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/* Generate random permutations.
Copyright (C) 2006 Free Software Foundation, Inc.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software Foundation,
Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */
/* Written by Paul Eggert. */
#include <config.h>
#include "randperm.h"
#include <limits.h>
#include "xalloc.h"
/* Return the ceiling of the log base 2 of N. If N is zero, return
an unspecified value. */
static size_t
ceil_lg (size_t n)
{
size_t b = 0;
for (n--; n != 0; n /= 2)
b++;
return b;
}
/* Return an upper bound on the number of random bytes needed to
generate the first H elements of a random permutation of N
elements. H must not exceed N. */
size_t
randperm_bound (size_t h, size_t n)
{
/* Upper bound on number of bits needed to generate the first number
of the permutation. */
size_t lg_n = ceil_lg (n);
/* Upper bound on number of bits needed to generated the first H elements. */
size_t ar = lg_n * h;
/* Convert the bit count to a byte count. */
size_t bound = (ar + CHAR_BIT - 1) / CHAR_BIT;
return bound;
}
/* From R, allocate and return the first H elements of a random
permutation of N elements. H must not exceed N. Return NULL if H
is zero. */
size_t *
randperm_new (struct randint_source *r, size_t h, size_t n)
{
size_t *v;
switch (h)
{
case 0:
v = NULL;
break;
case 1:
v = xmalloc (sizeof *v);
v[0] = randint_choose (r, n);
break;
default:
{
size_t i;
v = xnmalloc (n, sizeof *v);
for (i = 0; i < n; i++)
v[i] = i;
for (i = 0; i < h; i++)
{
size_t j = i + randint_choose (r, n - i);
size_t t = v[i];
v[i] = v[j];
v[j] = t;
}
v = xnrealloc (v, h, sizeof *v);
}
break;
}
return v;
}
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