org.apache.lucene.index.pruning
Class RIDFTermPruningPolicy

java.lang.Object
  org.apache.lucene.index.pruning.PruningPolicy
      org.apache.lucene.index.pruning.TermPruningPolicy
          org.apache.lucene.index.pruning.RIDFTermPruningPolicy

public class RIDFTermPruningPolicy
extends TermPruningPolicy

Implementation of TermPruningPolicy that uses "residual IDF" metric to determine the postings of terms to keep/remove, as defined in http://www.dc.fi.udc.es/~barreiro/publications/blanco_barreiro_ecir2007.pdf.

Residual IDF measures a difference between a collection-wide IDF of a term (which assumes a uniform distribution of occurrences) and the actual observed total number of occurrences of a term in all documents. Positive values indicate that a term is informative (e.g. for rare terms), negative values indicate that a term is not informative (e.g. too popular to offer good selectivity).

This metric produces small values close to [-1, 1], so useful ranges for thresholds under this metrics are somewhere between [0, 1]. The higher the threshold the more informative (and more rare) terms will be retained. For filtering of common words a value of close to or slightly below 0 (e.g. -0.1) should be a good starting point.

Field Summary

Fields inherited from class org.apache.lucene.index.pruning.TermPruningPolicy

fieldFlags, in

Fields inherited from class org.apache.lucene.index.pruning.PruningPolicy

DEL_ALL, DEL_PAYLOADS, DEL_POSTINGS, DEL_STORED, DEL_VECTOR

Constructor Summary

RIDFTermPruningPolicy(IndexReader in, Map<String,Integer> fieldFlags, Map<String,Double> thresholds, double defThreshold)

Method Summary

void	initPositionsTerm(TermPositions tp, Term t) Called when moving TermPositions to a new Term.
boolean	pruneAllPositions(TermPositions termPositions, Term t) Prune all postings per term (invoked once per term per doc)
int	pruneSomePositions(int docNum, int[] positions, Term curTerm) Prune some postings per term (invoked once per term per doc).
boolean	pruneTermEnum(TermEnum te) Pruning of all postings for a term (invoked once per term).
int	pruneTermVectorTerms(int docNumber, String field, String[] terms, int[] freqs, TermFreqVector v) Pruning of individual terms in term vectors.

Methods inherited from class org.apache.lucene.index.pruning.TermPruningPolicy

pruneAllFieldPostings, prunePayload, pruneWholeTermVector

Methods inherited from class java.lang.Object

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

Constructor Detail

RIDFTermPruningPolicy

public RIDFTermPruningPolicy(IndexReader in,
                             Map<String,Integer> fieldFlags,
                             Map<String,Double> thresholds,
                             double defThreshold)

Method Detail

initPositionsTerm

public void initPositionsTerm(TermPositions tp,
                              Term t)
                       throws IOException

Description copied from class: TermPruningPolicy

Called when moving TermPositions to a new Term.

Specified by:

initPositionsTerm in class TermPruningPolicy

Parameters:

tp - input term positions

t - current term

Throws:

IOException

pruneTermEnum

public boolean pruneTermEnum(TermEnum te)
                      throws IOException

Description copied from class: TermPruningPolicy

Pruning of all postings for a term (invoked once per term).

Specified by:

pruneTermEnum in class TermPruningPolicy

Parameters:

te - positioned term enum.

Returns:

true if all postings for this term should be removed, false otherwise.

Throws:

IOException

pruneAllPositions

public boolean pruneAllPositions(TermPositions termPositions,
                                 Term t)
                          throws IOException

Description copied from class: TermPruningPolicy

Prune all postings per term (invoked once per term per doc)

Specified by:

pruneAllPositions in class TermPruningPolicy

Parameters:

termPositions - positioned term positions. Implementations MUST NOT advance this by calling TermPositions methods that advance either the position pointer (next, skipTo) or term pointer (seek).

t - current term

Returns:

true if the current posting should be removed, false otherwise.

Throws:

IOException

pruneTermVectorTerms

public int pruneTermVectorTerms(int docNumber,
                                String field,
                                String[] terms,
                                int[] freqs,
                                TermFreqVector v)
                         throws IOException

Description copied from class: TermPruningPolicy

Pruning of individual terms in term vectors.

Specified by:

pruneTermVectorTerms in class TermPruningPolicy

Parameters:

docNumber - document number

field - field name

terms - array of terms

freqs - array of term frequencies

v - the original term frequency vector

Returns:

0 if no terms are to be removed, positive number to indicate how many terms need to be removed. The same number of entries in the terms array must be set to null to indicate which terms to remove.

Throws:

IOException

pruneSomePositions

public int pruneSomePositions(int docNum,
                              int[] positions,
                              Term curTerm)

Description copied from class: TermPruningPolicy

Prune some postings per term (invoked once per term per doc).

Specified by:

pruneSomePositions in class TermPruningPolicy

Parameters:

docNum - current document number

positions - original term positions in the document (and indirectly term frequency)

curTerm - current term

Returns:

0 if no postings are to be removed, or positive number to indicate how many postings need to be removed. The same number of entries in the positions array must be set to -1 to indicate which positions to remove.