57 lines
1.5 KiB
C
57 lines
1.5 KiB
C
#include "tommath_private.h"
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#ifdef S_MP_SUB_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 subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
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mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
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{
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int oldused = c->used, min = b->used, max = a->used, i;
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mp_digit u;
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mp_err err;
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/* init result */
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if ((err = mp_grow(c, max)) != MP_OKAY) {
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return err;
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}
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c->used = max;
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/* set carry to zero */
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u = 0;
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for (i = 0; i < min; i++) {
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/* 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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* Note this saves performing an AND operation since
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* if a carry does occur it will propagate all the way to the
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* MSB. As a result a single shift is enough to get the carry
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*/
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u = c->dp[i] >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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/* Clear carry 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, e.g. if A has more digits than B */
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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_SIZEOF_BITS(mp_digit) - 1u);
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/* Clear carry from T[i] */
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c->dp[i] &= MP_MASK;
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}
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/* clear digits above used (since we may not have grown result above) */
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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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