pt.tumba.ngram.compression
Class ArithDecoder

java.lang.Object
  pt.tumba.ngram.compression.ArithDecoder

public final class ArithDecoder
extends java.lang.Object

Performs arithmetic decoding, converting bit input into cumulative probability interval output. Returns probabilities as integer counts low, high and total, with the range being [low/total,high/total).

Author:

Bruno Martins

See Also:

ArithEncoder, BitInput

Field Summary

private int	_bufferedBits Number of bits that have been buffered.
private boolean	_endOfStream Value will be true if the end of stream has been reached.
protected long	_high The high bound on the current interval for coding.
private BitInput	_in Input stream from which to read bits.
protected long	_low The low bound on the current interval for coding.
private long	_value Current bits for decoding.
protected static int	CODE_VALUE_BITS Precision of coding, expressed in number of bits used for arithmetic before shifting out partial results.
protected static long	FIRST_QUARTER 1/4 of the largest possible value plus one.
protected static long	HALF 1/2 of the largest possible value; 2 * FIRST_QUARTER
protected static long	THIRD_QUARTER 3/4 of the largest possible value; 3 * FIRST_QUARTER
protected static long	TOP_VALUE The largest possible interval value.

Constructor Summary

ArithDecoder(BitInput in)
Construct an arithmetic decoder that reads from the given bit input.

ArithDecoder(java.io.InputStream in)
Construct an arithmetic decoder that reads from the given input stream.

Method Summary

private void	bufferBit() Reads a bit from the underlying bit input stream and buffers it.
void	close() Closes underlying bit output.
boolean	endOfStream() Returns true if the end of stream has been reached and there are no more symbols to decode.
int	getCurrentSymbolCount(int totalCount) Returns a count for the current symbol that will be between the low and high counts for the symbol in the model given the total count.
void	removeSymbolFromStream(int[] counts) Removes a symbol from the input stream that was coded with counts { low, high, total }.
void	removeSymbolFromStream(long lowCount, long highCount, long totalCount) Removes a symbol from the input stream.

Methods inherited from class java.lang.Object

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

Field Detail

_low

protected long _low

The low bound on the current interval for coding. Initialized to zero.

_high

protected long _high

The high bound on the current interval for coding. Initialized to top value possible.

CODE_VALUE_BITS

protected static final int CODE_VALUE_BITS

Precision of coding, expressed in number of bits used for arithmetic before shifting out partial results.

See Also:

Constant Field Values

TOP_VALUE

protected static final long TOP_VALUE

The largest possible interval value. All 1s.

See Also:

Constant Field Values

FIRST_QUARTER

protected static final long FIRST_QUARTER

1/4 of the largest possible value plus one.

See Also:

Constant Field Values

HALF

protected static final long HALF

1/2 of the largest possible value; 2 * FIRST_QUARTER

See Also:

Constant Field Values

THIRD_QUARTER

protected static final long THIRD_QUARTER

3/4 of the largest possible value; 3 * FIRST_QUARTER

See Also:

Constant Field Values

_in

private final BitInput _in

Input stream from which to read bits.

_value

private long _value

Current bits for decoding.

_endOfStream

private boolean _endOfStream

Value will be true if the end of stream has been reached.

_bufferedBits

private int _bufferedBits

Number of bits that have been buffered.

Constructor Detail

ArithDecoder

public ArithDecoder(BitInput in)
             throws java.io.IOException

Construct an arithmetic decoder that reads from the given bit input.

Parameters:

in - Bit input from which to read bits.

Throws:

java.io.IOException - If there is an exception buffering from the bit input stream.

ArithDecoder

public ArithDecoder(java.io.InputStream in)
             throws java.io.IOException

Construct an arithmetic decoder that reads from the given input stream.

Parameters:

in - Input stream from which to read.

Throws:

java.io.IOException - If there is an exception buffering from input stream.

Method Detail

endOfStream

public boolean endOfStream()

Returns true if the end of stream has been reached and there are no more symbols to decode.

Returns:

true if the end of stream has been reached.

getCurrentSymbolCount

public int getCurrentSymbolCount(int totalCount)

Returns a count for the current symbol that will be between the low and high counts for the symbol in the model given the total count. Once symbol is retrieved, the model is used to compute the actual low, high and total counts and removeSymbolFromStream(int[]) is called.

Parameters:

totalCount - The current total count for the model.

Returns:

A count that is in the range above or equal to the low count and less than the high count of the next symbol decoded.

removeSymbolFromStream

public void removeSymbolFromStream(int[] counts)
                            throws java.io.IOException

Removes a symbol from the input stream that was coded with counts { low, high, total }. Called after getCurrentSymbolCount(int).

Parameters:

counts - Array of low, high and total count used to code the symbol.

Throws:

java.io.IOException - If there is an exception in buffering input from the underlying input stream.

See Also:

removeSymbolFromStream(long,long,long)

removeSymbolFromStream

public void removeSymbolFromStream(long lowCount,
                                   long highCount,
                                   long totalCount)
                            throws java.io.IOException

Removes a symbol from the input stream. Called after getCurrentSymbolCount(int).

Parameters:

lowCount - Cumulative count for symbols indexed below symbol to be removed.

highCount - lowCount plus count for this symbol.

totalCount - Total count for all symbols seen.

Throws:

java.io.IOException - If there is an exception in buffering input from the underlying input stream.

close

public void close()
           throws java.io.IOException

Closes underlying bit output.

Throws:

java.io.IOException - If there is an underlying I/O exception in the bit input.

bufferBit

private void bufferBit()
                throws java.io.IOException

Reads a bit from the underlying bit input stream and buffers it.

Throws:

java.io.IOException - If there is an IOException buffering from the underlying bit stream.