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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.
 */

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

/**
 * The ChunkIteratorImpl is the 64 bit implementation of the ChunkIterator
 * interface, which efficiently returns the chunks of ones and zeros represented by an
 * EWAHIterator.
 *
 * @author Gregory Ssi-Yan-Kai
 */
final class ChunkIteratorImpl implements ChunkIterator {

    private final EWAHIterator ewahIter;
    private final int sizeInBits;
    private final Buffer buffer;
    private int position;
    private boolean runningBit;
    private int runningLength;
    private long word;
    private long wordMask;
    private int wordPosition;
    private int wordLength;
    private boolean hasNext;
    private Boolean nextBit;
    private int nextLength;

    ChunkIteratorImpl(EWAHIterator ewahIter, int sizeInBits) {
        this.ewahIter = ewahIter;
        this.sizeInBits = sizeInBits;
        this.buffer = ewahIter.buffer();
        this.hasNext = moveToNextRLW();
    }

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

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

    @Override
    public int nextLength() {
        return this.nextLength;
    }

    @Override
    public void move() {
        move(this.nextLength);
    }

    @Override
    public void move(int bits) {
        this.nextLength -= bits;
        if(this.nextLength <= 0) {
            do {
                this.nextBit = null;
                updateNext();
                this.hasNext = moveToNextRLW();
            } while(this.nextLength <= 0 && this.hasNext);
        }
    }

    private boolean moveToNextRLW() {
        while (!runningHasNext() && !literalHasNext()) {
            if (!hasNextRLW()) {
                return this.nextBit!=null;
            }
            setRLW(nextRLW());
            updateNext();
        }
        return true;
    }

    private void setRLW(RunningLengthWord rlw) {
        this.runningLength = Math.min(this.sizeInBits,
                                      this.position + WORD_IN_BITS * (int) rlw.getRunningLength());
        this.runningBit = rlw.getRunningBit();
        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.wordMask == 0 && this.wordPosition < this.wordLength) {
            this.word = this.buffer.getWord(this.wordPosition++);
            this.wordMask = 1l;
        }
        return (this.word != 0 || this.wordMask != 0 || !hasNextRLW()) && this.position < this.sizeInBits;
    }

    private boolean hasNextRLW() {
        return this.ewahIter.hasNext();
    }

    private RunningLengthWord nextRLW() {
        return this.ewahIter.next();
    }

    private void updateNext() {
        if(runningHasNext()) {
            if(this.nextBit == null || this.nextBit == this.runningBit) {
                this.nextBit = this.runningBit;
                int offset = runningOffset();
                this.nextLength += offset;
                movePosition(offset);
                updateNext();
            }
        } else if (literalHasNext()) {
            boolean b = currentWordBit();
            if(this.nextBit == null || this.nextBit == b) {
                this.nextBit = b;
                this.nextLength++;
                movePosition(1);
                shiftWordMask();
                updateNext();
            }
        } else {
            moveToNextRLW();
        }
    }

    private int runningOffset() {
        return this.runningLength - this.position;
    }

    private void movePosition(int offset) {
        this.position += offset;
    }

    private boolean currentWordBit() {
        return (this.word & this.wordMask) != 0;
    }

    private void shiftWordMask() {
        this.word &= ~this.wordMask;
        this.wordMask = this.wordMask << 1;
    }

}