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