contrib/gmp/mpn/generic/random2.c

   1 /* mpn_random2 -- Generate random numbers with relatively long strings
   2    of ones and zeroes.  Suitable for border testing.
   3
   4 Copyright 1992, 1993, 1994, 1996, 2000, 2001, 2002, 2004 Free Software
   5 Foundation, Inc.
   6
   7 This file is part of the GNU MP Library.
   8
   9 The GNU MP Library is free software; you can redistribute it and/or modify
  10 it under the terms of the GNU Lesser General Public License as published by
  11 the Free Software Foundation; either version 3 of the License, or (at your
  12 option) any later version.
  13
  14 The GNU MP Library is distributed in the hope that it will be useful, but
  15 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  16 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  17 License for more details.
  18
  19 You should have received a copy of the GNU Lesser General Public License
  20 along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
  21
  22 #include "gmp.h"
  23 #include "gmp-impl.h"
  24
  25 static void gmp_rrandomb __GMP_PROTO ((mp_ptr, gmp_randstate_t, unsigned long int));
  26
  27 /* Ask _gmp_rand for 32 bits per call unless that's more than a limb can hold.
  28    Thus, we get the same random number sequence in the common cases.
  29    FIXME: We should always generate the same random number sequence!  */
  30 #if GMP_NUMB_BITS < 32
  31 #define BITS_PER_RANDCALL GMP_NUMB_BITS
  32 #else
  33 #define BITS_PER_RANDCALL 32
  34 #endif
  35
  36 void
  37 mpn_random2 (mp_ptr rp, mp_size_t n)
  38 {
  39   gmp_randstate_ptr rstate = RANDS;
  40   int bit_pos;                  /* bit number of least significant bit where
  41                                    next bit field to be inserted */
  42   mp_limb_t ran, ranm;          /* buffer for random bits */
  43
  44   /* FIXME: Is n==0 supposed to be allowed? */
  45   ASSERT (n >= 0);
  46
  47   _gmp_rand (&ranm, rstate, BITS_PER_RANDCALL);
  48   ran = ranm;
  49
  50   /* Start off at a random bit position in the most significant limb.  */
  51   bit_pos = ran % GMP_NUMB_BITS;
  52
  53   gmp_rrandomb (rp, rstate, n * GMP_NUMB_BITS - bit_pos);
  54 }
  55
  56 static void
  57 gmp_rrandomb (mp_ptr rp, gmp_randstate_t rstate, unsigned long int nbits)
  58 {
  59   unsigned long int bi;
  60   mp_limb_t ranm;               /* buffer for random bits */
  61   unsigned cap_chunksize, chunksize;
  62   mp_size_t i;
  63
  64   /* Set entire result to 111..1  */
  65   i = (nbits + GMP_NUMB_BITS - 1) / GMP_NUMB_BITS - 1;
  66   rp[i] = GMP_NUMB_MAX >> (GMP_NUMB_BITS - (nbits % GMP_NUMB_BITS)) % GMP_NUMB_BITS;
  67   for (i = i - 1; i >= 0; i--)
  68     rp[i] = GMP_NUMB_MAX;
  69
  70   _gmp_rand (&ranm, rstate, BITS_PER_RANDCALL);
  71   cap_chunksize = nbits / (ranm % 4 + 1);
  72   cap_chunksize += cap_chunksize == 0; /* make it at least 1 */
  73
  74   bi = nbits;
  75
  76   for (;;)
  77     {
  78       _gmp_rand (&ranm, rstate, BITS_PER_RANDCALL);
  79       chunksize = 1 + ranm % cap_chunksize;
  80       bi = (bi < chunksize) ? 0 : bi - chunksize;
  81
  82       if (bi == 0)
  83         break;                  /* low chunk is ...1 */
  84
  85       rp[bi / GMP_NUMB_BITS] ^= CNST_LIMB (1) << bi % GMP_NUMB_BITS;
  86
  87       _gmp_rand (&ranm, rstate, BITS_PER_RANDCALL);
  88       chunksize = 1 + ranm % cap_chunksize;
  89       bi = (bi < chunksize) ? 0 : bi - chunksize;
  90
  91       mpn_incr_u (rp + bi / GMP_NUMB_BITS, CNST_LIMB (1) << bi % GMP_NUMB_BITS);
  92
  93       if (bi == 0)
  94         break;                  /* low chunk is ...0 */
  95     }
  96 }