/*
 * 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.
 */
package org.apache.lucene.search;


import java.io.IOException;

/**
 * This abstract class defines methods to iterate over a set of non-decreasing
 * doc ids. Note that this class assumes it iterates on doc Ids, and therefore
 * {@link #NO_MORE_DOCS} is set to {@value #NO_MORE_DOCS} in order to be used as
 * a sentinel object. Implementations of this class are expected to consider
 * {@link Integer#MAX_VALUE} as an invalid value.
 */
public abstract class DocIdSetIterator {
  
  /** An empty {@code DocIdSetIterator} instance */
  public static final DocIdSetIterator empty() {
    return new DocIdSetIterator() {
      boolean exhausted = false;
      
      @Override
      public int advance(int target) {
        assert !exhausted;
        assert target >= 0;
        exhausted = true;
        return NO_MORE_DOCS;
      }
      
      @Override
      public int docID() {
        return exhausted ? NO_MORE_DOCS : -1;
      }
      @Override
      public int nextDoc() {
        assert !exhausted;
        exhausted = true;
        return NO_MORE_DOCS;
      }
      
      @Override
      public long cost() {
        return 0;
      }
    };
  }

  /** A {@link DocIdSetIterator} that matches all documents up to
   *  {@code maxDoc - 1}. */
  public static final DocIdSetIterator all(int maxDoc) {
    return new DocIdSetIterator() {
      int doc = -1;

      @Override
      public int docID() {
        return doc;
      }

      @Override
      public int nextDoc() throws IOException {
        return advance(doc + 1);
      }

      @Override
      public int advance(int target) throws IOException {
        doc = target;
        if (doc >= maxDoc) {
          doc = NO_MORE_DOCS;
        }
        return doc;
      }

      @Override
      public long cost() {
        return maxDoc;
      }
    };
  }

  /**
   * When returned by {@link #nextDoc()}, {@link #advance(int)} and
   * {@link #docID()} it means there are no more docs in the iterator.
   */
  public static final int NO_MORE_DOCS = Integer.MAX_VALUE;

  /**
   * Returns the following:
   * <ul>
   * <li><code>-1</code> if {@link #nextDoc()} or
   * {@link #advance(int)} were not called yet.
   * <li>{@link #NO_MORE_DOCS} if the iterator has exhausted.
   * <li>Otherwise it should return the doc ID it is currently on.
   * </ul>
   * <p>
   * 
   * @since 2.9
   */
  public abstract int docID();

  /**
   * Advances to the next document in the set and returns the doc it is
   * currently on, or {@link #NO_MORE_DOCS} if there are no more docs in the
   * set.<br>
   * 
   * <b>NOTE:</b> after the iterator has exhausted you should not call this
   * method, as it may result in unpredicted behavior.
   * 
   * @since 2.9
   */
  public abstract int nextDoc() throws IOException;

 /**
   * Advances to the first beyond the current whose document number is greater 
   * than or equal to <i>target</i>, and returns the document number itself. 
   * Exhausts the iterator and returns {@link #NO_MORE_DOCS} if <i>target</i> 
   * is greater than the highest document number in the set.
   * <p>
   * The behavior of this method is <b>undefined</b> when called with
   * <code> target ≤ current</code>, or after the iterator has exhausted.
   * Both cases may result in unpredicted behavior.
   * <p>
   * When <code> target > current</code> it behaves as if written:
   * 
   * <pre class="prettyprint">
   * int advance(int target) {
   *   int doc;
   *   while ((doc = nextDoc()) < target) {
   *   }
   *   return doc;
   * }
   * </pre>
   * 
   * Some implementations are considerably more efficient than that.
   * <p>
   * <b>NOTE:</b> this method may be called with {@link #NO_MORE_DOCS} for
   * efficiency by some Scorers. If your implementation cannot efficiently
   * determine that it should exhaust, it is recommended that you check for that
   * value in each call to this method.
   * <p>
   *
   * @since 2.9
   */
  public abstract int advance(int target) throws IOException;

  /** Slow (linear) implementation of {@link #advance} relying on
   *  {@link #nextDoc()} to advance beyond the target position. */
  protected final int slowAdvance(int target) throws IOException {
    assert docID() < target;
    int doc;
    do {
      doc = nextDoc();
    } while (doc < target);
    return doc;
  }

  /**
   * Returns the estimated cost of this {@link DocIdSetIterator}.
   * <p>
   * This is generally an upper bound of the number of documents this iterator
   * might match, but may be a rough heuristic, hardcoded value, or otherwise
   * completely inaccurate.
   */
  public abstract long cost();
  
}