1/* gmp_urandomm_ui -- uniform random number 0 to N-1 for ulong N.
2
3Copyright 2003, 2004 Free Software Foundation, Inc.
4
5This file is part of the GNU MP Library.
6
7The GNU MP Library is free software; you can redistribute it and/or modify
8it under the terms of the GNU Lesser General Public License as published by
9the Free Software Foundation; either version 3 of the License, or (at your
10option) any later version.
11
12The GNU MP Library is distributed in the hope that it will be useful, but
13WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
14or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
15License for more details.
16
17You should have received a copy of the GNU Lesser General Public License
18along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
19
20#include "gmp.h"
21#include "gmp-impl.h"
22#include "longlong.h"
23
24
25/* If n is a power of 2 then the test ret<n is always true and the loop is
26   unnecessary, but there's no need to add special code for this.  Just get
27   the "bits" calculation correct and let it go through normally.
28
29   If n is 1 then will have bits==0 and _gmp_rand will produce no output and
30   we always return 0.  Again there seems no need for a special case, just
31   initialize a[0]=0 and let it go through normally.  */
32
33#define MAX_URANDOMM_ITER  80
34
35unsigned long
36gmp_urandomm_ui (gmp_randstate_ptr rstate, unsigned long n)
37{
38  mp_limb_t      a[LIMBS_PER_ULONG];
39  unsigned long  ret, bits, leading;
40  int            i;
41
42  if (UNLIKELY (n == 0))
43    DIVIDE_BY_ZERO;
44
45  /* start with zeros, since if bits==0 then _gmp_rand will store nothing at
46     all (bits==0 arises when n==1), or if bits <= GMP_NUMB_BITS then it
47     will store only a[0].  */
48  a[0] = 0;
49#if LIMBS_PER_ULONG > 1
50  a[1] = 0;
51#endif
52
53  count_leading_zeros (leading, (mp_limb_t) n);
54  bits = GMP_LIMB_BITS - leading - (POW2_P(n) != 0);
55
56  for (i = 0; i < MAX_URANDOMM_ITER; i++)
57    {
58      _gmp_rand (a, rstate, bits);
59#if LIMBS_PER_ULONG == 1
60      ret = a[0];
61#else
62      ret = a[0] | (a[1] << GMP_NUMB_BITS);
63#endif
64      if (LIKELY (ret < n))   /* usually one iteration suffices */
65        goto done;
66    }
67
68  /* Too many iterations, there must be something degenerate about the
69     rstate algorithm.  Return r%n.  */
70  ret -= n;
71  ASSERT (ret < n);
72
73 done:
74  return ret;
75}
76