randmui.c
上传用户:qaz666999
上传日期:2022-08-06
资源大小:2570k
文件大小:2k
- /* 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;
- }