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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.
package com.googlecode.javaewah32;
/*
* 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.
*/
/**
* Mostly for internal use. Similar to BufferedRunningLengthWord32, but
* automatically advances to the next BufferedRunningLengthWord32 as words are
* discarded.
*
* @author Daniel Lemire and David McIntosh
* @since 0.5.0
*/
public final class IteratingBufferedRunningLengthWord32 implements
IteratingRLW32, Cloneable {
/**
* Instantiates a new iterating buffered running length word.
*
* @param iterator iterator
*/
public IteratingBufferedRunningLengthWord32(
final EWAHIterator32 iterator) {
this.iterator = iterator;
this.brlw = new BufferedRunningLengthWord32(
this.iterator.next());
this.literalWordStartPosition = this.iterator.literalWords()
+ this.brlw.literalWordOffset;
this.buffer = this.iterator.buffer();
}
/**
* Instantiates a new iterating buffered running length word.
*
* @param bitmap over which we want to iterate
*/
public IteratingBufferedRunningLengthWord32(
final EWAHCompressedBitmap32 bitmap) {
this(EWAHIterator32.getEWAHIterator(bitmap));
}
/**
* Discard first words, iterating to the next running length word if
* needed.
*
* @param x the x
*/
@Override
public void discardFirstWords(int x) {
while (x > 0) {
if (this.brlw.RunningLength > x) {
this.brlw.RunningLength -= x;
return;
}
x -= this.brlw.RunningLength;
this.brlw.RunningLength = 0;
int toDiscard = x > this.brlw.NumberOfLiteralWords ? this.brlw.NumberOfLiteralWords
: x;
this.literalWordStartPosition += toDiscard;
this.brlw.NumberOfLiteralWords -= toDiscard;
x -= toDiscard;
if ((x > 0) || (this.brlw.size() == 0)) {
if (!this.iterator.hasNext()) {
break;
}
this.brlw.reset(this.iterator.next());
this.literalWordStartPosition = this.iterator
.literalWords();
}
}
}
@Override
public void discardRunningWords() {
this.brlw.RunningLength = 0;
if (this.brlw.getNumberOfLiteralWords() == 0)
this.next();
}
/**
* Write out up to max words, returns how many were written
*
* @param container target for writes
* @param max maximal number of writes
* @return how many written
*/
public int discharge(BitmapStorage32 container, int max) {
int index = 0;
while ((index < max) && (size() > 0)) {
// first run
int pl = getRunningLength();
if (index + pl > max) {
pl = max - index;
}
container.addStreamOfEmptyWords(getRunningBit(), pl);
index += pl;
int pd = getNumberOfLiteralWords();
if (pd + index > max) {
pd = max - index;
}
writeLiteralWords(pd, container);
discardFirstWords(pl + pd);
index += pd;
}
return index;
}
/**
* Write out up to max words (negated), returns how many were written
*
* @param container target for writes
* @param max maximal number of writes
* @return how many written
*/
public int dischargeNegated(BitmapStorage32 container, int max) {
int index = 0;
while ((index < max) && (size() > 0)) {
// first run
int pl = getRunningLength();
if (index + pl > max) {
pl = max - index;
}
container.addStreamOfEmptyWords(!getRunningBit(), pl);
index += pl;
int pd = getNumberOfLiteralWords();
if (pd + index > max) {
pd = max - index;
}
writeNegatedLiteralWords(pd, container);
discardFirstWords(pl + pd);
index += pd;
}
return index;
}
/**
* Move to the next RunningLengthWord
*
* @return whether the move was possible
*/
@Override
public boolean next() {
if (!this.iterator.hasNext()) {
this.brlw.NumberOfLiteralWords = 0;
this.brlw.RunningLength = 0;
return false;
}
this.brlw.reset(this.iterator.next());
this.literalWordStartPosition = this.iterator.literalWords(); // +
// this.brlw.literalWordOffset
// ==0
return true;
}
/**
* Write out the remain words, transforming them to zeroes.
*
* @param container target for writes
*/
public void dischargeAsEmpty(BitmapStorage32 container) {
while (size() > 0) {
container.addStreamOfEmptyWords(false, size());
discardFirstWords(size());
}
}
/**
* Write out the remaining words
*
* @param container target for writes
*/
public void discharge(BitmapStorage32 container) {
// fix the offset
this.brlw.literalWordOffset = this.literalWordStartPosition
- this.iterator.literalWords();
discharge(this.brlw, this.iterator, container);
}
/**
* Get the nth literal word for the current running length word
*
* @param index zero based index
* @return the literal word
*/
@Override
public int getLiteralWordAt(int index) {
return this.buffer.getWord(this.literalWordStartPosition + index);
}
/**
* Gets the number of literal words for the current running length word.
*
* @return the number of literal words
*/
@Override
public int getNumberOfLiteralWords() {
return this.brlw.NumberOfLiteralWords;
}
/**
* Gets the running bit.
*
* @return the running bit
*/
@Override
public boolean getRunningBit() {
return this.brlw.RunningBit;
}
/**
* Gets the running length.
*
* @return the running length
*/
@Override
public int getRunningLength() {
return this.brlw.RunningLength;
}
/**
* Size in uncompressed words of the current running length word.
*
* @return the int
*/
@Override
public int size() {
return this.brlw.size();
}
/**
* write the first N literal words to the target bitmap. Does not
* discard the words or perform iteration.
*
* @param numWords number of words to be written
* @param container where we write the data
*/
public void writeLiteralWords(int numWords, BitmapStorage32 container) {
container.addStreamOfLiteralWords(this.buffer,
this.literalWordStartPosition, numWords);
}
/**
* write the first N literal words (negated) to the target bitmap. Does
* not discard the words or perform iteration.
*
* @param numWords number of words to be written
* @param container where we write the data
*/
public void writeNegatedLiteralWords(int numWords,
BitmapStorage32 container) {
container.addStreamOfNegatedLiteralWords(this.buffer,
this.literalWordStartPosition, numWords);
}
/**
* For internal use. (One could use the non-static discharge method
* instead, but we expect them to be slower.)
*
* @param initialWord the initial word
* @param iterator the iterator
* @param container the container
*/
protected static void discharge(
final BufferedRunningLengthWord32 initialWord,
final EWAHIterator32 iterator, final BitmapStorage32 container) {
BufferedRunningLengthWord32 runningLengthWord = initialWord;
for (; ; ) {
final int runningLength = runningLengthWord
.getRunningLength();
container.addStreamOfEmptyWords(
runningLengthWord.getRunningBit(),
runningLength);
container.addStreamOfLiteralWords(iterator.buffer(),
iterator.literalWords()
+ runningLengthWord.literalWordOffset,
runningLengthWord.getNumberOfLiteralWords()
);
if (!iterator.hasNext())
break;
runningLengthWord = new BufferedRunningLengthWord32(
iterator.next());
}
}
@Override
public IteratingBufferedRunningLengthWord32 clone()
throws CloneNotSupportedException {
IteratingBufferedRunningLengthWord32 answer = (IteratingBufferedRunningLengthWord32) super
.clone();
answer.brlw = this.brlw.clone();
answer.iterator = this.iterator.clone();
return answer;
}
private BufferedRunningLengthWord32 brlw;
private final Buffer32 buffer;
private int literalWordStartPosition;
private EWAHIterator32 iterator;
}
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