lib/randperm.c

   1 /* Generate random permutations.
   2
   3    Copyright (C) 2006, 2007 Free Software Foundation, Inc.
   4
   5    This program is free software; you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 2, or (at your option)
   8    any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program; if not, write to the Free Software Foundation,
  17    Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */
  18
  19 /* Written by Paul Eggert.  */
  20
  21 #include <config.h>
  22
  23 #include "randperm.h"
  24
  25 #include <limits.h>
  26
  27 #include "xalloc.h"
  28
  29 /* Return the ceiling of the log base 2 of N.  If N is zero, return
  30    an unspecified value.  */
  31
  32 static size_t
  33 ceil_lg (size_t n)
  34 {
  35   size_t b = 0;
  36   for (n--; n != 0; n /= 2)
  37     b++;
  38   return b;
  39 }
  40
  41 /* Return an upper bound on the number of random bytes needed to
  42    generate the first H elements of a random permutation of N
  43    elements.  H must not exceed N.  */
  44
  45 size_t
  46 randperm_bound (size_t h, size_t n)
  47 {
  48   /* Upper bound on number of bits needed to generate the first number
  49      of the permutation.  */
  50   size_t lg_n = ceil_lg (n);
  51
  52   /* Upper bound on number of bits needed to generated the first H elements.  */
  53   size_t ar = lg_n * h;
  54
  55   /* Convert the bit count to a byte count.  */
  56   size_t bound = (ar + CHAR_BIT - 1) / CHAR_BIT;
  57
  58   return bound;
  59 }
  60
  61 /* From R, allocate and return a malloc'd array of the first H elements
  62    of a random permutation of N elements.  H must not exceed N.
  63    Return NULL if H is zero.  */
  64
  65 size_t *
  66 randperm_new (struct randint_source *r, size_t h, size_t n)
  67 {
  68   size_t *v;
  69
  70   switch (h)
  71     {
  72     case 0:
  73       v = NULL;
  74       break;
  75
  76     case 1:
  77       v = xmalloc (sizeof *v);
  78       v[0] = randint_choose (r, n);
  79       break;
  80
  81     default:
  82       {
  83         size_t i;
  84
  85         v = xnmalloc (n, sizeof *v);
  86         for (i = 0; i < n; i++)
  87           v[i] = i;
  88
  89         for (i = 0; i < h; i++)
  90           {
  91             size_t j = i + randint_choose (r, n - i);
  92             size_t t = v[i];
  93             v[i] = v[j];
  94             v[j] = t;
  95           }
  96
  97         v = xnrealloc (v, h, sizeof *v);
  98       }
  99       break;
 100     }
 101
 102   return v;
 103 }