[jira] [Created] (LUCENE-5527) Make the Collector API work per-segment

["Adrien Grand (JIRA)" <ji...@apache.org>]

Adrien Grand created LUCENE-5527:
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             Summary: Make the Collector API work per-segment
                 Key: LUCENE-5527
                 URL: https://issues.apache.org/jira/browse/LUCENE-5527
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Adrien Grand
            Priority: Minor


Spin-off of LUCENE-5299.

LUCENE-5229 is large and proposes lots of different changes, some of them being controversial, but there is one of them that I really really like that consists in refactoring the {{Collector}} API in order to have a different Collector per segment.

The idea is, instead of having a single Collector object that needs to be able to take care of all segments, to have a top-level Collector:
{code}
public interface Collector {

  AtomicCollector setNextReader(AtomicReaderContext context) throws IOException;
  
}
{code}

and a per-AtomicReaderContext collector:

{code}
public interface AtomicCollector {

  void setScorer(Scorer scorer) throws IOException;

  void collect(int doc) throws IOException;

  boolean acceptsDocsOutOfOrder();

}
{code}

I think it makes the API clearer since it is now obious {{setScorer}} and {{acceptDocsOutOfOrder}} need to be called after {{setNextReader}} which is otherwise unclear.

It also makes things more flexible. For example, a collector could much more easily decide to use different strategies on different segments. In particular, it makes the early-termination collector much cleaner since it can return different atomic collectors implementations depending on whether the current segment is sorted or not.

Even if we have lots of collectors all over the place, we could make it easier to migrate by having a Collector that would implement both Collector and AtomicCollector, return {{this}} in setNextReader and make current concrete Collector implementations extend this class instead of directly extending Collector.



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