1/* mpn_random2 -- Generate random numbers with relatively long strings
2   of ones and zeroes.  Suitable for border testing.
3
4Copyright 1992-1994, 1996, 2000-2002, 2004, 2012 Free Software Foundation, Inc.
5
6This file is part of the GNU MP Library.
7
8The GNU MP Library is free software; you can redistribute it and/or modify
9it under the terms of either:
10
11  * the GNU Lesser General Public License as published by the Free
12    Software Foundation; either version 3 of the License, or (at your
13    option) any later version.
14
15or
16
17  * the GNU General Public License as published by the Free Software
18    Foundation; either version 2 of the License, or (at your option) any
19    later version.
20
21or both in parallel, as here.
22
23The GNU MP Library is distributed in the hope that it will be useful, but
24WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
25or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
26for more details.
27
28You should have received copies of the GNU General Public License and the
29GNU Lesser General Public License along with the GNU MP Library.  If not,
30see https://www.gnu.org/licenses/.  */
31
32#include "gmp-impl.h"
33
34static void gmp_rrandomb (mp_ptr, gmp_randstate_t, mp_bitcnt_t);
35
36/* Ask _gmp_rand for 32 bits per call unless that's more than a limb can hold.
37   Thus, we get the same random number sequence in the common cases.
38   FIXME: We should always generate the same random number sequence!  */
39#if GMP_NUMB_BITS < 32
40#define BITS_PER_RANDCALL GMP_NUMB_BITS
41#else
42#define BITS_PER_RANDCALL 32
43#endif
44
45void
46mpn_random2 (mp_ptr rp, mp_size_t n)
47{
48  gmp_randstate_ptr rstate = RANDS;
49  int bit_pos;			/* bit number of least significant bit where
50				   next bit field to be inserted */
51  mp_limb_t ran, ranm;		/* buffer for random bits */
52
53  /* FIXME: Is n==0 supposed to be allowed? */
54  ASSERT (n >= 0);
55
56  _gmp_rand (&ranm, rstate, BITS_PER_RANDCALL);
57  ran = ranm;
58
59  /* Start off at a random bit position in the most significant limb.  */
60  bit_pos = ran % GMP_NUMB_BITS;
61
62  gmp_rrandomb (rp, rstate, n * GMP_NUMB_BITS - bit_pos);
63}
64
65static void
66gmp_rrandomb (mp_ptr rp, gmp_randstate_t rstate, mp_bitcnt_t nbits)
67{
68  mp_bitcnt_t bi;
69  mp_limb_t ranm;		/* buffer for random bits */
70  unsigned cap_chunksize, chunksize;
71  mp_size_t i;
72
73  /* Set entire result to 111..1  */
74  i = BITS_TO_LIMBS (nbits) - 1;
75  rp[i] = GMP_NUMB_MAX >> (GMP_NUMB_BITS - (nbits % GMP_NUMB_BITS)) % GMP_NUMB_BITS;
76  for (i = i - 1; i >= 0; i--)
77    rp[i] = GMP_NUMB_MAX;
78
79  _gmp_rand (&ranm, rstate, BITS_PER_RANDCALL);
80  cap_chunksize = nbits / (ranm % 4 + 1);
81  cap_chunksize += cap_chunksize == 0; /* make it at least 1 */
82
83  bi = nbits;
84
85  for (;;)
86    {
87      _gmp_rand (&ranm, rstate, BITS_PER_RANDCALL);
88      chunksize = 1 + ranm % cap_chunksize;
89      bi = (bi < chunksize) ? 0 : bi - chunksize;
90
91      if (bi == 0)
92	break;			/* low chunk is ...1 */
93
94      rp[bi / GMP_NUMB_BITS] ^= CNST_LIMB (1) << bi % GMP_NUMB_BITS;
95
96      _gmp_rand (&ranm, rstate, BITS_PER_RANDCALL);
97      chunksize = 1 + ranm % cap_chunksize;
98      bi = (bi < chunksize) ? 0 : bi - chunksize;
99
100      mpn_incr_u (rp + bi / GMP_NUMB_BITS, CNST_LIMB (1) << bi % GMP_NUMB_BITS);
101
102      if (bi == 0)
103	break;			/* low chunk is ...0 */
104    }
105}
106