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The bit array data structure is implemented in Java as the BitSet class. Unfortunately, this fails to scale without compression. JavaEWAH is a word-aligned compressed variant of the Java bitset class. It uses a 64-bit run-length encoding (RLE) compression scheme. The goal of word-aligned compression is not to achieve the best compression, but rather to improve query processing time. Hence, we try to save CPU cycles, maybe at the expense of storage. However, the EWAH scheme we implemented is always more efficient storage-wise than an uncompressed bitmap (implemented in Java as the BitSet class). Unlike some alternatives, javaewah does not rely on a patented scheme.

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package com.googlecode.javaewah;

/*
 * Copyright 2009-2016, Daniel Lemire, Cliff Moon, David McIntosh, Robert Becho, Google Inc., Veronika Zenz, Owen Kaser, Gregory Ssi-Yan-Kai, Rory Graves
 * Licensed under the Apache License, Version 2.0.
 */

/**
 * Like a standard Java iterator, except that you can clone it.
 *
 * @param  the data type of the iterator
 */
public interface CloneableIterator extends Cloneable {

    /**
     * @return whether there is more
     */
    boolean hasNext();

    /**
     * @return the next element
     */
    E next();

    /**
     * @return a copy
     * @throws CloneNotSupportedException this should never happen in practice
     */
    CloneableIterator clone() throws CloneNotSupportedException;

}