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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.
 */

/**
 * This class can be used to iterate over blocks of bitmap data.
 *
 * @author Daniel Lemire
 */
public class BufferedIterator implements IteratingRLW, Cloneable {

    /**
     * Instantiates a new iterating buffered running length word.
     *
     * @param iterator iterator
     */
    public BufferedIterator(final CloneableIterator iterator) {
        this.masterIterator = iterator;
        if (this.masterIterator.hasNext()) {
            iteratingBrlw = new IteratingBufferedRunningLengthWord(this.masterIterator.next());
        }
    }

    /**
     * Discard first words, iterating to the next running length word if
     * needed.
     *
     * @param x the number of words to be discarded
     */
    @Override
    public void discardFirstWords(long x) {
        while (x > 0) {
            if (this.iteratingBrlw.getRunningLength() > x) {
                this.iteratingBrlw.discardFirstWords(x);
                return;
            }
            this.iteratingBrlw.discardFirstWords(this.iteratingBrlw.getRunningLength());
            x -= this.iteratingBrlw.getRunningLength();
            long toDiscard = x > this.iteratingBrlw.getNumberOfLiteralWords()
                    ? this.iteratingBrlw.getNumberOfLiteralWords()
                    : x;

            this.iteratingBrlw.discardFirstWords(toDiscard);
            x -= toDiscard;
            if ((x > 0) || (this.iteratingBrlw.size() == 0)) {
                if (!this.next()) {
                    break;
                }
            }
        }
    }

    @Override
    public void discardRunningWords() {
        this.iteratingBrlw.discardRunningWords();
        if (this.iteratingBrlw.getNumberOfLiteralWords() == 0)
            this.next();
    }

    /**
     * Move to the next RunningLengthWord
     *
     * @return whether the move was possible
     */
    @Override
    public boolean next() {
        if (!this.iteratingBrlw.next()) {
            if (!this.masterIterator.hasNext()) {
                return false;
            } else {
                this.iteratingBrlw = new IteratingBufferedRunningLengthWord(this.masterIterator.next());
            }
        }
        return true;
    }

    /**
     * Get the nth literal word for the current running length word
     *
     * @param index zero based index
     * @return the literal word
     */
    @Override
    public long getLiteralWordAt(int index) {
        return this.iteratingBrlw.getLiteralWordAt(index);
    }

    /**
     * Gets the number of literal words for the current running length word.
     *
     * @return the number of literal words
     */
    @Override
    public int getNumberOfLiteralWords() {
        return this.iteratingBrlw.getNumberOfLiteralWords();
    }

    /**
     * Gets the running bit.
     *
     * @return the running bit
     */
    @Override
    public boolean getRunningBit() {
        return this.iteratingBrlw.getRunningBit();
    }

    /**
     * Gets the running length.
     *
     * @return the running length
     */
    @Override
    public long getRunningLength() {
        return this.iteratingBrlw.getRunningLength();
    }

    /**
     * Size in uncompressed words of the current running length word.
     *
     * @return the size
     */
    @Override
    public long size() {
        return this.iteratingBrlw.size();
    }

    @Override
    public BufferedIterator clone() throws CloneNotSupportedException {
        BufferedIterator answer = (BufferedIterator) super.clone();
        answer.iteratingBrlw = this.iteratingBrlw.clone();
        answer.masterIterator = this.masterIterator.clone();
        return answer;
    }

    private IteratingBufferedRunningLengthWord iteratingBrlw;
    private CloneableIterator masterIterator;
}