89 lines
1.9 KiB
C
89 lines
1.9 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_digit *tmpa, *tmpc;
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mp_err err;
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int ix, 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 (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 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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/* setup regs */
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oldused = c->used;
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tmpa = a->dp;
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tmpc = c->dp;
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/* if a <= b simply fix the single digit */
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if (((a->used == 1) && (a->dp[0] <= b)) || (a->used == 0)) {
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if (a->used == 1) {
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*tmpc++ = b - *tmpa;
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} else {
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*tmpc++ = b;
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}
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ix = 1;
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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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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 (ix = 0; ix < a->used; ix++) {
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*tmpc = *tmpa++ - mu;
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mu = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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*tmpc++ &= MP_MASK;
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}
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}
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/* zero excess digits */
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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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