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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.javaewah32;

/*
 * 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.
 */

/**
 * High-level iterator over a compressed bitmap.
 * 
 */
public interface IteratingRLW32 {
    /**
     * @return whether there is more
     */
    boolean next();

    /**
     * @param index where the literal word is
     * @return the literal word at the given index.
     */
    int getLiteralWordAt(int index);

    /**
     * @return the number of literal (non-fill) words
     */
    int getNumberOfLiteralWords();

    /**
     * @return the bit used for the fill bits
     */
    boolean getRunningBit();

    /**
     * @return sum of getRunningLength() and getNumberOfLiteralWords()
     */
    int size();

    /**
     * @return length of the run of fill words
     */
    int getRunningLength();

    /**
     * @param x the number of words to discard
     */
    void discardFirstWords(int x);

    /**
     * Discard all running words
     */
    void discardRunningWords();

    /**
     * @return a copy of the iterator
     * @throws CloneNotSupportedException this should not be thrown in theory
     */
    IteratingRLW32 clone() throws CloneNotSupportedException;
}