mp_add_d.c

   1 #include "tommath_private.h"
   2 #ifdef MP_ADD_D_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* single digit addition */
   7 mp_err mp_add_d(const mp_int *a, mp_digit b, mp_int *c)
   8 {
   9    mp_err err;
  10    int oldused;
  11
  12    /* fast path for a == c */
  13    if (a == c) {
  14       if (!mp_isneg(c) &&
  15           !mp_iszero(c) &&
  16           ((c->dp[0] + b) < MP_DIGIT_MAX)) {
  17          c->dp[0] += b;
  18          return MP_OKAY;
  19       }
  20       if (mp_isneg(c) &&
  21           (c->dp[0] > b)) {
  22          c->dp[0] -= b;
  23          return MP_OKAY;
  24       }
  25    }
  26
  27    /* grow c as required */
  28    if ((err = mp_grow(c, a->used + 1)) != MP_OKAY) {
  29       return err;
  30    }
  31
  32    /* if a is negative and |a| >= b, call c = |a| - b */
  33    if (mp_isneg(a) && ((a->used > 1) || (a->dp[0] >= b))) {
  34       mp_int a_ = *a;
  35       /* temporarily fix sign of a */
  36       a_.sign = MP_ZPOS;
  37
  38       /* c = |a| - b */
  39       err = mp_sub_d(&a_, b, c);
  40
  41       /* fix sign  */
  42       c->sign = MP_NEG;
  43
  44       /* clamp */
  45       mp_clamp(c);
  46
  47       return err;
  48    }
  49
  50    /* old number of used digits in c */
  51    oldused = c->used;
  52
  53    /* if a is positive */
  54    if (!mp_isneg(a)) {
  55       /* add digits, mu is carry */
  56       int i;
  57       mp_digit mu = b;
  58       for (i = 0; i < a->used; i++) {
  59          c->dp[i] = a->dp[i] + mu;
  60          mu = c->dp[i] >> MP_DIGIT_BIT;
  61          c->dp[i] &= MP_MASK;
  62       }
  63       /* set final carry */
  64       c->dp[i] = mu;
  65
  66       /* setup size */
  67       c->used = a->used + 1;
  68    } else {
  69       /* a was negative and |a| < b */
  70       c->used = 1;
  71
  72       /* the result is a single digit */
  73       c->dp[0] = (a->used == 1) ? b - a->dp[0] : b;
  74    }
  75
  76    /* sign always positive */
  77    c->sign = MP_ZPOS;
  78
  79    /* now zero to oldused */
  80    s_mp_zero_digs(c->dp + c->used, oldused - c->used);
  81    mp_clamp(c);
  82
  83    return MP_OKAY;
  84 }
  85
  86 #endif