third_party/gmp/mpz/prodlimbs.c

/* mpz_prodlimbs(RESULT, V, LEN) -- Set RESULT to V[0]*V[1]*...*V[LEN-1].

Contributed to the GNU project by Marco Bodrato.

THE FUNCTION IN THIS FILE IS INTERNAL WITH A MUTABLE INTERFACE.
IT IS ONLY SAFE TO REACH IT THROUGH DOCUMENTED INTERFACES.
IN FACT, IT IS ALMOST GUARANTEED THAT IT WILL CHANGE OR
DISAPPEAR IN A FUTURE GNU MP RELEASE.

Copyright 2010-2012 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of either:

  * the GNU Lesser General Public License as published by the Free
    Software Foundation; either version 3 of the License, or (at your
    option) any later version.

or

  * the GNU General Public License as published by the Free Software
    Foundation; either version 2 of the License, or (at your option) any
    later version.

or both in parallel, as here.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received copies of the GNU General Public License and the
GNU Lesser General Public License along with the GNU MP Library.  If not,
see https://www.gnu.org/licenses/.  */

#include "gmp-impl.h"

/*********************************************************/
/* Section list-prod: product of a list -> mpz_t         */
/*********************************************************/

/* FIXME: should be tuned */
#ifndef RECURSIVE_PROD_THRESHOLD
#define RECURSIVE_PROD_THRESHOLD (MUL_TOOM22_THRESHOLD)
#endif

/* Computes the product of the j>1 limbs pointed by factors, puts the
 * result in x. It assumes that all limbs are non-zero. Above
 * Karatsuba's threshold it uses a binary splitting strategy, to gain
 * speed by the asymptotically fast multiplication algorithms.
 *
 * The list in  {factors, j} is overwritten.
 * Returns the size of the result
 */

mp_size_t
mpz_prodlimbs (mpz_ptr x, mp_ptr factors, mp_size_t j)
{
  mp_limb_t cy;
  mp_size_t size, i;
  mp_ptr    prod;

  ASSERT (j > 1);
  ASSERT (RECURSIVE_PROD_THRESHOLD > 3);

  if (BELOW_THRESHOLD (j, RECURSIVE_PROD_THRESHOLD)) {
    j--;
    size = 1;

    for (i = 1; i < j; i++)
      {
	cy = mpn_mul_1 (factors, factors, size, factors[i]);
	factors[size] = cy;
	size += cy != 0;
      };

    prod = MPZ_NEWALLOC (x, size + 1);

    cy = mpn_mul_1 (prod, factors, size, factors[i]);
    prod[size] = cy;
    return SIZ (x) = size + (cy != 0);
  } else {
    mpz_t x1, x2;
    TMP_DECL;

    i = j >> 1;
    j -= i;
    TMP_MARK;

    MPZ_TMP_INIT (x2, j);

    PTR (x1) = factors + i;
    ALLOC (x1) = j;
    j = mpz_prodlimbs (x2, factors + i, j);
    i = mpz_prodlimbs (x1, factors, i);
    size = i + j;
    prod = MPZ_NEWALLOC (x, size);
    if (i >= j)
      cy = mpn_mul (prod, PTR(x1), i, PTR(x2), j);
    else
      cy = mpn_mul (prod, PTR(x2), j, PTR(x1), i);
    TMP_FREE;

    return SIZ (x) = size - (cy == 0);
  }
}