94 lines
2.0 KiB
C
94 lines
2.0 KiB
C
#include <tommath.h>
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#ifdef BN_MP_SUB_D_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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*/
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/* single digit subtraction */
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int
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mp_sub_d (mp_int * a, mp_digit b, mp_int * c)
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{
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mp_digit *tmpa, *tmpc, mu;
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int res, ix, oldused;
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/* grow c as required */
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if (c->alloc < a->used + 1) {
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if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
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return res;
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}
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}
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/* if a is negative just do an unsigned
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* addition [with fudged signs]
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*/
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if (a->sign == MP_NEG) {
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a->sign = MP_ZPOS;
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res = mp_add_d(a, b, c);
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a->sign = c->sign = MP_NEG;
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/* clamp */
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mp_clamp(c);
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return res;
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}
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/* setup regs */
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oldused = c->used;
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tmpa = a->dp;
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tmpc = c->dp;
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/* if a <= b simply fix the single digit */
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if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) {
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if (a->used == 1) {
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*tmpc++ = b - *tmpa;
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} else {
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*tmpc++ = b;
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}
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ix = 1;
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/* negative/1digit */
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c->sign = MP_NEG;
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c->used = 1;
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} else {
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/* positive/size */
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c->sign = MP_ZPOS;
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c->used = a->used;
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/* subtract first digit */
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*tmpc = *tmpa++ - b;
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mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
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*tmpc++ &= MP_MASK;
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/* handle rest of the digits */
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for (ix = 1; ix < a->used; ix++) {
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*tmpc = *tmpa++ - mu;
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mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
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*tmpc++ &= MP_MASK;
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}
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}
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/* zero excess digits */
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while (ix++ < oldused) {
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*tmpc++ = 0;
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}
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mp_clamp(c);
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return MP_OKAY;
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}
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#endif
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/* $Source$ */
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/* $Revision$ */
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/* $Date$ */
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