87 lines
1.8 KiB
C
87 lines
1.8 KiB
C
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#include "tommath_private.h"
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#ifdef 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 oldused;
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/* fast path for a == c */
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if (a == c) {
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if (!mp_isneg(c) &&
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!mp_iszero(c) &&
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((c->dp[0] + b) < MP_DIGIT_MAX)) {
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c->dp[0] += b;
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return MP_OKAY;
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}
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if (mp_isneg(c) &&
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(c->dp[0] > b)) {
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c->dp[0] -= b;
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return MP_OKAY;
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}
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}
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/* grow c as required */
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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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/* if a is negative and |a| >= b, call c = |a| - b */
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if (mp_isneg(a) && ((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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/* if a is positive */
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if (!mp_isneg(a)) {
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/* add digits, mu is carry */
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int i;
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mp_digit mu = b;
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for (i = 0; i < a->used; i++) {
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c->dp[i] = a->dp[i] + mu;
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mu = c->dp[i] >> MP_DIGIT_BIT;
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c->dp[i] &= MP_MASK;
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}
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/* set final carry */
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c->dp[i] = 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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c->dp[0] = (a->used == 1) ? b - a->dp[0] : b;
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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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s_mp_zero_digs(c->dp + c->used, oldused - c->used);
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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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