it.unimi.di.mg4j.search
Class DifferenceDocumentIterator

java.lang.Object
  it.unimi.di.mg4j.search.AbstractDocumentIterator
      it.unimi.di.mg4j.search.AbstractIntervalDocumentIterator
          it.unimi.di.mg4j.search.DifferenceDocumentIterator

All Implemented Interfaces:

DocumentIterator

public class DifferenceDocumentIterator
extends AbstractIntervalDocumentIterator
implements DocumentIterator

A document iterator that computes the Brouwerian difference between two given document iterators.

In the lattice of interval antichains, the Brouwerian difference is obtained by deleting from the first operand (the minuend) all intervals that contain some interval of the second operand (the subtrahend). Thus, Brouwerian difference can be fruitfully employed to kill intervals containing a term or, even more fruitfully, to change at query time the granularity of an index by subtracting from the results of a query those length-two intervals that cross the cutpoints between the desired parts of the index.

Additionally, this class provides interval enlargement—by using a suitable factory method each interval returned by the subtrahend will be enlarged to the left and to the right by the given amount (e.g., if the left margin is 1 and the right margin is 2 the interval [2..3] will turn into [1..5]).

Note that while accept(DocumentIteratorVisitor) will recursively visit both the minuend and the subtrahend, acceptOnTruePaths(DocumentIteratorVisitor) will visit only the minuend.

Since:

1.2

Author:

Sebastiano Vigna

Nested Class Summary

protected class

DifferenceDocumentIterator.DifferenceIntervalIterator

Field Summary

protected int	leftMargin A margin that will be added to the left of each interval.
protected DocumentIterator	minuendIterator The first operand.
protected int	rightMargin A margin that will be added to the right of each interval.
protected DocumentIterator	subtrahendIterator The second operand.

Fields inherited from class it.unimi.di.mg4j.search.AbstractIntervalDocumentIterator

currentIterators, indices, intervalIterators, soleIndex, soleIntervalIterator, unmodifiableCurrentIterators

Fields inherited from class it.unimi.di.mg4j.search.AbstractDocumentIterator

curr, weight

Fields inherited from interface it.unimi.di.mg4j.search.DocumentIterator

END_OF_LIST

Constructor Summary

protected

DifferenceDocumentIterator(DocumentIterator minuendIterator, DocumentIterator subtrahendIterator, int leftMargin, int rightMargin) Creates a new difference document iterator given a minuend and a subtrahend iterator.

Method Summary

<T> T	accept(DocumentIteratorVisitor<T> visitor) Accepts a visitor.
<T> T	acceptOnTruePaths(DocumentIteratorVisitor<T> visitor) Accepts a visitor after a call to DocumentIterator.nextDocument(), limiting recursion to true paths.
void	dispose() Disposes this document iterator, releasing all resources.
static DocumentIterator	getInstance(DocumentIterator minuendIterator, DocumentIterator subtrahendIterator) Returns new difference document iterator given a minuend and a subtrahend iterator.
static DocumentIterator	getInstance(DocumentIterator minuendIterator, DocumentIterator subtrahendIterator, int leftMargin, int rightMargin) Returns new difference document iterator given a minuend and a subtrahend iterator.
protected IntervalIterator	getIntervalIterator(Index index, int n, boolean allIndexIterators, Object arg) Creates an interval iterator suitable for this AbstractIntervalDocumentIterator.
IntervalIterator	intervalIterator() Returns the interval iterator of this document iterator for single-index queries.
IntervalIterator	intervalIterator(Index index) Returns the interval iterator of this document iterator for the given index.
Reference2ReferenceMap<Index,IntervalIterator>	intervalIterators() Returns an unmodifiable map from indices to interval iterators.
boolean	mayHaveNext() Returns whether there may be a next document, possibly with false positives.
int	nextDocument() Returns the next document provided by this document iterator, or DocumentIterator.END_OF_LIST if no more documents are available.
int	skipTo(int n) Skips all documents smaller than n.
String	toString()

Methods inherited from class it.unimi.di.mg4j.search.AbstractIntervalDocumentIterator

allIndexIterators, indices, indices

Methods inherited from class it.unimi.di.mg4j.search.AbstractDocumentIterator

document, ensureOnADocument, weight, weight

Methods inherited from class java.lang.Object

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

Methods inherited from interface it.unimi.di.mg4j.search.DocumentIterator

document, indices, weight, weight

Field Detail

minuendIterator

protected final DocumentIterator minuendIterator

The first operand.

subtrahendIterator

protected final DocumentIterator subtrahendIterator

The second operand.

leftMargin

protected final int leftMargin

A margin that will be added to the left of each interval.

rightMargin

protected final int rightMargin

A margin that will be added to the right of each interval.

Constructor Detail

DifferenceDocumentIterator

protected DifferenceDocumentIterator(DocumentIterator minuendIterator,
                                     DocumentIterator subtrahendIterator,
                                     int leftMargin,
                                     int rightMargin)

Creates a new difference document iterator given a minuend and a subtrahend iterator.

Parameters:

minuendIterator - the minuend.

subtrahendIterator - the subtrahend.

Method Detail

getInstance

public static DocumentIterator getInstance(DocumentIterator minuendIterator,
                                           DocumentIterator subtrahendIterator)

Returns new difference document iterator given a minuend and a subtrahend iterator.

Parameters:

minuendIterator - the minuend.

subtrahendIterator - the subtrahend.

getInstance

public static DocumentIterator getInstance(DocumentIterator minuendIterator,
                                           DocumentIterator subtrahendIterator,
                                           int leftMargin,
                                           int rightMargin)

Returns new difference document iterator given a minuend and a subtrahend iterator.

Parameters:

minuendIterator - the minuend.

subtrahendIterator - the subtrahend.

leftMargin - a margin that will be added to the left of each interval.

rightMargin - a margin that will be added to the right of each interval.

getIntervalIterator

protected IntervalIterator getIntervalIterator(Index index,
                                               int n,
                                               boolean allIndexIterators,
                                               Object arg)

Description copied from class: AbstractIntervalDocumentIterator

Creates an interval iterator suitable for this AbstractIntervalDocumentIterator.

Specified by:

getIntervalIterator in class AbstractIntervalDocumentIterator

Parameters:

index - the reference index for the iterator, or null.

n - the number of underlying or component iterators.

allIndexIterators - whether all underlying or component iterators are index iterators.

arg - an optional argument.

Returns:

an interval iterator suitable for this AbstractIntervalDocumentIterator.

nextDocument

public int nextDocument()
                 throws IOException

Description copied from interface: DocumentIterator

Returns the next document provided by this document iterator, or DocumentIterator.END_OF_LIST if no more documents are available.

Specified by:

nextDocument in interface DocumentIterator

Returns:

the next document, or DocumentIterator.END_OF_LIST if no more documents are available.

Throws:

IOException

mayHaveNext

public boolean mayHaveNext()

Description copied from interface: DocumentIterator

Returns whether there may be a next document, possibly with false positives.

Specified by:

mayHaveNext in interface DocumentIterator

Overrides:

mayHaveNext in class AbstractDocumentIterator

Returns:

true there may be a next document; false if certainly there is no next document.

skipTo

public int skipTo(int n)
           throws IOException

Description copied from interface: DocumentIterator

Skips all documents smaller than n.

Define the current document k associated with this document iterator as follows:

• -1, if DocumentIterator.nextDocument() and this method have never been called;
• DocumentIterator.END_OF_LIST, if a call to this method or to DocumentIterator.nextDocument() returned DocumentIterator.END_OF_LIST;
• the last value returned by a call to DocumentIterator.nextDocument() or this method, otherwise.

If k is larger than or equal to n, then this method does nothing and returns k. Otherwise, a call to this method is equivalent to

 while( ( k = nextDocument() ) < n );
 return k;
 

Thus, when a result k ≠ DocumentIterator.END_OF_LIST is returned, the state of this iterator will be exactly the same as after a call to DocumentIterator.nextDocument() that returned k. In particular, the first document larger than or equal to n (when returned by this method) will not be returned by the next call to DocumentIterator.nextDocument().

Specified by:

skipTo in interface DocumentIterator

Parameters:

n - a document pointer.

Returns:

a document pointer larger than or equal to n if available, DocumentIterator.END_OF_LIST otherwise.

Throws:

IOException

intervalIterators

public Reference2ReferenceMap<Index,IntervalIterator> intervalIterators()
                                                                 throws IOException

Description copied from interface: DocumentIterator

Returns an unmodifiable map from indices to interval iterators.

After a call to DocumentIterator.nextDocument(), this map can be used to retrieve the intervals in the current document. An invocation of Map.get(java.lang.Object) on this map with argument index yields the same result as intervalIterator(index).

Specified by:

intervalIterators in interface DocumentIterator

Returns:

a map from indices to interval iterators over the current document.

Throws:

IOException

See Also:

DocumentIterator.intervalIterator(Index)

intervalIterator

public IntervalIterator intervalIterator()
                                  throws IOException

Description copied from interface: DocumentIterator

Returns the interval iterator of this document iterator for single-index queries.

This is a commodity method that can be used only for queries built over a single index.

Specified by:

intervalIterator in interface DocumentIterator

Returns:

an interval iterator.

Throws:

IOException

See Also:

DocumentIterator.intervalIterator(Index)

intervalIterator

public IntervalIterator intervalIterator(Index index)
                                  throws IOException

Description copied from interface: DocumentIterator

Returns the interval iterator of this document iterator for the given index.

After a call to DocumentIterator.nextDocument(), this iterator can be used to retrieve the intervals in the current document (the one returned by DocumentIterator.nextDocument()) for the index index.

Note that if all indices have positions, it is guaranteed that at least one index will return an interval. However, for disjunctive queries it cannot be guaranteed that all indices will return an interval.

Indices without positions always return IntervalIterators.TRUE. Thus, in presence of indices without positions it is possible that no intervals at all are available.

Specified by:

intervalIterator in interface DocumentIterator

Parameters:

index - an index (must be one over which the query was built).

Returns:

an interval iterator over the current document in index.

Throws:

IOException

dispose

public void dispose()
             throws IOException

Description copied from interface: DocumentIterator

Disposes this document iterator, releasing all resources.

This method should propagate down to the underlying index iterators, where it should release resources such as open files and network connections. If you're doing your own resource tracking and pooling, then you do not need to call this method.

Specified by:

dispose in interface DocumentIterator

Throws:

IOException

accept

public <T> T accept(DocumentIteratorVisitor<T> visitor)
         throws IOException

Description copied from interface: DocumentIterator

Accepts a visitor.

A document iterator is usually structured as composite, with operators as internal nodes and IndexIterators as leaves. This method implements the visitor pattern.

Specified by:

accept in interface DocumentIterator

Parameters:

visitor - the visitor.

Returns:

an object resulting from the visit, or null if the visit was interrupted.

Throws:

IOException

acceptOnTruePaths

public <T> T acceptOnTruePaths(DocumentIteratorVisitor<T> visitor)
                    throws IOException

Description copied from interface: DocumentIterator

Accepts a visitor after a call to DocumentIterator.nextDocument(), limiting recursion to true paths.

After a call to DocumentIterator.nextDocument(), a document iterator is positioned over a document. This call is equivalent to DocumentIterator.accept(DocumentIteratorVisitor), but visits only along true paths.

We define a true path as a path from the root of the composite that passes only through nodes whose associated subtree is positioned on the same document of the root. Note that OrDocumentIterators detach exhausted iterators from the composite tree, so true paths define the subtree that is causing the current document to satisfy the query represented by this document iterator.

For more elaboration, and the main application of this method, see CounterCollectionVisitor.

Specified by:

acceptOnTruePaths in interface DocumentIterator

Parameters:

visitor - the visitor.

Returns:

an object resulting from the visit, or null if the visit was interrupted.

Throws:

IOException

See Also:

DocumentIterator.accept(DocumentIteratorVisitor), CounterCollectionVisitor

toString

public String toString()

Overrides:

toString in class Object