tommath/bn_mp_mul.c

#include <tommath_private.h>
#ifdef BN_MP_MUL_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
 *
 * LibTomMath is a library that provides multiple-precision
 * integer arithmetic as well as number theoretic functionality.
 *
 * The library was designed directly after the MPI library by
 * Michael Fromberger but has been written from scratch with
 * additional optimizations in place.
 *
 * The library is free for all purposes without any express
 * guarantee it works.
 *
 * Tom St Denis, tstdenis82@gmail.com, http://libtom.org
 */

/* high level multiplication (handles sign) */
int mp_mul (mp_int * a, mp_int * b, mp_int * c)
{
  int     res, neg;
  neg = (a->sign == b->sign) ? MP_ZPOS : MP_NEG;

  /* use Toom-Cook? */
#ifdef BN_MP_TOOM_MUL_C
  if (MIN(a->used, b->used) >= TOOM_MUL_CUTOFF) {
    res = mp_toom_mul(a, b, c);
  } else
#endif
#ifdef BN_MP_KARATSUBA_MUL_C
  /* use Karatsuba? */
  if (MIN(a->used, b->used) >= KARATSUBA_MUL_CUTOFF) {
    res = mp_karatsuba_mul(a, b, c);
  } else
#endif
  {
    /* can we use the fast multiplier?
     *
     * The fast multiplier can be used if the output will
     * have less than MP_WARRAY digits and the number of
     * digits won't affect carry propagation
     */
    int     digs = a->used + b->used + 1;

#ifdef BN_FAST_S_MP_MUL_DIGS_C
    if ((digs < MP_WARRAY) &&
        (MIN(a->used, b->used) <=
         (1 << ((CHAR_BIT * sizeof(mp_word)) - (2 * DIGIT_BIT))))) {
      res = fast_s_mp_mul_digs(a, b, c, digs);
    } else
#endif
    {
#ifdef BN_S_MP_MUL_DIGS_C
      res = s_mp_mul(a, b, c); /* uses s_mp_mul_digs */
#else
      res = MP_VAL;
#endif
    }
  }
  c->sign = (c->used > 0) ? neg : MP_ZPOS;
  return res;
}
#endif

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