69 lines
1.8 KiB
C
69 lines
1.8 KiB
C
/*
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Copyright 2009 Jason Moxham
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This file is part of the MPIR Library.
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The MPIR Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published
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by the Free Software Foundation; either version 2.1 of the License, or (at
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your option) any later version.
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The MPIR Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the MPIR Library; see the file COPYING.LIB. If not, write
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to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
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Boston, MA 02110-1301, USA.
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*/
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#include "mpir.h"
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#include "gmp-impl.h"
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/*
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Returns smallest d such that d|N, start <= d < stop, d != 1. If no such d exists
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return 0. The name trial_division implies a method, perhaps call it
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mpz_smallest_divisor.
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N must have no divisors < start.
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*/
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unsigned long mpz_trial_division(mpz_srcptr N, unsigned long start, unsigned long stop)
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{
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unsigned long i, dd;
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/* ASSERT N has no divisors < start excluding 1 */
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ASSERT(mpz_cmp_ui(N, 0) != 0);
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ASSERT(mpz_cmp_ui(N, 1) != 0);
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ASSERT(mpz_cmp_si(N, -1) != 0);
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if (start <= 2 && 2 < stop && mpz_even_p(N))
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return 2;
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if (start <= 3 && 3 < stop && mpz_divisible_ui_p(N, 3))
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return 3;
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if (start < 5)
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start = 5; /* dont be silly */
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if (start%2 == 0)
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start+=1;
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if (start%3 == 0)
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start+=2;
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ASSERT(start%2 != 0);
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ASSERT(start%3 != 0);
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dd = 2;
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if (start%6 == 1)
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dd = 4;
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for (i = start; i < stop; i += dd, dd = 6 - dd)
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if (mpz_divisible_ui_p(N, i))
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return i;
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return 0;
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}
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