Interface ICollector

Expert: Collectors are primarily meant to be used to gather raw results from a search, and implement sorting or custom result filtering, collation, etc.

Lucene's core collectors are derived from Collector. Likely your application can use one of these classes, or subclass TopDocsCollector<T>, instead of implementing ICollector directly:

• TopDocsCollector<T> is an abstract base class that assumes you will retrieve the top N docs, according to some criteria, after collection is done.
• TopScoreDocCollector is a concrete subclass TopDocsCollector<T> and sorts according to score + docID. This is used internally by the IndexSearcher search methods that do not take an explicit Sort. It is likely the most frequently used collector.
• TopFieldCollector subclasses TopDocsCollector<T> and sorts according to a specified Sort object (sort by field). This is used internally by the IndexSearcher search methods that take an explicit Sort.
• TimeLimitingCollector, which wraps any other Collector and aborts the search if it's taken too much time.
• PositiveScoresOnlyCollector wraps any other ICollector and prevents collection of hits whose score is <= 0.0

ICollector decouples the score from the collected doc: the score computation is skipped entirely if it's not needed. Collectors that do need the score should implement the SetScorer(Scorer) method, to hold onto the passed Scorer instance, and call GetScore() within the collect method to compute the current hit's score. If your collector may request the score for a single hit multiple times, you should use ScoreCachingWrappingScorer.

NOTE: The doc that is passed to the collect method is relative to the current reader. If your collector needs to resolve this to the docID space of the Multi*Reader, you must re-base it by recording the docBase from the most recent SetNextReader(AtomicReaderContext) call. Here's a simple example showing how to collect docIDs into an OpenBitSet:

private class MySearchCollector : ICollector
{
    private readonly OpenBitSet bits;
    private int docBase;

    public MySearchCollector(OpenBitSet bits)
    {
        if (bits == null) throw new ArgumentNullException("bits");
        this.bits = bits;
    }

    // ignore scorer
    public void SetScorer(Scorer scorer)
    { 
    }

    // accept docs out of order (for a BitSet it doesn't matter)
    public bool AcceptDocsOutOfOrder
    {
        get { return true; }
    }

    public void Collect(int doc)
    {
        bits.Set(doc + docBase);
    }

    public void SetNextReader(AtomicReaderContext context)
    {
        this.docBase = context.DocBase;
    }
}

IndexSearcher searcher = new IndexSearcher(indexReader);
OpenBitSet bits = new OpenBitSet(indexReader.MaxDoc);
searcher.Search(query, new MySearchCollector(bits));

Not all collectors will need to rebase the docID. For example, a collector that simply counts the total number of hits would skip it.

NOTE: Prior to 2.9, Lucene silently filtered out hits with score <= 0. As of 2.9, the core ICollectors no longer do that. It's very unusual to have such hits (a negative query boost, or function query returning negative custom scores, could cause it to happen). If you need that behavior, use PositiveScoresOnlyCollector.

@lucene.experimental

@since 2.9

Namespace: Lucene.Net.Search

Assembly: Lucene.Net.dll

Syntax

public interface ICollector

Properties

AcceptsDocsOutOfOrder

Return true if this collector does not require the matching docIDs to be delivered in int sort order (smallest to largest) to Collect(Int32).

Most Lucene Query implementations will visit matching docIDs in order. However, some queries (currently limited to certain cases of BooleanQuery) can achieve faster searching if the ICollector allows them to deliver the docIDs out of order.

Many collectors don't mind getting docIDs out of order, so it's important to return true here.

Declaration

bool AcceptsDocsOutOfOrder { get; }

Property Value

Type	Description
System.Boolean

Methods

Collect(Int32)

Called once for every document matching a query, with the unbased document number.

Note: The collection of the current segment can be terminated by throwing a CollectionTerminatedException. In this case, the last docs of the current AtomicReaderContext will be skipped and IndexSearcher will swallow the exception and continue collection with the next leaf.

Note: this is called in an inner search loop. For good search performance, implementations of this method should not call Doc(Int32) or Document(Int32) on every hit. Doing so can slow searches by an order of magnitude or more.

Declaration

void Collect(int doc)

Parameters

Type	Name	Description
System.Int32	doc

SetNextReader(AtomicReaderContext)

Called before collecting from each AtomicReaderContext. All doc ids in Collect(Int32) will correspond to Reader.

Add DocBase to the current Reader's internal document id to re-base ids in Collect(Int32).

Declaration

void SetNextReader(AtomicReaderContext context)

Parameters

Type	Name	Description
AtomicReaderContext	context	next atomic reader context

SetScorer(Scorer)

Called before successive calls to Collect(Int32). Implementations that need the score of the current document (passed-in to Collect(Int32)), should save the passed-in Scorer and call scorer.GetScore() when needed.

Declaration

void SetScorer(Scorer scorer)

Parameters

Type	Name	Description
Scorer	scorer