Package org.apache.lucene.search

Class FieldComparator<T>

java.lang.Object

org.apache.lucene.search.FieldComparator<T>

Direct Known Subclasses:

DocComparator, FieldComparator.RelevanceComparator, FieldComparator.TermOrdValComparator, FieldComparator.TermValComparator, LatLonPointDistanceComparator, NumericComparator, SimpleFieldComparator, XYPointDistanceComparator

public abstract class FieldComparator<T>
extends Object

Expert: a FieldComparator compares hits so as to determine their sort order when collecting the top results with TopFieldCollector. The concrete public FieldComparator classes here correspond to the SortField types.

The document IDs passed to these methods must only move forwards, since they are using doc values iterators to retrieve sort values.

This API is designed to achieve high performance sorting, by exposing a tight interaction with FieldValueHitQueue as it visits hits. Whenever a hit is competitive, it's enrolled into a virtual slot, which is an int ranging from 0 to numHits-1. Segment transitions are handled by creating a dedicated per-segment LeafFieldComparator which also needs to interact with the FieldValueHitQueue but can optimize based on the segment to collect.

The following functions need to be implemented

• compare(int, int) Compare a hit at 'slot a' with hit 'slot b'.
• setTopValue(T) This method is called by TopFieldCollector to notify the FieldComparator of the top most value, which is used by future calls to LeafFieldComparator.compareTop(int).
• getLeafComparator(org.apache.lucene.index.LeafReaderContext) Invoked when the search is switching to the next segment. You may need to update internal state of the comparator, for example retrieving new values from DocValues.
• value(int) Return the sort value stored in the specified slot. This is only called at the end of the search, in order to populate FieldDoc.fields when returning the top results.

See Also:

• LeafFieldComparator

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static final class	FieldComparator.RelevanceComparator	Sorts by descending relevance.
static class	FieldComparator.TermOrdValComparator	Sorts by field's natural Term sort order, using ordinals.
static class	FieldComparator.TermValComparator	Sorts by field's natural Term sort order.

Constructor Summary

Constructors

Constructor	Description
FieldComparator()

Method Summary

Modifier and Type	Method	Description
abstract int	compare(int slot1, int slot2)	Compare hit at slot1 with hit at slot2.
int	compareValues(T first, T second)	Returns a negative integer if first is less than second, 0 if they are equal and a positive integer otherwise.
void	disableSkipping()	Informs the comparator that the skipping of documents should be disabled.
abstract LeafFieldComparator	getLeafComparator(LeafReaderContext context)	Get a per-segment LeafFieldComparator to collect the given LeafReaderContext.
void	setSingleSort()	Informs the comparator that sort is done on this single field.
abstract void	setTopValue(T value)	Record the top value, for future calls to LeafFieldComparator.compareTop(int).
abstract T	value(int slot)	Return the actual value in the slot.

Methods inherited from class java.lang.Object

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

Constructor Details

FieldComparator

public FieldComparator()

Method Details

compare

public abstract int compare(int slot1,
 int slot2)

Compare hit at slot1 with hit at slot2.

Parameters:

slot1 - first slot to compare

slot2 - second slot to compare

Returns:

any N < 0 if slot2's value is sorted after slot1, any N > 0 if the slot2's value is sorted before slot1 and 0 if they are equal

setTopValue

public abstract void setTopValue(T value)

Record the top value, for future calls to LeafFieldComparator.compareTop(int). This is only called for searches that use searchAfter (deep paging), and is called before any calls to getLeafComparator(LeafReaderContext).

value

public abstract T value(int slot)

Return the actual value in the slot.

Parameters:

slot - the value

Returns:

value in this slot

getLeafComparator

public abstract LeafFieldComparator getLeafComparator(LeafReaderContext context)
                                               throws IOException

Get a per-segment LeafFieldComparator to collect the given LeafReaderContext. All docIDs supplied to this LeafFieldComparator are relative to the current reader (you must add docBase if you need to map it to a top-level docID).

Parameters:

context - current reader context

Returns:

the comparator to use for this segment

Throws:

IOException - if there is a low-level IO error

compareValues

public int compareValues(T first,
 T second)

Returns a negative integer if first is less than second, 0 if they are equal and a positive integer otherwise. Default impl to assume the type implements Comparable and invoke .compareTo; be sure to override this method if your FieldComparator's type isn't a Comparable or if your values may sometimes be null

setSingleSort

public void setSingleSort()

Informs the comparator that sort is done on this single field. This is useful to enable some optimizations for skipping non-competitive documents.

disableSkipping

public void disableSkipping()

Informs the comparator that the skipping of documents should be disabled. This function is called in cases when the skipping functionality should not be applied or not necessary. One example for numeric comparators is when we don't know if the same numeric data has been indexed with docValues and points if these two fields have the same name. As the skipping functionality relies on these fields to have the same data and as we don't know if it is true, we have to disable it. Another example could be when search sort is a part of the index sort, and can be already efficiently handled by TopFieldCollector, and doing extra work for skipping in the comparator is redundant.