tommath/bn_s_mp_sub.c

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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is library that provides for multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library is designed directly after the MPI library by
* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* The library is free for all purposes without any express
* guarantee it works.
*
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* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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*/
#include <tommath.h>
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/* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
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int
s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
{
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int olduse, res, min, max;
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/* find sizes */
min = b->used;
max = a->used;
/* init result */
if (c->alloc < max) {
if ((res = mp_grow (c, max)) != MP_OKAY) {
return res;
}
}
olduse = c->used;
c->used = max;
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{
register mp_digit u, *tmpa, *tmpb, *tmpc;
register int i;
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/* alias for digit pointers */
tmpa = a->dp;
tmpb = b->dp;
tmpc = c->dp;
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/* set carry to zero */
u = 0;
for (i = 0; i < min; i++) {
/* T[i] = A[i] - B[i] - U */
*tmpc = *tmpa++ - *tmpb++ - u;
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/* U = carry bit of T[i]
* Note this saves performing an AND operation since
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* if a carry does occur it will propagate all the way to the
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* MSB. As a result a single shift is enough to get the carry
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*/
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u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
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/* Clear carry from T[i] */
*tmpc++ &= MP_MASK;
}
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/* now copy higher words if any, e.g. if A has more digits than B */
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for (; i < max; i++) {
/* T[i] = A[i] - U */
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*tmpc = *tmpa++ - u;
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/* U = carry bit of T[i] */
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u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
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/* Clear carry from T[i] */
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*tmpc++ &= MP_MASK;
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}
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/* clear digits above used (since we may not have grown result above) */
for (i = c->used; i < olduse; i++) {
*tmpc++ = 0;
}
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}
mp_clamp (c);
return MP_OKAY;
}
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