78 lines
2.4 KiB
C
78 lines
2.4 KiB
C
/* gmp_urandomm_ui -- uniform random number 0 to N-1 for ulong N.
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Copyright 2003, 2004 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP 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 by
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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The GNU MP 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 GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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MA 02110-1301, USA. */
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#include "mpir.h"
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#include "gmp-impl.h"
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#include "longlong.h"
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/* If n is a power of 2 then the test ret<n is always true and the loop is
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unnecessary, but there's no need to add special code for this. Just get
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the "bits" calculation correct and let it go through normally.
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If n is 1 then will have bits==0 and _gmp_rand will produce no output and
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we always return 0. Again there seems no need for a special case, just
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initialize a[0]=0 and let it go through normally. */
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#define MAX_URANDOMM_ITER 80
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gmp_ui
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gmp_urandomm_ui (gmp_randstate_ptr rstate, gmp_ui n)
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{
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mp_limb_t a[LIMBS_PER_UI];
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unsigned long ret, bits, leading;
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int i;
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if (UNLIKELY (n == 0))
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DIVIDE_BY_ZERO;
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/* start with zeros, since if bits==0 then _gmp_rand will store nothing at
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all (bits==0 arises when n==1), or if bits <= GMP_NUMB_BITS then it
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will store only a[0]. */
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a[0] = 0;
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#if LIMBS_PER_UI > 1
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a[1] = 0;
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#endif
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count_leading_zeros (leading, (mp_limb_t) n);
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bits = GMP_LIMB_BITS - leading - (POW2_P(n) != 0);
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for (i = 0; i < MAX_URANDOMM_ITER; i++)
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{
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_gmp_rand (a, rstate, bits);
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#if LIMBS_PER_UI == 1
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ret = a[0];
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#else
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ret = a[0] | (a[1] << GMP_NUMB_BITS);
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#endif
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if (LIKELY (ret < n)) /* usually one iteration suffices */
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goto done;
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}
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/* Too many iterations, there must be something degenerate about the
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rstate algorithm. Return r%n. */
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ret -= n;
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ASSERT (ret < n);
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done:
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return ret;
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}
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