source/libs/gmp/gmp-src/rand/randmui.c

   1 /* gmp_urandomm_ui -- uniform random number 0 to N-1 for ulong N.
   2
   3 Copyright 2003, 2004 Free Software Foundation, Inc.
   4
   5 This file is part of the GNU MP Library.
   6
   7 The GNU MP Library is free software; you can redistribute it and/or modify
   8 it under the terms of either:
   9
  10   * the GNU Lesser General Public License as published by the Free
  11     Software Foundation; either version 3 of the License, or (at your
  12     option) any later version.
  13
  14 or
  15
  16   * the GNU General Public License as published by the Free Software
  17     Foundation; either version 2 of the License, or (at your option) any
  18     later version.
  19
  20 or both in parallel, as here.
  21
  22 The GNU MP Library is distributed in the hope that it will be useful, but
  23 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  24 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  25 for more details.
  26
  27 You should have received copies of the GNU General Public License and the
  28 GNU Lesser General Public License along with the GNU MP Library.  If not,
  29 see https://www.gnu.org/licenses/.  */
  30
  31 #include "gmp.h"
  32 #include "gmp-impl.h"
  33 #include "longlong.h"
  34
  35
  36 /* If n is a power of 2 then the test ret<n is always true and the loop is
  37    unnecessary, but there's no need to add special code for this.  Just get
  38    the "bits" calculation correct and let it go through normally.
  39
  40    If n is 1 then will have bits==0 and _gmp_rand will produce no output and
  41    we always return 0.  Again there seems no need for a special case, just
  42    initialize a[0]=0 and let it go through normally.  */
  43
  44 #define MAX_URANDOMM_ITER  80
  45
  46 unsigned long
  47 gmp_urandomm_ui (gmp_randstate_ptr rstate, unsigned long n)
  48 {
  49   mp_limb_t      a[LIMBS_PER_ULONG];
  50   unsigned long  ret, bits, leading;
  51   int            i;
  52
  53   if (UNLIKELY (n == 0))
  54     DIVIDE_BY_ZERO;
  55
  56   /* start with zeros, since if bits==0 then _gmp_rand will store nothing at
  57      all (bits==0 arises when n==1), or if bits <= GMP_NUMB_BITS then it
  58      will store only a[0].  */
  59   a[0] = 0;
  60 #if LIMBS_PER_ULONG > 1
  61   a[1] = 0;
  62 #endif
  63
  64   count_leading_zeros (leading, (mp_limb_t) n);
  65   bits = GMP_LIMB_BITS - leading - (POW2_P(n) != 0);
  66
  67   for (i = 0; i < MAX_URANDOMM_ITER; i++)
  68     {
  69       _gmp_rand (a, rstate, bits);
  70 #if LIMBS_PER_ULONG == 1
  71       ret = a[0];
  72 #else
  73       ret = a[0] | (a[1] << GMP_NUMB_BITS);
  74 #endif
  75       if (LIKELY (ret < n))   /* usually one iteration suffices */
  76         goto done;
  77     }
  78
  79   /* Too many iterations, there must be something degenerate about the
  80      rstate algorithm.  Return r%n.  */
  81   ret -= n;
  82   ASSERT (ret < n);
  83
  84  done:
  85   return ret;
  86 }