1af0de1f91
* same behavior for positive numbers * generalisation for negative numbers, treating them as two complement * improve algorithm, iterate once over the digits, manually perform two complement * simplify mp_add_d, mp_sub_d * functions are safe in case of a==c or b==c * renamed mp_tc_div_2d to mp_signed_rsh (signed right shift)
90 lines
1.8 KiB
C
90 lines
1.8 KiB
C
#include "tommath_private.h"
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#ifdef BN_MP_ADD_D_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/* single digit addition */
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mp_err mp_add_d(const mp_int *a, mp_digit b, mp_int *c)
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{
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mp_err err;
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int ix, oldused;
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mp_digit *tmpa, *tmpc;
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/* grow c as required */
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if (c->alloc < (a->used + 1)) {
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if ((err = mp_grow(c, a->used + 1)) != MP_OKAY) {
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return err;
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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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mp_int a_ = *a;
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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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err = mp_sub_d(&a_, b, c);
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/* fix sign */
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c->sign = MP_NEG;
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/* clamp */
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mp_clamp(c);
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return err;
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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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/* 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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/* add digits, mu is carry */
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mp_digit mu = b;
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for (ix = 0; ix < a->used; ix++) {
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*tmpc = *tmpa++ + mu;
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mu = *tmpc >> MP_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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/* setup size */
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c->used = a->used + 1;
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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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/* sign always positive */
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c->sign = MP_ZPOS;
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/* now zero to oldused */
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MP_ZERO_DIGITS(tmpc, oldused - ix);
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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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