kc3-lang/libtommath/bn_s_mp_sub.c

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• Hash : 8b2daf78
  Author :
  Date : 2019-05-19T17:16:13
  

  always use varname err with mp_err

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• bn_s_mp_sub.c

• #include "tommath_private.h"
  #ifdef BN_S_MP_SUB_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
  mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
  {
     int    olduse, min, max;
     mp_err err;
  
     /* find sizes */
     min = b->used;
     max = a->used;
  
     /* init result */
     if (c->alloc < max) {
        if ((err = mp_grow(c, max)) != MP_OKAY) {
           return err;
        }
     }
     olduse = c->used;
     c->used = max;
  
     {
        mp_digit u, *tmpa, *tmpb, *tmpc;
        int i;
  
        /* alias for digit pointers */
        tmpa = a->dp;
        tmpb = b->dp;
        tmpc = c->dp;
  
        /* set carry to zero */
        u = 0;
        for (i = 0; i < min; i++) {
           /* T[i] = A[i] - B[i] - U */
           *tmpc = (*tmpa++ - *tmpb++) - u;
  
           /* U = carry bit of T[i]
            * Note this saves performing an AND operation since
            * if a carry does occur it will propagate all the way to the
            * MSB.  As a result a single shift is enough to get the carry
            */
           u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
  
           /* Clear carry from T[i] */
           *tmpc++ &= MP_MASK;
        }
  
        /* now copy higher words if any, e.g. if A has more digits than B  */
        for (; i < max; i++) {
           /* T[i] = A[i] - U */
           *tmpc = *tmpa++ - u;
  
           /* U = carry bit of T[i] */
           u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
  
           /* Clear carry from T[i] */
           *tmpc++ &= MP_MASK;
        }
  
        /* clear digits above used (since we may not have grown result above) */
        MP_ZERO_DIGITS(tmpc, olduse - c->used);
     }
  
     mp_clamp(c);
     return MP_OKAY;
  }
  
  #endif
  

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