lucene按某个字段排序代码解析

对于lucene来说，索引目录下有多个索引段，那么对应的每个索引段有相对应的reader.

这些reader它们之间是完全独立的，数据是独立的，搜索是独立的。

现在看一下按某个字段排序的收集器是如何做的

 private static class OneComparatorScoringMaxScoreCollector extends
      OneComparatorNonScoringCollector {

    Scorer scorer;
    
    public OneComparatorScoringMaxScoreCollector(FieldValueHitQueue<Entry> queue,
        int numHits, boolean fillFields) throws IOException {
      super(queue, numHits, fillFields);
      // Must set maxScore to NEG_INF, or otherwise Math.max always returns NaN.
      maxScore = Float.NEGATIVE_INFINITY;
    }
    
    final void updateBottom(int doc, float score) {
      bottom.doc = docBase + doc;
      bottom.score = score;
      bottom =  pq.updateTop();
    }

    @Override
    public void collect(int doc) throws IOException {
      final float score = scorer.score();
      if (score > maxScore) {
        maxScore = score;
      }
      ++totalHits;
      if (queueFull) {
        if ((reverseMul * comparator.compareBottom(doc)) <= 0) {
          // since docs are visited in doc Id order, if compare is 0, it means
          // this document is largest than anything else in the queue, and
          // therefore not competitive.
          return;
        }
        
        // This hit is competitive - replace bottom element in queue & adjustTop
        comparator.copy(bottom.slot, doc);
        updateBottom(doc, score);
        comparator.setBottom(bottom.slot);
      } else {
        // Startup transient: queue hasn't gathered numHits yet
        final int slot = totalHits - 1;
        // Copy hit into queue
        comparator.copy(slot, doc);
        add(slot, doc, score);
        if (queueFull) {
          comparator.setBottom(bottom.slot);
        }
      }

    }
    
    @Override
    public void setScorer(Scorer scorer) throws IOException {
      this.scorer = scorer;
      super.setScorer(scorer);
    }
  }

进入这个优先级队列的时候，当队列未满的时候可以直接拷贝在values数组中，并设置好一个bottom，当然队列满的时候只须跟bottom比较就可以了

小于bottom的直接不要。否则放入队列，并修改bottom的值。

我们先暂时使用数字字段的比较器IntComparator

IntComparator(int numHits, String field, FieldCache.Parser parser, Integer missingValue) {
      super(field, missingValue);
      values = new int[numHits];
      this.parser = (IntParser) parser;
    }

生成一个长度为numHits的数组values，保存放入堆里的值，优先级队列比较的时候是使用values来比较。但命中的doc可以放入优先级队列的时候，需要得到当前doc对应的字段的值。。如何获取呢：

@Override
    public void setNextReader(IndexReader reader, int docBase) throws IOException {
      // NOTE: must do this before calling super otherwise
      // we compute the docsWithField Bits twice!
      currentReaderValues = FieldCache.DEFAULT.getInts(reader, field, parser, missingValue != null);
      super.setNextReader(reader, docBase);
    }

这里可以从缓存中得到该reader下某字段的所有值放在currentReaderValues数组中。。

currentReaderValues[doc]直接得到该doc对应字段的值，并放在value数组中：

  public void copy(int slot, int doc) {
      int v2 = currentReaderValues[doc];
      // Test for v2 == 0 to save Bits.get method call for
      // the common case (doc has value and value is non-zero):
      if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
        v2 = missingValue;
      }
      values[slot] = v2;
    }