//******************************************************************************
//
// File: RandomSample.java
// Package: edu.rit.util
// Unit: Class edu.rit.util.RandomSample
//
// This Java source file is copyright (C) 2010 by Alan Kaminsky. All rights
// reserved. For further information, contact the author, Alan Kaminsky, at
// ark@cs.rit.edu.
//
// This Java source file is part of the Parallel Java Library ("PJ"). PJ 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 3 of the License, or (at your option) any later version.
//
// PJ 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.
//
// A copy of the GNU General Public License is provided in the file gpl.txt. You
// may also obtain a copy of the GNU General Public License on the World Wide
// Web at http://www.gnu.org/licenses/gpl.html.
//
//******************************************************************************
package edu.rit.util;
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* Class RandomSample provides objects that generate random samples from
* discrete sets.
*
* @author Alan Kaminsky
* @version 15-Feb-2010
*/
public class RandomSample
{
// Prevent construction.
private RandomSample()
{
}
// Exported operations.
/**
* Create an iterator that generates a random sample of ints
* without replacement. The set to be sampled consists of the integers from
* 0 through N−1 inclusive. The sample consists of n
* items. Each item in the set is equally likely to occur in the sample. The
* iterator returns the items in the sample in ascending order. The iterator
* uses the given prng to sample items at random. For each item
* returned, the iterator consumes one random number from the prng.
*
* As a special case, if n ≥ N, then the iterator returns
* all items in the set in ascending order, and the iterator does not use
* the prng.
*
* The iterator uses the algorithm in A. Bissell, Ordered random selection
* without replacement, Applied Statistics, 35(1):73-75, 1986.
*
* @param prng Pseudorandom number generator.
* @param n Number of items in the sample.
* @param N Number of items in the set.
*
* @return Iterator over the sample.
*
* @exception IllegalArgumentException
* (unchecked exception) Thrown if n < 0 or N < 0.
*/
public static Iterator withoutReplacement
(final Random prng,
final int n,
final int N)
{
if (n < 0)
{
throw new IllegalArgumentException
("RandomSample.withoutReplacement(): n < 0 illegal");
}
else if (N < 0)
{
throw new IllegalArgumentException
("RandomSample.withoutReplacement(): N < 0 illegal");
}
else if (n < N)
{
return new Iterator()
{
private int i = 0;
private int M = N;
private int r = N - n;
public boolean hasNext()
{
return i < n;
}
public Integer next()
{
if (i >= n) throw new NoSuchElementException();
++ i;
double x = prng.nextDouble();
double p = 1.0;
for (;;)
{
p = p*r/M;
if (p <= x)
{
int result = N - M;
-- M;
return result;
}
else
{
-- M;
-- r;
}
}
}
public void remove()
{
throw new UnsupportedOperationException();
}
};
}
else
{
return new Iterator()
{
private int i = 0;
public boolean hasNext()
{
return i < N;
}
public Integer next()
{
if (i >= N) throw new NoSuchElementException();
return i ++;
}
public void remove()
{
throw new UnsupportedOperationException();
}
};
}
}
/**
* Generate a random sample of ints without replacement. The set to
* be sampled consists of the integers from 0 through N−1
* inclusive. The sample consists of n items. Each item in the set is
* equally likely to occur in the sample. The items in the sample are stored
* in ascending order in the given buf starting at index 0. The
* iterator uses the given prng to sample items at random. For each
* item sampled, the iterator consumes one random number from the
* prng.
*
* As a special case, if n ≥ N, then all items in the set
* are stored in ascending order in the given buf starting at index
* 0, and the iterator does not use the prng.
*
* This method uses the algorithm in A. Bissell, Ordered random selection
* without replacement, Applied Statistics, 35(1):73-75, 1986.
*
* @param prng Pseudorandom number generator.
* @param n Number of items in the sample.
* @param N Number of items in the set.
* @param buf Array in which to store the sampled items.
*
* @return Number of sampled items actually stored in buf.
*
* @exception IllegalArgumentException
* (unchecked exception) Thrown if n < 0 or N < 0.
* @exception NullPointerException
* (unchecked exception) Thrown if buf is null.
* @exception IndexOutOfBoundsException
* (unchecked exception) Thrown if buf.length < n.
*/
public static int withoutReplacement
(Random prng,
int n,
int N,
int[] buf)
{
if (n < 0)
{
throw new IllegalArgumentException
("RandomSample.withoutReplacement(): n < 0 illegal");
}
else if (N < 0)
{
throw new IllegalArgumentException
("RandomSample.withoutReplacement(): N < 0 illegal");
}
else if (n < N)
{
int M = N;
int r = N - n;
for (int i = 0; i < n; ++ i)
{
double x = prng.nextDouble();
double p = 1.0;
for (;;)
{
p = p*r/M;
if (p <= x)
{
buf[i] = N - M;
-- M;
break;
}
else
{
-- M;
-- r;
}
}
}
return n;
}
else
{
for (int i = 0; i < N; ++ i)
{
buf[i] = i;
}
return N;
}
}
/**
* Create an iterator that generates a random sample of longs
* without replacement. The set to be sampled consists of the integers from
* 0 through N−1 inclusive. The sample consists of n
* items. Each item in the set is equally likely to occur in the sample. The
* iterator returns the items in the sample in ascending order. The iterator
* uses the given prng to sample items at random. For each item
* returned, the iterator consumes one random number from the prng.
*
* As a special case, if n ≥ N, then the iterator returns
* all items in the set in ascending order, and the iterator does not use
* the prng.
*
* The iterator uses the algorithm in A. Bissell, Ordered random selection
* without replacement, Applied Statistics, 35(1):73-75, 1986.
*
* @param prng Pseudorandom number generator.
* @param n Number of items in the sample.
* @param N Number of items in the set.
*
* @return Iterator over the sample.
*
* @exception IllegalArgumentException
* (unchecked exception) Thrown if n < 0 or N < 0.
*/
public static Iterator withoutReplacement
(final Random prng,
final int n,
final long N)
{
if (n < 0)
{
throw new IllegalArgumentException
("RandomSample.withoutReplacement(): n < 0 illegal");
}
else if (N < 0L)
{
throw new IllegalArgumentException
("RandomSample.withoutReplacement(): N < 0 illegal");
}
else if (n < N)
{
return new Iterator()
{
private int i = 0;
private long M = N;
private long r = N - n;
public boolean hasNext()
{
return i < n;
}
public Long next()
{
if (i >= n) throw new NoSuchElementException();
++ i;
double x = prng.nextDouble();
double p = 1.0;
for (;;)
{
p = p*r/M;
if (p <= x)
{
long result = N - M;
-- M;
return result;
}
else
{
-- M;
-- r;
}
}
}
public void remove()
{
throw new UnsupportedOperationException();
}
};
}
else
{
return new Iterator()
{
private long i = 0L;
public boolean hasNext()
{
return i < N;
}
public Long next()
{
if (i >= N) throw new NoSuchElementException();
return i ++;
}
public void remove()
{
throw new UnsupportedOperationException();
}
};
}
}
/**
* Generate a random sample of longs without replacement. The set
* to be sampled consists of the integers from 0 through N−1
* inclusive. The sample consists of n items. Each item in the set is
* equally likely to occur in the sample. The items in the sample are stored
* in ascending order in the given buf starting at index 0. The
* iterator uses the given prng to sample items at random. For each
* item sampled, the iterator consumes one random number from the
* prng.
*
* As a special case, if n ≥ N, then all items in the set
* are stored in ascending order in the given buf starting at index
* 0, and the iterator does not use the prng.
*
* This method uses the algorithm in A. Bissell, Ordered random selection
* without replacement, Applied Statistics, 35(1):73-75, 1986.
*
* @param prng Pseudorandom number generator.
* @param n Number of items in the sample.
* @param N Number of items in the set.
* @param buf Array in which to store the sampled items.
*
* @return Number of sampled items actually stored in buf.
*
* @exception IllegalArgumentException
* (unchecked exception) Thrown if n < 0 or N < 0.
* @exception NullPointerException
* (unchecked exception) Thrown if buf is null.
* @exception IndexOutOfBoundsException
* (unchecked exception) Thrown if buf.length < n.
*/
public static int withoutReplacement
(Random prng,
int n,
long N,
long[] buf)
{
if (n < 0)
{
throw new IllegalArgumentException
("RandomSample.withoutReplacement(): n < 0 illegal");
}
else if (N < 0)
{
throw new IllegalArgumentException
("RandomSample.withoutReplacement(): N < 0 illegal");
}
else if (n < N)
{
long M = N;
long r = N - n;
for (int i = 0; i < n; ++ i)
{
double x = prng.nextDouble();
double p = 1.0;
for (;;)
{
p = p*r/M;
if (p <= x)
{
buf[i] = N - M;
-- M;
break;
}
else
{
-- M;
-- r;
}
}
}
return n;
}
else
{
for (int i = 0; i < N; ++ i)
{
buf[i] = i;
}
return (int) N;
}
}
// Unit test main program.
// /**
// * Unit test main program.
// */
// public static void main
// (String[] args)
// {
// if (args.length != 3)
// {
// System.err.println ("Usage: java edu.rit.util.RandomSample ");
// System.exit (1);
// }
// int n = Integer.parseInt (args[0]);
// int N = Integer.parseInt (args[1]);
// long seed = Long.parseLong (args[2]);
// Random prng = Random.getInstance (seed);
// Iterator iter = withoutReplacement (prng, n, N);
// while (iter.hasNext()) System.out.printf (" %d", iter.next());
// System.out.println();
// }
}
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