contrib/mpfr/src/urandom.c

   1 /* mpfr_urandom (rop, state, rnd_mode) -- Generate a uniform pseudorandom
   2    real number between 0 and 1 (exclusive) and round it to the precision of rop
   3    according to the given rounding mode.
   4
   5 Copyright 2000, 2001, 2002, 2003, 2004, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
   6 Contributed by the AriC and Caramel projects, INRIA.
   7
   8 This file is part of the GNU MPFR Library.
   9
  10 The GNU MPFR Library is free software; you can redistribute it and/or modify
  11 it under the terms of the GNU Lesser General Public License as published by
  12 the Free Software Foundation; either version 3 of the License, or (at your
  13 option) any later version.
  14
  15 The GNU MPFR Library is distributed in the hope that it will be useful, but
  16 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  17 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  18 License for more details.
  19
  20 You should have received a copy of the GNU Lesser General Public License
  21 along with the GNU MPFR Library; see the file COPYING.LESSER.  If not, see
  22 http://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
  23 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
  24
  25
  26 #define MPFR_NEED_LONGLONG_H
  27 #include "mpfr-impl.h"
  28
  29
  30 /* generate one random bit */
  31 static int
  32 random_rounding_bit (gmp_randstate_t rstate)
  33 {
  34   mp_limb_t r;
  35
  36   mpfr_rand_raw (&r, rstate, 1);
  37   return r & MPFR_LIMB_ONE;
  38 }
  39
  40
  41 int
  42 mpfr_urandom (mpfr_ptr rop, gmp_randstate_t rstate, mpfr_rnd_t rnd_mode)
  43 {
  44   mpfr_limb_ptr rp;
  45   mpfr_prec_t nbits;
  46   mp_size_t nlimbs;
  47   mp_size_t n;
  48   mpfr_exp_t exp;
  49   mpfr_exp_t emin;
  50   int cnt;
  51   int inex;
  52
  53   rp = MPFR_MANT (rop);
  54   nbits = MPFR_PREC (rop);
  55   nlimbs = MPFR_LIMB_SIZE (rop);
  56   MPFR_SET_POS (rop);
  57   exp = 0;
  58   emin = mpfr_get_emin ();
  59   if (MPFR_UNLIKELY (emin > 0))
  60     {
  61       if (rnd_mode == MPFR_RNDU || rnd_mode == MPFR_RNDA
  62           || (emin == 1 && rnd_mode == MPFR_RNDN
  63               && random_rounding_bit (rstate)))
  64         {
  65           mpfr_set_ui_2exp (rop, 1, emin - 1, rnd_mode);
  66           return +1;
  67         }
  68       else
  69         {
  70           MPFR_SET_ZERO (rop);
  71           return -1;
  72         }
  73     }
  74
  75   /* Exponent */
  76 #define DRAW_BITS 8 /* we draw DRAW_BITS at a time */
  77   cnt = DRAW_BITS;
  78   MPFR_ASSERTN(DRAW_BITS <= GMP_NUMB_BITS);
  79   while (cnt == DRAW_BITS)
  80     {
  81       /* generate DRAW_BITS in rp[0] */
  82       mpfr_rand_raw (rp, rstate, DRAW_BITS);
  83       if (MPFR_UNLIKELY (rp[0] == 0))
  84         cnt = DRAW_BITS;
  85       else
  86         {
  87           count_leading_zeros (cnt, rp[0]);
  88           cnt -= GMP_NUMB_BITS - DRAW_BITS;
  89         }
  90
  91       if (MPFR_UNLIKELY (exp < emin + cnt))
  92         {
  93           /* To get here, we have been drawing more than -emin zeros
  94              in a row, then return 0 or the smallest representable
  95              positive number.
  96
  97              The rounding to nearest mode is subtle:
  98              If exp - cnt == emin - 1, the rounding bit is set, except
  99              if cnt == DRAW_BITS in which case the rounding bit is
 100              outside rp[0] and must be generated. */
 101           if (rnd_mode == MPFR_RNDU || rnd_mode == MPFR_RNDA
 102               || (rnd_mode == MPFR_RNDN && cnt == exp - emin - 1
 103                   && (cnt != DRAW_BITS || random_rounding_bit (rstate))))
 104             {
 105               mpfr_set_ui_2exp (rop, 1, emin - 1, rnd_mode);
 106               return +1;
 107             }
 108           else
 109             {
 110               MPFR_SET_ZERO (rop);
 111               return -1;
 112             }
 113         }
 114       exp -= cnt;
 115     }
 116   MPFR_EXP (rop) = exp; /* Warning: may be outside the current
 117                            exponent range */
 118
 119
 120   /* Significand: we need generate only nbits-1 bits, since the most
 121      significant is 1 */
 122   mpfr_rand_raw (rp, rstate, nbits - 1);
 123   n = nlimbs * GMP_NUMB_BITS - nbits;
 124   if (MPFR_LIKELY (n != 0)) /* this will put the low bits to zero */
 125     mpn_lshift (rp, rp, nlimbs, n);
 126
 127   /* Set the msb to 1 since it was fixed by the exponent choice */
 128   rp[nlimbs - 1] |= MPFR_LIMB_HIGHBIT;
 129
 130   /* Rounding */
 131   if (rnd_mode == MPFR_RNDU || rnd_mode == MPFR_RNDA
 132       || (rnd_mode == MPFR_RNDN && random_rounding_bit (rstate)))
 133     {
 134       /* Take care of the exponent range: it may have been reduced */
 135       if (exp < emin)
 136         mpfr_set_ui_2exp (rop, 1, emin - 1, rnd_mode);
 137       else if (exp > mpfr_get_emax ())
 138         mpfr_set_inf (rop, +1); /* overflow, flag set by mpfr_check_range */
 139       else
 140         mpfr_nextabove (rop);
 141       inex = +1;
 142     }
 143   else
 144     inex = -1;
 145
 146   return mpfr_check_range (rop, inex, rnd_mode);
 147 }