ivory.bloomir.data

Class CompressedPostings

java.lang.Object

ivory.bloomir.data.CompressedPostings

All Implemented Interfaces:

Writable

Direct Known Subclasses:

CompressedPositionalPostings

public class CompressedPostings
extends Object
implements Writable

A compressed postings list representation. This class uses PForDelta to compress a given set of document ids into blocks of equal size.

Author:

Nima Asadi

Method Summary

Methods

Modifier and Type	Method and Description
int	decompressBlock(int[] outBlock, int blockNumber) Decompresses a block, stores the decompressed data in an array and returns the number of decompressed elements as output.
boolean	equals(Object o)
int	getBlockCount()
int	getBlockNumber(int index) Computes the block number of a given index.
static int	getBlockSize()
int	getBlockStartIndex(int blockNumber) Returns the index (in the original data) of the first element in the compressed block.
int	getPositionInBlock(int index)
boolean	isFirstElementInBlock(int index)
static CompressedPostings	newInstance(int[] data) Compresses the given input and generates a new instance of this class.
void	readFields(DataInput input)
static CompressedPostings	readInstance(DataInput input) Reads an instance of this class from a given input.
void	write(DataOutput output)

Methods inherited from class java.lang.Object

getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

decompressBlock

public int decompressBlock(int[] outBlock,
                  int blockNumber)

Decompresses a block, stores the decompressed data in an array and returns the number of decompressed elements as output. To read the n_th element of this block, add outBlock[n-1] to the current value. That is:

     original_data[0] = outBlock(0)

     original_data[1] = original_data[0] + outBlock[1]

     original_data[2] = original_data[1] + outBlock[2]

     ...

     original_data[n-1] = original_data[n-2] + outBlock[n-1]

     

Parameters:

outBlock - Array to store the decompressed values. Note that the size of this array must be at least equal to the size of each block.

blockNumber - The block index to decompress.

Returns:

Number of elements in the decompressed array, starting from index 0.

equals

public boolean equals(Object o)

Overrides:

equals in class Object

getBlockCount

public int getBlockCount()

Returns:

The number of blocks in this postings list.

getBlockNumber

public int getBlockNumber(int index)

Computes the block number of a given index. For efficiency purposes we do not check the range of the input index against the number of elements in this postings list.

Parameters:

index - Index of an element in the original data

Returns:

Block number.

getBlockSize

public static int getBlockSize()

Returns:

The actual block size.

getBlockStartIndex

public int getBlockStartIndex(int blockNumber)

Returns the index (in the original data) of the first element in the compressed block.

Parameters:

blockNumber - Block index.

Returns:

Index of the first value in the specified block.

getPositionInBlock

public int getPositionInBlock(int index)

Parameters:

index - Index of an element in the original data

Returns:

Index of the element in the block.

isFirstElementInBlock

public boolean isFirstElementInBlock(int index)

Parameters:

index - Index of an element in the original data

Returns:

Whether or not this element is the first element of a block. For more information on how to read the elements please refer to decompressBlock(int[], int)

newInstance

public static CompressedPostings newInstance(int[] data)

Compresses the given input and generates a new instance of this class.

Parameters:

data - Array of sorted document ids

Returns:

an instance of this class which contains the given data in a compresed format.

readFields

public void readFields(DataInput input)
                throws IOException

Specified by:

readFields in interface Writable

Throws:

IOException

readInstance

public static CompressedPostings readInstance(DataInput input)
                                       throws IOException

Reads an instance of this class from a given input.

Parameters:

input - Data input

Returns:

An instance of this class.

Throws:

IOException

write

public void write(DataOutput output)
           throws IOException

Specified by:

write in interface Writable

Throws:

IOException