package org.apache.lucene.search;

/**
 * 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 org.apache.lucene.util.ArrayUtil;
import java.io.IOException;
import java.util.Collection;
import java.util.Comparator;

/** Scorer for conjunctions, sets of queries, all of which are required. */
class ConjunctionScorer extends Scorer {
  
  private final Scorer[] scorers;
  private int lastDoc = -1;

  public ConjunctionScorer(Weight weight, Collection<Scorer> scorers) throws IOException {
    this(weight, scorers.toArray(new Scorer[scorers.size()]));
  }

  public ConjunctionScorer(Weight weight, Scorer... scorers) throws IOException {
    super(weight);
    this.scorers = scorers;
    
    for (int i = 0; i < scorers.length; i++) {
      if (scorers[i].nextDoc() == NO_MORE_DOCS) {
        // If even one of the sub-scorers does not have any documents, this
        // scorer should not attempt to do any more work.
        lastDoc = NO_MORE_DOCS;
        return;
      }
    }

    // Sort the array the first time...
    // We don't need to sort the array in any future calls because we know
    // it will already start off sorted (all scorers on same doc).
    
    // Note that this comparator is not consistent with equals!
    // Also we use mergeSort here to be stable (so order of Scoreres that
    // match on first document keeps preserved):
    ArrayUtil.mergeSort(scorers, new Comparator<Scorer>() { // sort the array
      public int compare(Scorer o1, Scorer o2) {
        return o1.docID() - o2.docID();
      }
    });

    // NOTE: doNext() must be called before the re-sorting of the array later on.
    // The reason is this: assume there are 5 scorers, whose first docs are 1,
    // 2, 3, 5, 5 respectively. Sorting (above) leaves the array as is. Calling
    // doNext() here advances all the first scorers to 5 (or a larger doc ID
    // they all agree on). 
    // However, if we re-sort before doNext() is called, the order will be 5, 3,
    // 2, 1, 5 and then doNext() will stop immediately, since the first scorer's
    // docs equals the last one. So the invariant that after calling doNext() 
    // all scorers are on the same doc ID is broken.
    if (doNext() == NO_MORE_DOCS) {
      // The scorers did not agree on any document.
      lastDoc = NO_MORE_DOCS;
      return;
    }

    // If first-time skip distance is any predictor of
    // scorer sparseness, then we should always try to skip first on
    // those scorers.
    // Keep last scorer in it's last place (it will be the first
    // to be skipped on), but reverse all of the others so that
    // they will be skipped on in order of original high skip.
    int end = scorers.length - 1;
    int max = end >> 1;
    for (int i = 0; i < max; i++) {
      Scorer tmp = scorers[i];
      int idx = end - i - 1;
      scorers[i] = scorers[idx];
      scorers[idx] = tmp;
    }
  }

  private int doNext() throws IOException {
    int first = 0;
    int doc = scorers[scorers.length - 1].docID();
    Scorer firstScorer;
    while ((firstScorer = scorers[first]).docID() < doc) {
      doc = firstScorer.advance(doc);
      first = first == scorers.length - 1 ? 0 : first + 1;
    }
    return doc;
  }
  
  @Override
  public int advance(int target) throws IOException {
    if (lastDoc == NO_MORE_DOCS) {
      return lastDoc;
    } else if (scorers[(scorers.length - 1)].docID() < target) {
      scorers[(scorers.length - 1)].advance(target);
    }
    return lastDoc = doNext();
  }

  @Override
  public int docID() {
    return lastDoc;
  }
  
  @Override
  public int nextDoc() throws IOException {
    if (lastDoc == NO_MORE_DOCS) {
      return lastDoc;
    } else if (lastDoc == -1) {
      return lastDoc = scorers[scorers.length - 1].docID();
    }
    scorers[(scorers.length - 1)].nextDoc();
    return lastDoc = doNext();
  }
  
  @Override
  public float score() throws IOException {
    float sum = 0.0f;
    for (int i = 0; i < scorers.length; i++) {
      sum += scorers[i].score();
    }
    return sum;
  }

  @Override
  public float freq() throws IOException {
    return scorers.length;
  }
}
