it.unimi.dsi.mg4j.search
Class AbstractIntersectionDocumentIterator

java.lang.Object
  it.unimi.dsi.fastutil.ints.AbstractIntIterator
      it.unimi.dsi.mg4j.search.AbstractDocumentIterator
          it.unimi.dsi.mg4j.search.AbstractCompositeDocumentIterator
              it.unimi.dsi.mg4j.search.AbstractIntersectionDocumentIterator

All Implemented Interfaces:

IntIterator, DocumentIterator, Iterable<Interval>, Iterator<Integer>

Direct Known Subclasses:

AbstractOrderedIntervalDocumentIterator, AndDocumentIterator

public abstract class AbstractIntersectionDocumentIterator
extends AbstractCompositeDocumentIterator

An abstract iterator on documents, generating the intersection of the documents returned by a number of document iterators.

To be usable, this class must be subclassed so to provide also an iterator on intervals. Such iterators must be instantiable using getComposedIntervalIterator(Index). The latter is an example of a non-static factory method, that is, a factory method which depends on the enclosing instance. This pattern allows to easily specialise this class to iterators that do different things, such as AndDocumentIterator and ConsecutiveDocumentIterator, but that have a similar semantics at the document level (the semantics may in fact be slightly different: for instance, not all document belonging to all components will actually appear in a consecutive iterator, as there may be documents filtered at the interval level).

The important invariant is that only after a call to AbstractDocumentIterator.nextDocument(), a call to intervalIterator(Index) will return an interval iterator over the document just returned, and that for at least one index in AbstractCompositeDocumentIterator.indices() the iterator will not be empty or TRUE.

The intersection algorithm

Since MG4J 1.1, this class implements a new intersection algorithm that should be significantly faster than the previous one. The main idea is that of letting sparser iterator interact as much as possible to obtain a candidate common document, and then trying to align the others. At construction time, the component iterators are sorted so that index iterators are separated, and sorted by frequency. Then, each time we have to align the iterators we align them greedily starting from the index iterators, in frequency order. This has the effect of skipping very quickly (and usually by large jumps, which are handled nicely by indices with skips), as the main interaction happens between low-frequency index iterators.

Moreover, this class treats in a special way index iterators coming from payload-based indices. Such iterators are checked at the end of the alignment process, after all standard index iterators (and general document iterators) are aligned. At that point, the special method PayloadPredicateDocumentIterator.skipUnconditionallyTo(int) is used to position unconditionally such iterators and check whether the payload predicate is satisfied. If this doesn't happen, the current candidate (obtained by alignment of standard iterators) is increased and the whole process is restarted. This procedure guarantees that we will never search exhaustively in a payload-based index a document record satisfying the predicate (unless, of course, we have a query containing just PayloadPredicateDocumentIterators), which is very efficient if the payload-based index uses skipping.

Nested Class Summary

Nested classes/interfaces inherited from class it.unimi.dsi.mg4j.search.AbstractCompositeDocumentIterator

AbstractCompositeDocumentIterator.AbstractCompositeIndexIntervalIterator, AbstractCompositeDocumentIterator.AbstractCompositeIntervalIterator

Nested classes/interfaces inherited from class it.unimi.dsi.mg4j.search.AbstractDocumentIterator

AbstractDocumentIterator.AbstractIntervalIterator

Field Summary

protected Reference2ReferenceArrayMap<Index,IntervalIterator>	currentIterators A map from indices to the iterators returned for the current document.
protected Reference2ReferenceArrayMap<Index,IntervalIterator>	intervalIterators A map from indices to interval iterators.
protected Reference2ReferenceMap<Index,IntervalIterator>	unmodifiableCurrentIterators An unmodifiable wrapper around currentIterators.

Fields inherited from class it.unimi.dsi.mg4j.search.AbstractCompositeDocumentIterator

documentIterator, indexIterator, indices, n, soleIndex

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

ahead, curr, weight

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

END_OF_LIST

Constructor Summary

protected	AbstractIntersectionDocumentIterator(DocumentIterator[] documentIterator) Creates a new intersection iterator using a given array of iterators.
protected	AbstractIntersectionDocumentIterator(Index index, DocumentIterator[] documentIterator) Creates a new intersection iterator using a given array of iterators and a given index.

Method Summary

protected abstract IntervalIterator	getComposedIntervalIterator(Index index)
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.
protected int	nextDocumentInternal()
int	skipTo(int n) Skips all documents smaller than n.

Methods inherited from class it.unimi.dsi.mg4j.search.AbstractCompositeDocumentIterator

accept, acceptOnTruePaths, dispose, indices, intervalIterator, toString

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

document, ensureOnADocument, fromNextDocument, hasNext, iterator, nextDocument, nextInt, toNextDocument, weight, weight

Methods inherited from class it.unimi.dsi.fastutil.ints.AbstractIntIterator

next, remove, skip

Methods inherited from class java.lang.Object

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

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

document, iterator, nextDocument, nextInt, weight, weight

Methods inherited from interface it.unimi.dsi.fastutil.ints.IntIterator

skip

Methods inherited from interface java.util.Iterator

hasNext, next, remove

Field Detail

intervalIterators

protected final Reference2ReferenceArrayMap<Index,IntervalIterator> intervalIterators

A map from indices to interval iterators.

currentIterators

protected final Reference2ReferenceArrayMap<Index,IntervalIterator> currentIterators

A map from indices to the iterators returned for the current document. The key set may not contain an index because the related iterator has never been requested. Moreover, the iterator in this map for a given index may differ from the one in intervalIterators because it could be TRUE (in fact, in that case it may even happen that intervalIterators does not contain the index).

unmodifiableCurrentIterators

protected final Reference2ReferenceMap<Index,IntervalIterator> unmodifiableCurrentIterators

An unmodifiable wrapper around currentIterators.

Constructor Detail

AbstractIntersectionDocumentIterator

protected AbstractIntersectionDocumentIterator(Index index,
                                               DocumentIterator[] documentIterator)
                                        throws IOException

Creates a new intersection iterator using a given array of iterators and a given index.

Parameters:

index - an index that will be passed to AbstractCompositeDocumentIterator.AbstractCompositeDocumentIterator(Index, DocumentIterator...).

documentIterator - the iterators to be intersected (at least one).

Throws:

IOException

AbstractIntersectionDocumentIterator

protected AbstractIntersectionDocumentIterator(DocumentIterator[] documentIterator)
                                        throws IOException

Creates a new intersection iterator using a given array of iterators.

Parameters:

documentIterator - the iterators to be insersected (at least one).

Throws:

IOException

Method Detail

skipTo

public int skipTo(int n)
           throws IOException

Description copied from interface: DocumentIterator

Skips all documents smaller than n. If Iterator.hasNext() has been called returning true but DocumentIterator.nextDocument() has not been called afterwards, then a call to DocumentIterator.skipTo(int) will be implicitly preceded by a call to DocumentIterator.nextDocument() (the only consequence is that skipping to the current document after a call to Iterator.hasNext() will return the next document).

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 returned DocumentIterator.END_OF_LIST, or DocumentIterator.nextDocument() returned -1;
• 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 && k != -1 );
 return k == -1 ? END_OF_LIST : 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().

Parameters:

n - a document pointer.

Returns:

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

Throws:

IOException

nextDocumentInternal

protected int nextDocumentInternal()
                            throws IOException

Specified by:

nextDocumentInternal in class AbstractDocumentIterator

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).

Returns:

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

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.

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

getComposedIntervalIterator

protected abstract IntervalIterator getComposedIntervalIterator(Index index)