74 lines
2.0 KiB
C
74 lines
2.0 KiB
C
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/*=============================================================================
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This file is part of FLINT.
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FLINT is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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(at your option) any later version.
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FLINT is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with FLINT; if not, write to the Free Software
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Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
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=============================================================================*/
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/******************************************************************************
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Copyright (C) 2013 Mike Hansen
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******************************************************************************/
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#include <stdio.h>
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#include <stdlib.h>
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#include <math.h>
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#include "flint.h"
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#include "ulong_extras.h"
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mp_limb_t n_discrete_log_bsgs(mp_limb_t b, mp_limb_t a, mp_limb_t n)
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{
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int i, j;
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double ninv;
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mp_limb_t m, a_m, c;
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mp_limb_t * table;
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ninv = n_precompute_inverse(n);
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m = ceil(sqrt((double) n));
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table = (mp_limb_t*)flint_malloc(m * sizeof(mp_limb_t));
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table[0] = 1;
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for (j = 1; j < m; j++)
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{
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table[j] = n_mulmod_precomp(table[j-1], a, n, ninv);
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}
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a_m = n_invmod(a, n);
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a_m = n_powmod_precomp(a_m, m, n, ninv);
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c = b;
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for (i = 0; i < m; i++)
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{
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for (j = 0; j < m; j++)
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{
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if (c == table[j])
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{
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flint_free(table);
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return i * m + j;
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}
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}
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c = n_mulmod_precomp(c, a_m, n, ninv);
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}
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flint_free(table);
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flint_printf("Exception (n_discrete_log_bsgs). discrete log not found.\n");
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abort();
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}
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