src.it.unimi.dsi.bits.PrefixCoderTransformationStrategy Maven / Gradle / Ivy

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package it.unimi.dsi.bits;

/*
 * DSI utilities
 *
 * Copyright (C) 2007-2017 Sebastiano Vigna
 *
 *  This library is free software; you can redistribute it and/or modify it
 *  under the terms of the GNU Lesser General Public License as published by the Free
 *  Software Foundation; either version 3 of the License, or (at your option)
 *  any later version.
 *
 *  This library is distributed in the hope that it will be useful, but
 *  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 *  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
 *  for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public License
 *  along with this program; if not, see .
 *
 */

import it.unimi.dsi.compression.PrefixCodec;
import it.unimi.dsi.compression.PrefixCoder;
import it.unimi.dsi.fastutil.chars.Char2IntMap;
import it.unimi.dsi.fastutil.chars.Char2IntOpenHashMap;

/** A transformation strategy mapping strings using a {@linkplain PrefixCodec prefix-free encoder}.
 *
 * The actual encoding must be provided via a map from characters to symbols, and a set
 * of codewords. The default return value of the map will be used for unknown characters.
 *
 * This strategy creates a new {@link LongArrayBitVector} each time {@link #toBitVector(CharSequence)} is invoked.
 */

public class PrefixCoderTransformationStrategy implements TransformationStrategy {
	private static final long serialVersionUID = 1;

	protected final BitVector[] codeWord;
	protected final Char2IntOpenHashMap char2symbol;
	protected final boolean prefixFree;

	/** Create a new transformation strategy based on a prefix-free coder.
	 *
	 * @param coder a prefix-free coder.
	 * @param char2symbol a map from character to symbols (the default returned value will be used for unknown symbols).
	 * @param prefixFree whether it is required that the resulting bit vectors are prefix-free: in this case, symbol 0 will
	 * be appended to each string, and will not be allowed to appear in any string.
	 */

	public PrefixCoderTransformationStrategy(final PrefixCoder coder, final Char2IntOpenHashMap char2symbol, final boolean prefixFree) {
		this(coder.codeWords(), char2symbol, prefixFree);
	}

	protected PrefixCoderTransformationStrategy(final PrefixCoderTransformationStrategy transformationStrategy) {
		this(transformationStrategy.codeWord, transformationStrategy.char2symbol, transformationStrategy.prefixFree);
	}

	protected PrefixCoderTransformationStrategy(final BitVector[] codeWord, final Char2IntOpenHashMap char2symbol, final boolean prefixFree) {
		this.codeWord = codeWord;
		this.char2symbol = char2symbol;
		this.prefixFree = prefixFree;
	}

	@Override
	public LongArrayBitVector toBitVector(final CharSequence s) {
		final BitVector[] codeWord = this.codeWord;
		final Char2IntMap char2symbol = this.char2symbol;
		final int length = s.length();
		int numBits = (int) (prefixFree ? codeWord[0].length() : 0);

		for(int i = length; i-- != 0;) numBits += codeWord[char2symbol.get(s.charAt(i))].length();
		final LongArrayBitVector result = LongArrayBitVector.getInstance(numBits);
		for(int i = 0; i < s.length(); i++) result.append(codeWord[char2symbol.get(s.charAt(i))]);
		if (prefixFree) result.append(codeWord[0]);
		return result;
	}

	@Override
	public long length(final CharSequence s) {
		final BitVector[] codeWord = this.codeWord;
		final Char2IntMap char2symbol = this.char2symbol;
		final int length = s.length();
		int numBits = (int) (prefixFree ? codeWord[0].length() : 0);

		for(int i = length; i-- != 0;) numBits += codeWord[char2symbol.get(s.charAt(i))].length();
		return numBits;
	}

	@Override
	public long numBits() {
		long numBits = 0;
		for(int i = codeWord.length; i-- != 0;) numBits += codeWord[i].length();
		return numBits;
	}

	@Override
	public PrefixCoderTransformationStrategy copy() {
		return new PrefixCoderTransformationStrategy(this);
	}
}