#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