contrib/gmp/mpz/urandomm.c

   1 /* mpz_urandomm (rop, state, n) -- Generate a uniform pseudorandom
   2    integer in the range 0 to N-1, using STATE as the random state
   3    previously initialized by a call to gmp_randinit().
   4
   5 Copyright 2000, 2002  Free Software 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 #include "longlong.h" /* for count_leading_zeros */
  25
  26
  27 #define MAX_URANDOMM_ITER  80
  28
  29 void
  30 mpz_urandomm (mpz_ptr rop, gmp_randstate_t rstate, mpz_srcptr n)
  31 {
  32   mp_ptr rp, np, nlast;
  33   mp_size_t nbits, size;
  34   int count;
  35   int pow2;
  36   int cmp;
  37   TMP_DECL;
  38
  39   size = ABSIZ (n);
  40   if (size == 0)
  41     DIVIDE_BY_ZERO;
  42
  43   nlast = &PTR (n)[size - 1];
  44
  45   /* Detect whether n is a power of 2.  */
  46   pow2 = POW2_P (*nlast);
  47   if (pow2 != 0)
  48     for (np = PTR (n); np < nlast; np++)
  49       if (*np != 0)
  50         {
  51           pow2 = 0;             /* Mark n as `not a power of two'.  */
  52           break;
  53         }
  54
  55   count_leading_zeros (count, *nlast);
  56   nbits = size * GMP_NUMB_BITS - (count - GMP_NAIL_BITS) - pow2;
  57   if (nbits == 0)               /* nbits == 0 means that n was == 1.  */
  58     {
  59       SIZ (rop) = 0;
  60       return;
  61     }
  62
  63   TMP_MARK;
  64   np = PTR (n);
  65   if (rop == n)
  66     {
  67       mp_ptr tp;
  68       tp = TMP_ALLOC_LIMBS (size);
  69       MPN_COPY (tp, np, size);
  70       np = tp;
  71     }
  72
  73   /* Here the allocated size can be one too much if n is a power of
  74      (2^GMP_NUMB_BITS) but it's convenient for using mpn_cmp below.  */
  75   rp = MPZ_REALLOC (rop, size);
  76   /* Clear last limb to prevent the case in which size is one too much.  */
  77   rp[size - 1] = 0;
  78
  79   count = MAX_URANDOMM_ITER;    /* Set iteration count limit.  */
  80   do
  81     {
  82       _gmp_rand (rp, rstate, nbits);
  83       MPN_CMP (cmp, rp, np, size);
  84     }
  85   while (cmp >= 0 && --count != 0);
  86
  87   if (count == 0)
  88     /* Too many iterations; return result mod n == result - n */
  89     mpn_sub_n (rp, rp, np, size);
  90
  91   MPN_NORMALIZE (rp, size);
  92   SIZ (rop) = size;
  93   TMP_FREE;
  94 }