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MG4J (Managing Gigabytes for Java) is a free full-text search engine for large document collections written in Java. The big version is a fork of the original MG4J that can handle more than 2^31 terms and documents.
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package it.unimi.dsi.big.mg4j.search;

/*		 
 * MG4J: Managing Gigabytes for Java (big)
 *
 * Copyright (C) 2003-2011 Paolo Boldi and Sebastiano Vigna 
 *
 *  This library is free software; you can redistribute it and/or modify it
 *  under the terms of the GNU Lesser General Public License as published by the Free
 *  Software Foundation; either version 3 of the License, or (at your option)
 *  any later version.
 *
 *  This library 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 Lesser General Public License
 *  for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public License
 *  along with this program; if not, see .
 *
 */


import it.unimi.dsi.fastutil.longs.LongSet;
import it.unimi.dsi.fastutil.objects.AbstractObjectIterator;
import it.unimi.dsi.big.mg4j.index.IndexIterator;
import it.unimi.dsi.util.Interval;
import it.unimi.dsi.util.Intervals;

import java.util.Collections;
import java.util.NoSuchElementException;
                                                                                                                                                           
/** A class providing static methods and objects that do useful things with interval iterators. 
 * 
 */

public class IntervalIterators {
	private IntervalIterators() {}
	
	/** An iterator that throws an exception on all advancing method calls, except for {@link #hasNext()},
	 * which has a settable value. The extent is 0.
	 * 
	 * The only purpose of this class is to instantiate singleton iterators
	 * such as {@link #TRUE} and {@link #FALSE}.
	 */

	protected static class FakeIterator extends AbstractObjectIterator implements IntervalIterator {
		final boolean hasNext;
		private FakeIterator( final boolean hasNext ) { this.hasNext = hasNext; }
		public boolean hasNext() { return hasNext; }
		public void reset() {}
		public Interval next() { throw hasNext ? new UnsupportedOperationException() : new NoSuchElementException(); }
		public Interval nextInterval() { if ( ! hasNext ) new UnsupportedOperationException(); return null; }
		public int extent() { return 0; } // TODO: this is not correct for FALSE (but it is unlikely to be a problem)
		public String toString() { return this.getClass().getName() + "." + ( hasNext ? "TRUE" : "FALSE" ); }
		public void intervalTerms( final LongSet terms ) {} 
	}
                                                                 
	/** A singleton iterator representing maximum truth.
	 * 
	 * 
This iterator is a placeholder for an iterator returning just {@link Intervals#EMPTY_INTERVAL}.
	 * The antichain formed by the empty interval is the top element of the lattice of antichains, and
	 * thus represents the highest truth. Since, however, EMPTY_INTERVAL
	 * is a singleton that slightly violates the {@link Interval} invariants, an iterator actually
	 * returning EMPTY_INTERVAL would cause severe problems in all algorithms manipulating
	 * intervals. Rather, {@link #TRUE} is treated separately and is never actually used in
	 * an algorithm on interval antichains (also because, albeit it claims to have elements,
	 * it will return null on {@link IntervalIterator#nextInterval()}).
	 * 
	 * 
A most natural appearance of {@link #TRUE} is due to negation: all documents satisfying
	 * a negative query return {@link #TRUE} as interval iterator.
	 * 
	 * 
Finally, an {@link IndexIterator} by convention returns {@link #TRUE} 
	 * when {@link DocumentIterator#intervalIterator(it.unimi.dsi.big.mg4j.index.Index)} 
	 * is called with an argument that is not the {@linkplain IndexIterator#index() key index}.
	 * The idea is that there is no witness to be returned, but the query is nonetheless true.
	 */

	public final static IntervalIterator TRUE = new FakeIterator( true );
	
	/** A singleton empty interval iterator.
	 * 
	 * The main usefulness of this iterator is as a singleton: in some circumstances you have
	 * to return an empty iterator, and since it is by definition stateless, it is a pity
	 * to create a new object (the same considerations led to {@link Collections#emptySet()}).
	 */
	public final static IntervalIterator FALSE = new FakeIterator( false );

}