kc3-lang/libtommath/mp_sub_d.c

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• Hash : 1cc289d2
  Author :
  Date : 2019-11-09T20:23:03
  

  better use of mp_isneg() and mp_iszero()

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• mp_sub_d.c

• #include "tommath_private.h"
  #ifdef MP_SUB_D_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* single digit subtraction */
  mp_err mp_sub_d(const mp_int *a, mp_digit b, mp_int *c)
  {
     mp_err err;
     int oldused;
  
     /* fast path for a == c */
     if (a == c) {
        if ((c->sign == MP_NEG) &&
            ((c->dp[0] + b) < MP_DIGIT_MAX)) {
           c->dp[0] += b;
           return MP_OKAY;
        }
        if ((c->sign == MP_ZPOS) &&
            (c->dp[0] > b)) {
           c->dp[0] -= b;
           return MP_OKAY;
        }
     }
  
     /* grow c as required */
     if ((err = mp_grow(c, a->used + 1)) != MP_OKAY) {
        return err;
     }
  
     /* if a is negative just do an unsigned
      * addition [with fudged signs]
      */
     if (a->sign == MP_NEG) {
        mp_int a_ = *a;
        a_.sign = MP_ZPOS;
        err     = mp_add_d(&a_, b, c);
        c->sign = MP_NEG;
  
        /* clamp */
        mp_clamp(c);
  
        return err;
     }
  
     oldused = c->used;
  
     /* if a <= b simply fix the single digit */
     if (((a->used == 1) && (a->dp[0] <= b)) || mp_iszero(a)) {
        c->dp[0] = (a->used == 1) ? b - a->dp[0] : b;
  
        /* negative/1digit */
        c->sign = MP_NEG;
        c->used = 1;
     } else {
        int i;
        mp_digit mu = b;
  
        /* positive/size */
        c->sign = MP_ZPOS;
        c->used = a->used;
  
        /* subtract digits, mu is carry */
        for (i = 0; i < a->used; i++) {
           c->dp[i] = a->dp[i] - mu;
           mu = c->dp[i] >> (MP_SIZEOF_BITS(mp_digit) - 1u);
           c->dp[i] &= MP_MASK;
        }
     }
  
     /* zero excess digits */
     s_mp_zero_digs(c->dp + c->used, oldused - c->used);
  
     mp_clamp(c);
     return MP_OKAY;
  }
  
  #endif
  

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