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41 lines
1.2 KiB
41 lines
1.2 KiB
#include "tommath_private.h"
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#ifdef BN_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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/* high level subtraction (handles signs) */
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mp_err mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
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{
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mp_sign sa = a->sign, sb = b->sign;
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mp_err err;
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if (sa != sb) {
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/* subtract a negative from a positive, OR */
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/* subtract a positive from a negative. */
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/* In either case, ADD their magnitudes, */
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/* and use the sign of the first number. */
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c->sign = sa;
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err = s_mp_add(a, b, c);
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} else {
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/* subtract a positive from a positive, OR */
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/* subtract a negative from a negative. */
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/* First, take the difference between their */
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/* magnitudes, then... */
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if (mp_cmp_mag(a, b) != MP_LT) {
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/* Copy the sign from the first */
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c->sign = sa;
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/* The first has a larger or equal magnitude */
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err = s_mp_sub(a, b, c);
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} else {
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/* The result has the *opposite* sign from */
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/* the first number. */
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c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS;
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/* The second has a larger magnitude */
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err = s_mp_sub(b, a, c);
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}
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}
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return err;
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}
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#endif
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