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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 java.util.Stack;

/**
 * The class ReverseEWAHIterator represents a special type of efficient iterator
 * iterating over (uncompressed) words of bits in reverse order.
 *
 * @author Gregory Ssi-Yan-Kai
 */
final class ReverseEWAHIterator {

    /**
     * Instantiates a new reverse EWAH iterator.
     *
     * @param buffer      the buffer
     */
    public ReverseEWAHIterator(final Buffer buffer) {
        this.pointer = 0;
        this.rlw = new RunningLengthWord(buffer, this.pointer);
        this.positions = new Stack();
        this.positions.ensureCapacity(buffer.sizeInWords());
        while(this.pointer < buffer.sizeInWords()) {
            this.positions.push(this.pointer);
            this.rlw.position = this.pointer;
            this.pointer += this.rlw.getNumberOfLiteralWords() + 1;
        }
    }

    /**
     * Access to the buffer
     *
     * @return the buffer
     */
    public Buffer buffer() {
        return this.rlw.buffer;
    }

    /**
     * Position of the current running length word.
     *
     * @return the int
     */
    public int position() {
        return this.pointer;
    }

    /**
     * Checks for previous.
     *
     * @return true, if successful
     */
    public boolean hasPrevious() {
        return !this.positions.isEmpty();
    }

    /**
     * Previous running length word.
     *
     * @return the running length word
     */
    public RunningLengthWord previous() {
        this.pointer = this.positions.pop();
        this.rlw.position = this.pointer;
        return this.rlw;
    }

    /**
     * The positions of running length words (embedded in the rlw attribute).
     */
    private Stack positions;

    /**
     * The pointer representing the location of the current running length word
     * in the array of words (embedded in the rlw attribute).
     */
    private int pointer;

    /**
     * The current running length word.
     */
    protected RunningLengthWord rlw;

}