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https://issues.apache.org/jira/browse/LUCENE-5527?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13959218#comment-13959218
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Adrien Grand commented on LUCENE-5527:
--------------------------------------
The patch only moves 3 methods of the {{Collector}} class into a new
{{LeafCollector}} class that gets instantiated per-segment. It doesn't add any
functionnality but just changes the way documents are collected.
I think having callbacks upon collection completion would be useful and help
clean up code as [~shikhar] mentionned. But this {{LeafCollector}} refactoring
has already been stalled in LUCENE-5299 because LUCENE-5299 also discussed
parallelizing collection so I would like to keep this change as small as
possible and get it in.
> 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
> Fix For: 5.0
>
> Attachments: LUCENE-5527.patch
>
>
> Spin-off of LUCENE-5299.
> LUCENE-5229 proposes 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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