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/* gmp_urandomm_ui -- uniform random number 0 to N-1 for ulong N.
Copyright 2003, 2004 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 either:
* the GNU Lesser General Public License as published by the Free
Software Foundation; either version 3 of the License, or (at your
option) any later version.
or
* the GNU General Public License as published by the Free Software
Foundation; either version 2 of the License, or (at your option) any
later version.
or both in parallel, as here.
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 General Public License
for more details.
You should have received copies of the GNU General Public License and the
GNU Lesser General Public License along with the GNU MP Library. If not,
see https://www.gnu.org/licenses/. */
#include "gmp-impl.h"
#include "longlong.h"
/* If n is a power of 2 then the test ret<n is always true and the loop is
unnecessary, but there's no need to add special code for this. Just get
the "bits" calculation correct and let it go through normally.
If n is 1 then will have bits==0 and _gmp_rand will produce no output and
we always return 0. Again there seems no need for a special case, just
initialize a[0]=0 and let it go through normally. */
#define MAX_URANDOMM_ITER 80
unsigned long
gmp_urandomm_ui (gmp_randstate_ptr rstate, unsigned long n)
{
mp_limb_t a[LIMBS_PER_ULONG];
unsigned long ret, bits, leading;
int i;
if (UNLIKELY (n == 0))
DIVIDE_BY_ZERO;
/* start with zeros, since if bits==0 then _gmp_rand will store nothing at
all (bits==0 arises when n==1), or if bits <= GMP_NUMB_BITS then it
will store only a[0]. */
a[0] = 0;
#if LIMBS_PER_ULONG > 1
a[1] = 0;
#endif
count_leading_zeros (leading, (mp_limb_t) n);
bits = GMP_LIMB_BITS - leading - (POW2_P(n) != 0);
for (i = 0; i < MAX_URANDOMM_ITER; i++)
{
_gmp_rand (a, rstate, bits);
#if LIMBS_PER_ULONG == 1
ret = a[0];
#else
ret = a[0] | (a[1] << GMP_NUMB_BITS);
#endif
if (LIKELY (ret < n)) /* usually one iteration suffices */
goto done;
}
/* Too many iterations, there must be something degenerate about the
rstate algorithm. Return r%n. */
ret -= n;
ASSERT (ret < n);
done:
return ret;
}
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