104 lines
2.2 KiB
C
104 lines
2.2 KiB
C
/* 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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#include <tommath.h>
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/* single digit addition */
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int
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mp_add_d (mp_int * a, mp_digit b, mp_int * c)
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{
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int res, ix, oldused;
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mp_digit *tmpa, *tmpc, mu;
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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 and |a| >= b, call c = |a| - b */
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if (a->sign == MP_NEG && (a->used > 1 || a->dp[0] >= b)) {
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/* temporarily fix sign of a */
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a->sign = MP_ZPOS;
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/* c = |a| - b */
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res = mp_sub_d(a, b, c);
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/* fix sign */
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a->sign = c->sign = MP_NEG;
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return res;
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}
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/* old number of used digits in c */
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oldused = c->used;
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/* sign always positive */
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c->sign = MP_ZPOS;
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/* source alias */
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tmpa = a->dp;
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/* destination alias */
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tmpc = c->dp;
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/* if a is positive */
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if (a->sign == MP_ZPOS) {
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/* setup size */
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c->used = a->used + 1;
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/* add digit, after this we're propagating
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* the carry.
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*/
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*tmpc = *tmpa++ + b;
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mu = *tmpc >> DIGIT_BIT;
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*tmpc++ &= MP_MASK;
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/* now 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 >> DIGIT_BIT;
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*tmpc++ &= MP_MASK;
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}
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/* set final carry */
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ix++;
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*tmpc++ = mu;
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} else {
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/* a was negative and |a| < b */
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c->used = 1;
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/* the result is a single digit */
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if (a->used == 1) {
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*tmpc++ = b - a->dp[0];
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} else {
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*tmpc++ = b;
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}
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/* setup count so the clearing of oldused
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* can fall through correctly
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*/
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ix = 1;
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}
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/* now zero to oldused */
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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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