71 lines
1.6 KiB
C
71 lines
1.6 KiB
C
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#include "tommath_private.h"
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#ifdef S_MP_ADD_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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/* low level addition, based on HAC pp.594, Algorithm 14.7 */
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mp_err s_mp_add(const mp_int *a, const mp_int *b, mp_int *c)
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{
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int oldused, min, max, i;
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mp_digit u;
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mp_err err;
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/* find sizes, we let |a| <= |b| which means we have to sort
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* them. "x" will point to the input with the most digits
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*/
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if (a->used < b->used) {
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MP_EXCH(const mp_int *, a, b);
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}
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min = b->used;
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max = a->used;
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/* init result */
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if ((err = mp_grow(c, max + 1)) != MP_OKAY) {
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return err;
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}
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/* get old used digit count and set new one */
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oldused = c->used;
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c->used = max + 1;
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/* zero the carry */
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u = 0;
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for (i = 0; i < min; i++) {
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/* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
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c->dp[i] = a->dp[i] + b->dp[i] + u;
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/* U = carry bit of T[i] */
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u = c->dp[i] >> (mp_digit)MP_DIGIT_BIT;
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/* take away carry bit from T[i] */
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c->dp[i] &= MP_MASK;
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}
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/* now copy higher words if any, that is in A+B
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* if A or B has more digits add those in
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*/
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if (min != max) {
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for (; i < max; i++) {
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/* T[i] = A[i] + U */
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c->dp[i] = a->dp[i] + u;
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/* U = carry bit of T[i] */
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u = c->dp[i] >> (mp_digit)MP_DIGIT_BIT;
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/* take away carry bit from T[i] */
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c->dp[i] &= MP_MASK;
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}
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}
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/* add carry */
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c->dp[i] = u;
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/* clear digits above 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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