functions realizable with word-parallel logical and two's-complement
 * addition instructions", CACM 20.6 (June 1977), p.439.
 */
int				/* predicate */
goodmask(mask)
struct in_addr mask;
{
	unsigned long x = ntohl(mask.s_addr);
	/* clear rightmost contiguous string of 1-bits */
#	define	CRCS1B(x)	(((x|(x-1))+1)&x)
#	define	TOPBIT		(1UL << 31)

	/* either zero, or has one string of 1-bits which is left-justified */
	if (x == 0 || (CRCS1B(x) == 0 && (x&TOPBIT)))
		return 1;
	return 0;
}

/*
 - masktobits - how many bits in this mask?
 * The algorithm is essentially a binary search, but highly optimized
 * for this particular task.
 */
int				/* -1 means !goodmask() */
masktobits(mask)
struct in_addr mask;
{
	unsigned long m = ntohl(mask.s_addr);
	int masklen;

	if (!goodmask(mask))
		return -1;

	if (m&0x00000001UL)
		return 32;
	masklen = 0;
	if (m&(0x0000ffffUL<<1)) {	/* <<1 for 1-origin numbering */
		masklen |= 0x10;
		m <<= 16;
	}
	if (m&(0x00ff0000UL<<1)) {
		masklen |= 0x08;
		m <<= 8;
	}
	if (m&(0x0f000000UL<<1)) {
		masklen |= 0x04;
		m <<= 4;
	}
	if (m&(0x30000000UL<<1)) {
		masklen |= 0x02;
		m <<= 2;
	}
	if (m&(0x40000000UL<<1))
		masklen |= 0x01;

	return masklen;
}

/*
 - bitstomask - return a mask with this many high bits on
 */
struct in_addr
bitstomask(n)
int n;
{
	struct in_addr result;

	if (n > 0 && n <= ABITS)
		result.s_addr = htonl(~((1UL << (ABITS - n)) - 1));
	else if (n == 0)
		result.s_addr = 0;
	else
		result.s_addr = 0;	/* best error report we can do */
	return result;
}