content: 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.Comparator;
  import java.util.Collections;
  import java.util.List;
  import java.util.RandomAccess;
  
  /**
   * Methods for manipulating (sorting) collections.
   * Sort methods work directly on the supplied lists and don't copy to/from arrays
   * before/after. For medium size collections as used in the Lucene indexer that is
   * much more efficient.
   *
   * @lucene.internal
   */
  
  public final class CollectionUtil {
  
    private CollectionUtil() {} // no instance
    
    /** SorterTemplate with custom {@link Comparator} */
    private static <T> SorterTemplate getSorter(final List<T> list, final Comparator<? super T> comp) {
      if (!(list instanceof RandomAccess))
        throw new IllegalArgumentException("CollectionUtil can only sort random access lists in-place.");
      return new SorterTemplate() {
        @Override
        protected void swap(int i, int j) {
          Collections.swap(list, i, j);
        }
        
        @Override
        protected int compare(int i, int j) {
          return comp.compare(list.get(i), list.get(j));
        }
  
        @Override
        protected void setPivot(int i) {
          pivot = list.get(i);
        }
    
        @Override
        protected int comparePivot(int j) {
          return comp.compare(pivot, list.get(j));
        }
        
        private T pivot;
      };
    }
    
    /** Natural SorterTemplate */
    private static <T extends Comparable<? super T>> SorterTemplate getSorter(final List<T> list) {
      if (!(list instanceof RandomAccess))
        throw new IllegalArgumentException("CollectionUtil can only sort random access lists in-place.");
      return new SorterTemplate() {
        @Override
        protected void swap(int i, int j) {
          Collections.swap(list, i, j);
        }
        
        @Override
        protected int compare(int i, int j) {
          return list.get(i).compareTo(list.get(j));
        }
  
        @Override
        protected void setPivot(int i) {
          pivot = list.get(i);
        }
    
        @Override
        protected int comparePivot(int j) {
          return pivot.compareTo(list.get(j));
        }
        
        private T pivot;
      };
    }
  
    /**
     * Sorts the given random access {@link List} using the {@link Comparator}.
     * The list must implement {@link RandomAccess}. This method uses the quick sort
     * algorithm, but falls back to insertion sort for small lists.
     * @throws IllegalArgumentException if list is e.g. a linked list without random access.
     */
    public static <T> void quickSort(List<T> list, Comparator<? super T> comp) {
      final int size = list.size();
      if (size <= 1) return;
      getSorter(list, comp).quickSort(0, size-1);
    }
    
    /**
     * Sorts the given random access {@link List} in natural order.
     * The list must implement {@link RandomAccess}. This method uses the quick sort
     * algorithm, but falls back to insertion sort for small lists.
     * @throws IllegalArgumentException if list is e.g. a linked list without random access.
     */
    public static <T extends Comparable<? super T>> void quickSort(List<T> list) {
      final int size = list.size();
      if (size <= 1) return;
      getSorter(list).quickSort(0, size-1);
    }
  
    // mergeSorts:
    
    /**
     * Sorts the given random access {@link List} using the {@link Comparator}.
     * The list must implement {@link RandomAccess}. This method uses the merge sort
     * algorithm, but falls back to insertion sort for small lists.
     * @throws IllegalArgumentException if list is e.g. a linked list without random access.
     */
    public static <T> void mergeSort(List<T> list, Comparator<? super T> comp) {
      final int size = list.size();
      if (size <= 1) return;
      getSorter(list, comp).mergeSort(0, size-1);
    }
    
    /**
     * Sorts the given random access {@link List} in natural order.
     * The list must implement {@link RandomAccess}. This method uses the merge sort
     * algorithm, but falls back to insertion sort for small lists.
     * @throws IllegalArgumentException if list is e.g. a linked list without random access.
     */
    public static <T extends Comparable<? super T>> void mergeSort(List<T> list) {
      final int size = list.size();
      if (size <= 1) return;
      getSorter(list).mergeSort(0, size-1);
    }
  
    // insertionSorts:
    
    /**
     * Sorts the given random access {@link List} using the {@link Comparator}.
     * The list must implement {@link RandomAccess}. This method uses the insertion sort
     * algorithm. It is only recommended to use this algorithm for partially sorted small lists!
     * @throws IllegalArgumentException if list is e.g. a linked list without random access.
     */
    public static <T> void insertionSort(List<T> list, Comparator<? super T> comp) {
      final int size = list.size();
      if (size <= 1) return;
      getSorter(list, comp).insertionSort(0, size-1);
    }
    
    /**
     * Sorts the given random access {@link List} in natural order.
     * The list must implement {@link RandomAccess}. This method uses the insertion sort
     * algorithm. It is only recommended to use this algorithm for partially sorted small lists!
     * @throws IllegalArgumentException if list is e.g. a linked list without random access.
     */
    public static <T extends Comparable<? super T>> void insertionSort(List<T> list) {
      final int size = list.size();
      if (size <= 1) return;
      getSorter(list).insertionSort(0, size-1);
    }
    
  }