urandomm.c revision 1.1.1.3 
1/* mpz_urandomm (rop, state, n) -- Generate a uniform pseudorandom
2   integer in the range 0 to N-1, using STATE as the random state
3   previously initialized by a call to gmp_randinit().
4
5Copyright 2000, 2002, 2012 Free Software Foundation, Inc.
6
7This file is part of the GNU MP Library.
8
9The GNU MP Library is free software; you can redistribute it and/or modify
10it under the terms of either:
11
12  * the GNU Lesser General Public License as published by the Free
13    Software Foundation; either version 3 of the License, or (at your
14    option) any later version.
15
16or
17
18  * the GNU General Public License as published by the Free Software
19    Foundation; either version 2 of the License, or (at your option) any
20    later version.
21
22or both in parallel, as here.
23
24The GNU MP Library is distributed in the hope that it will be useful, but
25WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
26or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
27for more details.
28
29You should have received copies of the GNU General Public License and the
30GNU Lesser General Public License along with the GNU MP Library.  If not,
31see https://www.gnu.org/licenses/.  */
32
33#include "gmp.h"
34#include "gmp-impl.h"
35#include "longlong.h" /* for count_leading_zeros */
36
37
38#define MAX_URANDOMM_ITER  80
39
40void
41mpz_urandomm (mpz_ptr rop, gmp_randstate_t rstate, mpz_srcptr n)
42{
43  mp_ptr rp, np, nlast;
44  mp_size_t nbits, size;
45  int count;
46  int pow2;
47  int cmp;
48  TMP_DECL;
49
50  size = ABSIZ (n);
51  if (UNLIKELY (size == 0))
52    DIVIDE_BY_ZERO;
53
54  nlast = &PTR (n)[size - 1];
55
56  /* Detect whether n is a power of 2.  */
57  pow2 = POW2_P (*nlast);
58  if (pow2 != 0)
59    for (np = PTR (n); np < nlast; np++)
60      if (*np != 0)
61	{
62	  pow2 = 0;		/* Mark n as `not a power of two'.  */
63	  break;
64	}
65
66  count_leading_zeros (count, *nlast);
67  nbits = size * GMP_NUMB_BITS - (count - GMP_NAIL_BITS) - pow2;
68  if (nbits == 0)		/* nbits == 0 means that n was == 1.  */
69    {
70      SIZ (rop) = 0;
71      return;
72    }
73
74  TMP_MARK;
75  np = PTR (n);
76  if (rop == n)
77    {
78      mp_ptr tp;
79      tp = TMP_ALLOC_LIMBS (size);
80      MPN_COPY (tp, np, size);
81      np = tp;
82    }
83
84  /* Here the allocated size can be one too much if n is a power of
85     (2^GMP_NUMB_BITS) but it's convenient for using mpn_cmp below.  */
86  rp = MPZ_REALLOC (rop, size);
87  /* Clear last limb to prevent the case in which size is one too much.  */
88  rp[size - 1] = 0;
89
90  count = MAX_URANDOMM_ITER;	/* Set iteration count limit.  */
91  do
92    {
93      _gmp_rand (rp, rstate, nbits);
94      MPN_CMP (cmp, rp, np, size);
95    }
96  while (cmp >= 0 && --count != 0);
97
98  if (count == 0)
99    /* Too many iterations; return result mod n == result - n */
100    mpn_sub_n (rp, rp, np, size);
101
102  MPN_NORMALIZE (rp, size);
103  SIZ (rop) = size;
104  TMP_FREE;
105}
106