Easier way to access ValueSource given docid [LUCENE-4541]

[asfimport]

I was working on improving some sample code and I needed to access a value via a ValueSource given a docid in search results. I was disappointed to see how many steps were needed. Incidentally, Solr had to solve the same problem, and it's got ValueSourceAugmenter for this. I propose a small utility class be created to do something similar, such that the client can merely create an instance and then call a method with a doc id to retrieve the value.

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Migrated from LUCENE-4541 by David Smiley (@dsmiley)

[asfimport]

David Smiley (@dsmiley) (migrated from JIRA)

  public class ValueSourceAccessor {
    // implement FunctionValues ?
    private final List<AtomicReaderContext> readerContexts;
    private final FunctionValues[] docValuesArr;
    private final ValueSource valueSource;
    private final Map fContext;

    private int localId;
    private FunctionValues values;

    public ValueSourceAccessor(IndexSearcher searcher, ValueSource valueSource) {
      readerContexts = searcher.getIndexReader().leaves();
      this.valueSource = valueSource;
      docValuesArr = new FunctionValues[readerContexts.size()];
      fContext = ValueSource.newContext(searcher);
    }

    private void setState(int docid) throws IOException {
      int idx = ReaderUtil.subIndex(docid, readerContexts);
      AtomicReaderContext rcontext = readerContexts.get(idx);
      values = docValuesArr[idx];
      if (values == null) {
        docValuesArr[idx] = values = valueSource.getValues(fContext, rcontext);
      }
      localId = docid - rcontext.docBase;
    }

    public double doubleVal(int docid) throws IOException {
      setState(docid);
      return values.doubleVal(localId);
    }

    public Object objectVal(int docid) throws IOException {
      setState(docid);
      return values.objectVal(localId);
    }
    
    //...
  }