mirror of
https://github.com/vale981/arb
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87 lines
2.5 KiB
C
87 lines
2.5 KiB
C
/*=============================================================================
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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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Copyright (C) 2016 Pascal Molin
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******************************************************************************/
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#include <stdlib.h>
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#include "dlog.h"
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#include "ulong_extras.h"
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static int
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apow_cmp(const apow_t * x, const apow_t * y)
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{
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return (x->ak < y->ak) ? -1 : (x->ak > y->ak);
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}
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/* set size of table m=sqrt(nk) to compute k logs in a group of size n */
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void
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dlog_bsgs_init(dlog_bsgs_t t, ulong a, ulong mod, ulong n, ulong m)
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{
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ulong k, ak;
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t->table = (apow_t *)flint_malloc(m * sizeof(apow_t));
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if (m > n) m = n;
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t->mod = mod;
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t->modinv = n_precompute_inverse(mod);
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t->m = m;
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t->g = n / m;
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for (k = 0, ak = 1; k < m; k++)
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{
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t->table[k].k = k;
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t->table[k].ak = ak;
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ak = n_mulmod_precomp(ak, a, mod, t->modinv);
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}
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t->am = n_invmod(ak, mod);
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qsort(t->table, m, sizeof(apow_t), (int(*)(const void*,const void*))apow_cmp);
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}
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void
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dlog_bsgs_clear(dlog_bsgs_t t)
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{
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flint_free(t->table);
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}
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ulong
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dlog_bsgs(const dlog_bsgs_t t, ulong b)
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{
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ulong i;
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apow_t c, * x;
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c.ak = b;
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for (i = 0; i <= t->g; i++)
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{
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x = bsearch(&c, t->table, t->m, sizeof(apow_t),
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(int(*)(const void*,const void*))apow_cmp);
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if (x != NULL)
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return i * t->m + x->k;
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c.ak = n_mulmod_precomp(c.ak, t->am, t->mod, t->modinv);
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}
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flint_printf("Exception (n_discrete_log_bsgs). discrete log not found.\n");
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flint_printf(" table size %wu, cosize %wu mod %wu. %wu not found\n",
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t->m, t->g, t->mod, b);
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abort();
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}
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