package org.apache.lucene.util;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.util.ArrayList;
import java.util.List;
import java.io.IOException;

import org.apache.lucene.index.IndexReader;

/**
 * Common util methods for dealing with {@link IndexReader}s.
 *
 * @lucene.internal
 */
public class ReaderUtil {

  public static class Slice {
    public static final Slice[] EMPTY_ARRAY = new Slice[0];
    public final int start;
    public final int length;
    public final int readerIndex;

    public Slice(int start, int length, int readerIndex) {
      this.start = start;
      this.length = length;
      this.readerIndex = readerIndex;
    }

    public String toString() {
      return "slice start=" + start + " length=" + length + " readerIndex=" + readerIndex;
    }
  }

  /**
   * Gathers sub-readers from reader into a List.  See
   * {@link Gather} for are more general way to gather
   * whatever you need to, per reader.
   *
   * @lucene.experimental
   * 
   * @param allSubReaders
   * @param reader
   */

  public static void gatherSubReaders(final List<IndexReader> allSubReaders, IndexReader reader) {
    try {
      new Gather(reader) {
        @Override
        protected void add(int base, IndexReader r) {
          allSubReaders.add(r);
        }
      }.run();
    } catch (IOException ioe) {
      // won't happen
      throw new RuntimeException(ioe);
    }
  }

  /** Recursively visits all sub-readers of a reader.  You
   *  should subclass this and override the add method to
   *  gather what you need.
   *
   * @lucene.experimental */
  public static abstract class Gather {
    private final IndexReader topReader;

    public Gather(IndexReader r) {
      topReader = r;
    }

    public int run() throws IOException {
      return run(0, topReader);
    }

    public int run(int docBase) throws IOException {
      return run(docBase, topReader);
    }

    private int run(int base, IndexReader reader) throws IOException {
      IndexReader[] subReaders = reader.getSequentialSubReaders();
      if (subReaders == null) {
        // atomic reader
        add(base, reader);
        base += reader.maxDoc();
      } else {
        // composite reader
        for (int i = 0; i < subReaders.length; i++) {
          base = run(base, subReaders[i]);
        }
      }

      return base;
    }

    protected abstract void add(int base, IndexReader r) throws IOException;
  }

  /**
   * Returns sub IndexReader that contains the given document id.
   *    
   * @param doc id of document
   * @param reader parent reader
   * @return sub reader of parent which contains the specified doc id
   */
  public static IndexReader subReader(int doc, IndexReader reader) {
    List<IndexReader> subReadersList = new ArrayList<IndexReader>();
    ReaderUtil.gatherSubReaders(subReadersList, reader);
    IndexReader[] subReaders = subReadersList
        .toArray(new IndexReader[subReadersList.size()]);
    int[] docStarts = new int[subReaders.length];
    int maxDoc = 0;
    for (int i = 0; i < subReaders.length; i++) {
      docStarts[i] = maxDoc;
      maxDoc += subReaders[i].maxDoc();
    }
    return subReaders[subIndex(doc, docStarts)];
  }
  
  /**
   * Returns sub-reader subIndex from reader.
   * 
   * @param reader parent reader
   * @param subIndex index of desired sub reader
   * @return the subreader at subIndex
   */
  public static IndexReader subReader(IndexReader reader, int subIndex) {
    List<IndexReader> subReadersList = new ArrayList<IndexReader>();
    ReaderUtil.gatherSubReaders(subReadersList, reader);
    IndexReader[] subReaders = subReadersList
        .toArray(new IndexReader[subReadersList.size()]);
    return subReaders[subIndex];
  }


  /**
   * Returns index of the searcher/reader for document <code>n</code> in the
   * array used to construct this searcher/reader.
   */
  public static int subIndex(int n, int[] docStarts) { // find
    // searcher/reader for doc n:
    int size = docStarts.length;
    int lo = 0; // search starts array
    int hi = size - 1; // for first element less than n, return its index
    while (hi >= lo) {
      int mid = (lo + hi) >>> 1;
      int midValue = docStarts[mid];
      if (n < midValue)
        hi = mid - 1;
      else if (n > midValue)
        lo = mid + 1;
      else { // found a match
        while (mid + 1 < size && docStarts[mid + 1] == midValue) {
          mid++; // scan to last match
        }
        return mid;
      }
    }
    return hi;
  }
}