78 lines
2.5 KiB
C
78 lines
2.5 KiB
C
/* 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 2.1 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; see the file COPYING.LIB. If not, write to
|
|
the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
|
|
MA 02110-1301, USA. */
|
|
|
|
#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;
|
|
}
|