mp_xor.c

   1 #include "tommath_private.h"
   2 #ifdef MP_XOR_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* two complement xor */
   7 mp_err mp_xor(const mp_int *a, const mp_int *b, mp_int *c)
   8 {
   9    int used = MP_MAX(a->used, b->used) + 1, i;
  10    mp_err err;
  11    mp_digit ac = 1, bc = 1, cc = 1;
  12    bool neg = (a->sign != b->sign);
  13
  14    if ((err = mp_grow(c, used)) != MP_OKAY) {
  15       return err;
  16    }
  17
  18    for (i = 0; i < used; i++) {
  19       mp_digit x, y;
  20
  21       /* convert to two complement if negative */
  22       if (mp_isneg(a)) {
  23          ac += (i >= a->used) ? MP_MASK : (~a->dp[i] & MP_MASK);
  24          x = ac & MP_MASK;
  25          ac >>= MP_DIGIT_BIT;
  26       } else {
  27          x = (i >= a->used) ? 0uL : a->dp[i];
  28       }
  29
  30       /* convert to two complement if negative */
  31       if (mp_isneg(b)) {
  32          bc += (i >= b->used) ? MP_MASK : (~b->dp[i] & MP_MASK);
  33          y = bc & MP_MASK;
  34          bc >>= MP_DIGIT_BIT;
  35       } else {
  36          y = (i >= b->used) ? 0uL : b->dp[i];
  37       }
  38
  39       c->dp[i] = x ^ y;
  40
  41       /* convert to to sign-magnitude if negative */
  42       if (neg) {
  43          cc += ~c->dp[i] & MP_MASK;
  44          c->dp[i] = cc & MP_MASK;
  45          cc >>= MP_DIGIT_BIT;
  46       }
  47    }
  48
  49    c->used = used;
  50    c->sign = (neg ? MP_NEG : MP_ZPOS);
  51    mp_clamp(c);
  52    return MP_OKAY;
  53 }
  54 #endif