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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.
 */

/**
 * Buffer interface.
 * Users should not be concerned by this class.
 *
 * @author Gregory Ssi-Yan-Kai
 */
interface Buffer {

    /**
     * Returns the actual size in words
     */
    int sizeInWords();

    /**
     * Increases the size of the buffer if necessary
     */
    void ensureCapacity(int capacity);

    /**
     * Returns the word at a given position
     * @param position
     * @return the word
     */
    long getWord(int position);

    /**
     * Returns the last word of the buffer
     * @return the last word
     */
    long getLastWord();

    /**
     * Resets the buffer
     * The buffer is not fully cleared and any new set operations should
     * overwrite stale data
     */
    void clear();

    /**
     * Reduces the internal buffer to its minimal allowable size.
     * This can free memory.
     */
    void trim();

    /**
     * Replaces the word at the given position in the buffer with
     * the specified word.
     * @param position
     * @param word
     */
    void setWord(int position, long word);

    /**
     * Replaces the last word in the buffer with
     * the specified word.
     * @param word
     */
    void setLastWord(long word);

    /**
     * Appends the specified word to the end of the buffer
     * @param word
     */
    void push_back(long word);

    /**
     * Appends the specified buffer words to the end of the buffer.
     * @param buffer the buffer
     * @param start  the position of the first word to add
     * @param number the number of words to add
     */
    void push_back(Buffer buffer, int start, int number);

    /**
     * Same as push_back, but the words are negated.
     *
     * @param buffer the buffer
     * @param start  the position of the first word to add
     * @param number the number of words to add
     */
    void negative_push_back(Buffer buffer, int start, int number);

    /**
     * Removes the last word from the buffer
     */
    void removeLastWord();

    /**
     * Negates the word at the given position in the buffer
     * @param position
     */
    void negateWord(int position);

    /**
     * Replaces the word at the given position in the buffer
     * with its bitwise-and with the given mask.
     * @param position
     * @param mask
     */
    void andWord(int position, long mask);

    /**
     * Replaces the word at the given position in the buffer
     * with its bitwise-or with the given mask.
     * @param position
     * @param mask
     */
    void orWord(int position, long mask);

    /**
     * Replaces the last word position in the buffer
     * with its bitwise-and with the given mask.
     * @param mask
     */
    void andLastWord(long mask);

    /**
     * Replaces the last word position in the buffer
     * with its bitwise-or with the given mask.
     * @param mask
     */
    void orLastWord(long mask);

    /**
     * Expands the buffer by adding the given number of words at the given position.
     * The added words may contain stale data.
     * @param position the position of the buffer where to add words
     * @param length   the number of words to add
     */
    void expand(int position, int length);

    /**
     * Removes a given number of words at the given position in the buffer.
     * The freed words at the end of the buffer are properly cleaned.
     * @param position the position of the buffer where to add words
     * @param length   the number of words to add
     */
    void collapse(int position, int length);

    /**
     * Creates and returns a copy of the buffer
     */
    Buffer clone() throws CloneNotSupportedException;

    /**
     * Swap the content of the buffer with another.
     *
     * @param other buffer to swap with
     */
    void swap(Buffer other);

}