org.jgrapht.util.ElementsSequenceGenerator Maven / Gradle / Ivy
/*
* (C) Copyright 2020-2021, by Timofey Chudakov and Contributors.
*
* JGraphT : a free Java graph-theory library
*
* See the CONTRIBUTORS.md file distributed with this work for additional
* information regarding copyright ownership.
*
* This program and the accompanying materials are made available under the
* terms of the Eclipse Public License 2.0 which is available at
* http://www.eclipse.org/legal/epl-2.0, or the
* GNU Lesser General Public License v2.1 or later
* which is available at
* http://www.gnu.org/licenses/old-licenses/lgpl-2.1-standalone.html.
*
* SPDX-License-Identifier: EPL-2.0 OR LGPL-2.1-or-later
*/
package org.jgrapht.util;
import java.util.*;
/**
* Generates elements from the input collection in random order.
*
* An element can be generated only once. After all elements have been generated, this generator
* halts. At every step, an element is generated uniformly at random, which means that every element
* has an equal probability to be generated. This implementation is based on the
* Fisher-Yates algorithm.
* The generator is unbiased meaning the every permutation is equally likely.
*
* @param element type
* @author Timofey Chudakov
*/
public class ElementsSequenceGenerator
implements
Iterator,
Iterable
{
/**
* Input elements ordered as a list. This list is being decreased in size as the elements are
* generated.
*/
private List elements;
/**
* Random instance used by this generator.
*/
private Random rng;
/**
* Constructs a new {@link ElementsSequenceGenerator}.
*
* @param elements a collection of elements to generate elements from.
*/
public ElementsSequenceGenerator(Collection elements)
{
this(elements, System.nanoTime());
}
/**
* Constructs a new {@link ElementsSequenceGenerator} using the specified {@code seed}. Two
* different generators with the same seed will produce identical sequences given that the same
* collection of elements is provided.
*
* @param elements a collection of elements to generate elements from.
* @param seed a seed for the random number generator
*/
public ElementsSequenceGenerator(Collection elements, long seed)
{
this(elements, new Random(seed));
}
/**
* Constructs a new {@link ElementsSequenceGenerator} using the specified random number
* generator {@code rng}. Two different generators will produce identical sequences from a
* collection of elements given that the random number generator produces the same sequence of
* numbers.
*
* @param elements a collection of elements to generate elements from.
* @param rng a random number generator
*/
public ElementsSequenceGenerator(Collection elements, Random rng)
{
this.elements = new ArrayList<>(elements);
this.rng = rng;
}
/**
* {@inheritDoc}
*/
@Override
public boolean hasNext()
{
return !elements.isEmpty();
}
/**
* {@inheritDoc}
*/
@Override
public T next()
{
if (!hasNext()) {
throw new NoSuchElementException();
}
int index = rng.nextInt(elements.size());
T result = elements.get(index);
elements.set(index, elements.get(elements.size() - 1));
elements.remove(elements.size() - 1);
return result;
}
/**
* {@inheritDoc}
*/
@Override
public Iterator iterator()
{
return this;
}
}