crypto/bn/bn_sub.c

/* crypto/bn/bn_sub.c */
/* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
 * All rights reserved.
 *
 * This package is an SSL implementation written
 * by Eric Young (eay@cryptsoft.com).
 * The implementation was written so as to conform with Netscapes SSL.
 * 
 * This library is free for commercial and non-commercial use as long as
 * the following conditions are aheared to.  The following conditions
 * apply to all code found in this distribution, be it the RC4, RSA,
 * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
 * included with this distribution is covered by the same copyright terms
 * except that the holder is Tim Hudson (tjh@cryptsoft.com).
 * 
 * Copyright remains Eric Young's, and as such any Copyright notices in
 * the code are not to be removed.
 * If this package is used in a product, Eric Young should be given attribution
 * as the author of the parts of the library used.
 * This can be in the form of a textual message at program startup or
 * in documentation (online or textual) provided with the package.
 * 
 * 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 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.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *    "This product includes cryptographic software written by
 *     Eric Young (eay@cryptsoft.com)"
 *    The word 'cryptographic' can be left out if the rouines from the library
 *    being used are not cryptographic related :-).
 * 4. If you include any Windows specific code (or a derivative thereof) from 
 *    the apps directory (application code) you must include an acknowledgement:
 *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
 * 
 * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``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 OR CONTRIBUTORS 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.
 * 
 * The licence and distribution terms for any publically available version or
 * derivative of this code cannot be changed.  i.e. this code cannot simply be
 * copied and put under another distribution licence
 * [including the GNU Public Licence.]
 */

#include <stdio.h>
#include "cryptlib.h"
#include "bn_lcl.h"

/* unsigned subtraction of b from a, a must be larger than b. */
void bn_qsub(r, a, b)
BIGNUM *r;
BIGNUM *a;
BIGNUM *b;
	{
	int max,min;
	register BN_ULONG t1,t2,*ap,*bp,*rp;
	int i,carry;
#if defined(IRIX_CC_BUG) && !defined(LINT)
	int dummy;
#endif

	max=a->top;
	min=b->top;
	ap=a->d;
	bp=b->d;
	rp=r->d;

	carry=0;
	for (i=0; i<min; i++)
		{
		t1= *(ap++);
		t2= *(bp++);
		if (carry)
			{
			carry=(t1 <= t2);
			t1=(t1-t2-1)&BN_MASK2;
			}
		else
			{
			carry=(t1 < t2);
			t1=(t1-t2)&BN_MASK2;
			}
#if defined(IRIX_CC_BUG) && !defined(LINT)
		dummy=t1;
#endif
		*(rp++)=t1&BN_MASK2;
		}
	if (carry) /* subtracted */
		{
		while (i < max)
			{
			i++;
			t1= *(ap++);
			t2=(t1-1)&BN_MASK2;
			*(rp++)=t2;
			if (t1 > t2) break;
			}
		}
#if 0
	memcpy(rp,ap,sizeof(*rp)*(max-i));
#else
	for (; i<max; i++)
		*(rp++)= *(ap++);
#endif

	r->top=max;
	bn_fix_top(r);
	}

int BN_sub(r, a, b)
BIGNUM *r;
BIGNUM *a;
BIGNUM *b;
	{
	int max,i;
	int add=0,neg=0;
	BIGNUM *tmp;

	/*  a -  b	a-b
	 *  a - -b	a+b
	 * -a -  b	-(a+b)
	 * -a - -b	b-a
	 */
	if (a->neg)
		{
		if (b->neg)
			{ tmp=a; a=b; b=tmp; }
		else
			{ add=1; neg=1; }
		}
	else
		{
		if (b->neg) { add=1; neg=0; }
		}

	if (add)
		{
		/* As a fast max size, do a a->top | b->top */
		i=(a->top | b->top)+1;
	        if (bn_wexpand(r,i) == NULL)
			return(0);
		if (i)
			bn_qadd(r,a,b);
		else
			bn_qadd(r,b,a);
		r->neg=neg;
		return(1);
		}

	/* We are actually doing a - b :-) */

	max=(a->top > b->top)?a->top:b->top;
	if (bn_wexpand(r,max) == NULL) return(0);
	if (BN_ucmp(a,b) < 0)
		{
		bn_qsub(r,b,a);
		r->neg=1;
		}
	else
		{
		bn_qsub(r,a,b);
		r->neg=0;
		}
	return(1);
	}