Segment read

route ：

org.apache.lucene.search.IndexSearcher  IndexSearcher

org.apache.lucene.index.CompositeReader getContext

org.apache.lucene.index.CompositeReaderContext create

org.apache.lucene.index.CompositeReaderContext build 

    private IndexReaderContext build(CompositeReaderContext parent, IndexReader reader, int ord, int docBase) {
      if (reader instanceof LeafReader) {
        final LeafReader ar = (LeafReader) reader;
        final LeafReaderContext atomic = new LeafReaderContext(parent, ar, ord, docBase, leaves.size(), leafDocBase);
        leaves.add(atomic);
        leafDocBase += reader.maxDoc();
        return atomic;
      } else {
        final CompositeReader cr = (CompositeReader) reader;
        final List<? extends IndexReader> sequentialSubReaders = cr.getSequentialSubReaders();
        final List<IndexReaderContext> children = Arrays.asList(new IndexReaderContext[sequentialSubReaders.size()]);
        final CompositeReaderContext newParent;
        if (parent == null) {
          newParent = new CompositeReaderContext(cr, children, leaves);
        } else {
          newParent = new CompositeReaderContext(parent, cr, ord, docBase, children);
        }
        // docBase It's cumulative 
        int newDocBase = 0;
        for (int i = 0, c = sequentialSubReaders.size(); i < c; i++) {
          final IndexReader r = sequentialSubReaders.get(i);
            //  Here is a recursion 
          children.set(i, build(newParent, r, i, newDocBase));
          newDocBase += r.maxDoc();
        }
        assert newDocBase == cr.maxDoc();
        return newParent;
      }
    }

org.apache.lucene.index.OrdinalMap map

// weights： The same field In different segment Different from term The number of     
private static int[] map(final long[] weights) {
      final int[] newToOld = new int[weights.length];
      for (int i = 0; i < weights.length; ++i) {
        newToOld[i] = i;
      }
      new InPlaceMergeSorter() {
        @Override
        protected void swap(int i, int j) {
          final int tmp = newToOld[i];
          newToOld[i] = newToOld[j];
          newToOld[j] = tmp;
        }
        @Override
        protected int compare(int i, int j) {
          // j first since we actually want higher weights first
          return Long.compare(weights[newToOld[j]], weights[newToOld[i]]);
        }
      }.sort(0, weights.length);
      return newToOld;
    }