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https://issues.apache.org/jira/browse/LUCENE-5527?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Adrien Grand updated LUCENE-5527:
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Attachment: LUCENE-5527.patch
Here is a patch, all tests pass.
As much as possible, I tried to only change the API, not implementations so
top-docs collectors, etc. still work exactly the same way. The helper
{{SimpleCollector}} implements both {{Collector}} and {{AtomicCollector}} and
returns itself in {{setNextReader}}. It is useful for collectors that don't
need to change their behavior based on the current reader context such as
{{TotalHitCountCollector}}, but it is also quite handy for the migration since
migrating a {{Collector}} to the new API is just a matter of extending
{{SimpleCollector}} instead of {{Collector}} and renaming {{setNextReader}} to
{{doSetNextReader}}.
I had initially thought about doing this change for 5.0 only but since the
migration ends up being easy and since {{Collector}} is an experimental API, we
could also have it for 4.8.
Feedback is highly welcome!
bq. Maybe we can get rid of setScorer, passing Scorer to
setNextReader(AtomicReaderContext,Scorer)?
This can't be done today because there is a circular dependency between the
{{Collector}} and the {{Scorer}}. If you look at
{{IndexSearcher.searchsearch(List<AtomicReaderContext>, Weight, Collector)}},
it needs to know whether the collector supports out-of-order collection in
order to know which {{BulkScorer}} to instantiate, and then the {{BulkScorer}}
needs to let the {{Collector}} know about the {{Scorer}} that is being used.
> 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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