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/* mpz_bin_uiui - compute n over k.
Copyright (C) 1998, 2000 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 the GNU Library General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at your
option) any later version.
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 Library General Public
License for more details.
You should have received a copy of the GNU Library General Public License
along with the GNU MP Library; see the file COPYING.LIB. If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA. */
#include "gmp.h"
#include "gmp-impl.h"
#include "longlong.h"
/*
tege> Vilken är definitionen för negativa n?
teke> Den rekursiva definitionen funkar där också dvs
teke>
teke> k < 0 => bin(n,k) = 0
teke>
teke> k == 0 => bin(n,k) = 1
teke>
teke> 1 => bin(n,k) = bin(n-1,k-1)*n/k
(teke=Torsten Ekedahl)
*/
/* This is a poor implementation. Look at bin_uiui.c for improvement ideas. */
void
#if __STDC__
mpz_bin_ui (mpz_ptr r, mpz_srcptr n, unsigned long int k)
#else
mpz_bin_ui (r, n, k)
mpz_ptr r;
mpz_srcptr n;
unsigned long int k;
#endif
{
mpz_t ni;
unsigned long int i;
unsigned long int kacc;
mpz_t nacc;
mpz_init (ni);
mpz_sub_ui (ni, n, k);
/* Rewrite bin(n,k) as bin(n,n-k) if that is simpler to compute. */
if (mpz_cmp_ui (ni, k) < 0)
{
k = mpz_get_ui (ni);
mpz_sub_ui (ni, n, k);
}
mpz_set_ui (r, 1);
kacc = 1;
mpz_init_set_ui (nacc, 1);
for (i = 1; i <= k; i++)
{
mp_limb_t k1, k0;
#if 0
mp_limb_t nacclow;
int c;
nacclow = PTR(nacc)[0];
for (c = 0; (((kacc | nacclow) & 1) == 0); c++)
{
kacc >>= 1;
nacclow >>= 1;
}
mpz_div_2exp (nacc, nacc, c);
#endif
mpz_add_ui (ni, ni, 1);
mpz_mul (nacc, nacc, ni);
umul_ppmm (k1, k0, kacc, i);
if (k1 != 0)
{
/* Accumulator overflow. Perform bignum step. */
mpz_mul (r, r, nacc);
mpz_set_ui (nacc, 1);
mpz_tdiv_q_ui (r, r, kacc);
kacc = i;
}
else
{
/* Save new products in accumulators to keep accumulating. */
kacc = k0;
}
}
mpz_mul (r, r, nacc);
mpz_set_ui (nacc, 1);
mpz_tdiv_q_ui (r, r, kacc);
mpz_clear (nacc);
mpz_clear (ni);
}
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