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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 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.
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 Lesser General Public
License for more details.
You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
#include "gmp.h"
#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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