mp_sub.c

   1 #include "tommath_private.h"
   2 #ifdef MP_SUB_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* high level subtraction (handles signs) */
   7 mp_err mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
   8 {
   9    if (a->sign != b->sign) {
  10       /* subtract a negative from a positive, OR */
  11       /* subtract a positive from a negative. */
  12       /* In either case, ADD their magnitudes, */
  13       /* and use the sign of the first number. */
  14       c->sign = a->sign;
  15       return s_mp_add(a, b, c);
  16    }
  17
  18    /* subtract a positive from a positive, OR */
  19    /* subtract a negative from a negative. */
  20    /* First, take the difference between their */
  21    /* magnitudes, then... */
  22    if (mp_cmp_mag(a, b) == MP_LT) {
  23       /* The second has a larger magnitude */
  24       /* The result has the *opposite* sign from */
  25       /* the first number. */
  26       c->sign = (!mp_isneg(a) ? MP_NEG : MP_ZPOS);
  27       MP_EXCH(const mp_int *, a, b);
  28    } else {
  29       /* The first has a larger or equal magnitude */
  30       /* Copy the sign from the first */
  31       c->sign = a->sign;
  32    }
  33    return s_mp_sub(a, b, c);
  34 }
  35
  36 #endif