79 lines
1.7 KiB
C
79 lines
1.7 KiB
C
#include "tommath_private.h"
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#ifdef MP_SUB_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 subtraction */
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mp_err mp_sub_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 ((c->sign == MP_NEG) &&
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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 ((c->sign == MP_ZPOS) &&
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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 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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mp_int a_ = *a;
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a_.sign = MP_ZPOS;
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err = mp_add_d(&a_, b, c);
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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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oldused = c->used;
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/* if a <= b simply fix the single digit */
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if (((a->used == 1) && (a->dp[0] <= b)) || mp_iszero(a)) {
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c->dp[0] = (a->used == 1) ? b - a->dp[0] : b;
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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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int i;
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mp_digit mu = b;
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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 digits, mu is carry */
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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_SIZEOF_BITS(mp_digit) - 1u);
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c->dp[i] &= MP_MASK;
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}
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}
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/* zero excess digits */
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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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