1 #include "tommath_private.h"
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
4 /* SPDX-License-Identifier: Unlicense */
6 /* high level subtraction (handles signs) */
7 mp_err mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
9 mp_sign sa = a->sign, sb = b->sign;
13 /* subtract a negative from a positive, OR */
14 /* subtract a positive from a negative. */
15 /* In either case, ADD their magnitudes, */
16 /* and use the sign of the first number. */
18 err = s_mp_add(a, b, c);
20 /* subtract a positive from a positive, OR */
21 /* subtract a negative from a negative. */
22 /* First, take the difference between their */
23 /* magnitudes, then... */
24 if (mp_cmp_mag(a, b) != MP_LT) {
25 /* Copy the sign from the first */
27 /* The first has a larger or equal magnitude */
28 err = s_mp_sub(a, b, c);
30 /* The result has the *opposite* sign from */
31 /* the first number. */
32 c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS;
33 /* The second has a larger magnitude */
34 err = s_mp_sub(b, a, c);