#include "tommath_private.h"

#ifdef BN_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_digit *tmpa, *tmpc;

   mp_err    err;

   int       ix, oldused;

   /* grow c as required */

   if (c->alloc < (a->used + 1)) {

      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;

   }

   /* setup regs */

   oldused = c->used;

   tmpa    = a->dp;

   tmpc    = c->dp;

   /* if a <= b simply fix the single digit */

   if (((a->used == 1) && (a->dp[0] <= b)) || (a->used == 0)) {

      if (a->used == 1) {

         *tmpc++ = b - *tmpa;

      } else {

         *tmpc++ = b;

      }

      ix      = 1;

      /* negative/1digit */

      c->sign = MP_NEG;

      c->used = 1;

   } else {

      mp_digit mu = b;

      /* positive/size */

      c->sign = MP_ZPOS;

      c->used = a->used;

      /* subtract digits, mu is carry */

      for (ix = 0; ix < a->used; ix++) {

         *tmpc    = *tmpa++ - mu;

         mu       = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);

         *tmpc++ &= MP_MASK;

      }

   }

   /* zero excess digits */

   MP_ZERO_DIGITS(tmpc, oldused - ix);

   mp_clamp(c);

   return MP_OKAY;

}

#endif