it.unimi.dsi.compression

Class HuffmanCodec

java.lang.Object

it.unimi.dsi.compression.HuffmanCodec

All Implemented Interfaces:

Codec, PrefixCodec, Serializable

public class HuffmanCodec
extends Object
implements PrefixCodec, Serializable

An implementation of Huffman optimal prefix-free coding.

A Huffman coder is built starting from an array of frequencies corresponding to each symbol. Frequency 0 symbols are allowed, but they will degrade the resulting code.

Instances of this class compute a canonical Huffman code (Eugene S. Schwartz and Bruce Kallick, “Generating a Canonical Prefix Encoding”, Commun. ACM 7(3), pages 166−169, 1964), which can by quickly decoded using table lookups. The construction uses the most efficient one-pass in-place codelength computation procedure described by Alistair Moffat and Jyrki Katajainen in “In-Place Calculation of Minimum-Redundancy Codes”, Algorithms and Data Structures, 4th International Workshop, number 955 in Lecture Notes in Computer Science, pages 393−402, Springer-Verlag, 1995.

We note by passing that this coded uses a CanonicalFast64CodeWordDecoder, which does not support codelengths above 64. However, since the worst case for codelengths is given by Fibonacci numbers, and frequencies are to be provided as integers, no codeword longer than the base-[(5^1/2 + 1)/2] logarithm of 5^1/2 · 2³¹ (less than 47) will ever be generated.

See Also:

Serialized Form

Field Summary

Fields

Modifier and Type	Field and Description
int	size The number of symbols of this coder.

Constructor Summary

Constructors

Constructor and Description
HuffmanCodec(int[] frequency) Creates a new Huffman codec using the given vector of frequencies.
HuffmanCodec(long[] frequency) Creates a new Huffman codec using the given vector of frequencies.

Method Summary

Methods

Modifier and Type	Method and Description
CodeWordCoder	coder() Returns a coder for the compression technique represented by this coded.
BitVector[]	codeWords() Returns the vector of prefix-free codewords used by this prefix coder.
Decoder	decoder() Returns a decoder for the compression technique represented by this coded.
int	size() Returns the number of symbols handled by this codec.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

size

public final int size

The number of symbols of this coder.

Constructor Detail

HuffmanCodec

public HuffmanCodec(int[] frequency)

Creates a new Huffman codec using the given vector of frequencies.

Parameters:

frequency - a vector of nonnnegative frequencies.

HuffmanCodec

public HuffmanCodec(long[] frequency)

Creates a new Huffman codec using the given vector of frequencies.

Parameters:

frequency - a vector of nonnnegative frequencies.

Method Detail

coder

public CodeWordCoder coder()

Description copied from interface: Codec

Returns a coder for the compression technique represented by this coded.

Specified by:

coder in interface Codec

Specified by:

coder in interface PrefixCodec

Returns:

a coder for the compression technique represented by this codec.

decoder

public Decoder decoder()

Description copied from interface: Codec

Returns a decoder for the compression technique represented by this coded.

Specified by:

decoder in interface Codec

Returns:

a decoder for the compression technique represented by this codec.

size

public int size()

Description copied from interface: Codec

Returns the number of symbols handled by this codec.

Specified by:

size in interface Codec

Returns:

the number of symbols handled by this codec.

codeWords

public BitVector[] codeWords()

Description copied from interface: PrefixCodec

Returns the vector of prefix-free codewords used by this prefix coder.

Specified by:

codeWords in interface PrefixCodec

Returns:

the vector of prefix-free codewords used by this prefix coder.