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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.symmetric;

import com.googlecode.javaewah32.IteratingBufferedRunningLengthWord32;

/**
 * Wrapper around an IteratingBufferedRunningLengthWord used by the
 * RunningBitmapMerge class.
 *
 * @author Daniel Lemire
 * @since 0.8.2
 */
public final class EWAHPointer32 implements Comparable {
    private int endrun;
    private final int pos;
    private boolean isLiteral;
    private boolean value;
    private boolean dead = false;

    /**
     * Underlying iterator
     */
    public final IteratingBufferedRunningLengthWord32 iterator;

    /**
     * Construct a pointer over an IteratingBufferedRunningLengthWord.
     *
     * @param previousEndRun word where the previous run ended
     * @param rw             the iterator
     * @param pos            current position (in word)
     */
    public EWAHPointer32(final int previousEndRun,
                         final IteratingBufferedRunningLengthWord32 rw, final int pos) {
        this.pos = pos;
        this.iterator = rw;
        if (this.iterator.getRunningLength() > 0) {
            this.endrun = previousEndRun
                    + this.iterator.getRunningLength();
            this.isLiteral = false;
            this.value = this.iterator.getRunningBit();
        } else if (this.iterator.getNumberOfLiteralWords() > 0) {
            this.isLiteral = true;
            this.endrun = previousEndRun
                    + this.iterator.getNumberOfLiteralWords();
        } else {
            this.endrun = previousEndRun;
            this.dead = true;
        }
    }

    /**
     * @return the end of the current run
     */
    public int endOfRun() {
        return this.endrun;
    }

    /**
     * @return the beginning of the current run
     */
    public int beginOfRun() {
        if (this.isLiteral)
            return this.endrun
                    - this.iterator.getNumberOfLiteralWords();
        return (this.endrun - this.iterator.getRunningLength());
    }

    /**
     * Process the next run
     */
    public void parseNextRun() {
        if ((this.isLiteral)
                || (this.iterator.getNumberOfLiteralWords() == 0)) {
            // no choice, must load next runs
            this.iterator.discardFirstWords(this.iterator.size());
            if (this.iterator.getRunningLength() > 0) {
                this.endrun += this.iterator
                        .getRunningLength();
                this.isLiteral = false;
                this.value = this.iterator.getRunningBit();
            } else if (this.iterator.getNumberOfLiteralWords() > 0) {
                this.isLiteral = true;
                this.endrun += this.iterator
                        .getNumberOfLiteralWords();
            } else {
                this.dead = true;
            }

        } else {
            this.isLiteral = true;
            this.endrun += this.iterator.getNumberOfLiteralWords();
        }

    }

    /**
     * @return true if there is no more data
     */
    public boolean hasNoData() {
        return this.dead;
    }

    /**
     * @param f call the function with the current information
     */
    public void callbackUpdate(final UpdateableBitmapFunction32 f) {
        if (this.dead)
            f.setZero(this.pos);
        else if (this.isLiteral)
            f.setLiteral(this.pos);
        else if (this.value)
            f.setOne(this.pos);
        else
            f.setZero(this.pos);
    }

    @Override
    public int compareTo(EWAHPointer32 other) {
        return this.endrun - other.endrun;
    }
}