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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 2012, Google Inc.
 * Licensed under the Apache License, Version 2.0.
 */

import static com.googlecode.javaewah.EWAHCompressedBitmap.WORD_IN_BITS;

/**
 * The IntIteratorImpl is the 64 bit implementation of the IntIterator
 * interface, which efficiently returns the stream of integers represented by an
 * EWAHIterator.
 *
 * @author Colby Ranger
 * @since 0.5.6
 */
final class IntIteratorImpl implements IntIterator {

    private final EWAHIterator ewahIter;
    private final Buffer buffer;
    private int position;
    private int runningLength;
    private long word;
    private int wordPosition;
    private int wordLength;
    private int literalPosition;
    private boolean hasNext;

    IntIteratorImpl(EWAHIterator ewahIter) {
        this.ewahIter = ewahIter;
        this.buffer = ewahIter.buffer();
        this.hasNext = this.moveToNext();
    }

    public boolean moveToNext() {
        while (!runningHasNext() && !literalHasNext()) {
            if (!this.ewahIter.hasNext()) {
                return false;
            }
            setRunningLengthWord(this.ewahIter.next());
        }
        return true;
    }

    @Override
    public boolean hasNext() {
        return this.hasNext;
    }

    @Override
    public int next() {
        final int answer;
        if (runningHasNext()) {
            answer = this.position++;
        } else {
            final long t = this.word & -this.word;
            answer = this.literalPosition + Long.bitCount(t - 1);
            this.word ^= t;
        }
        this.hasNext = this.moveToNext();
        return answer;
    }

    private void setRunningLengthWord(RunningLengthWord rlw) {
        this.runningLength = WORD_IN_BITS * (int) rlw.getRunningLength() + this.position;
        if (!rlw.getRunningBit()) {
            this.position = this.runningLength;
        }

        this.wordPosition = this.ewahIter.literalWords();
        this.wordLength = this.wordPosition + rlw.getNumberOfLiteralWords();
    }

    private boolean runningHasNext() {
        return this.position < this.runningLength;
    }

    private boolean literalHasNext() {
        while (this.word == 0 && this.wordPosition < this.wordLength) {
            this.word = this.buffer.getWord(this.wordPosition++);
            this.literalPosition = this.position;
            this.position += WORD_IN_BITS;
        }
        return this.word != 0;
    }
}