content: /*
   * The Alluxio Open Foundation licenses this work under the Apache License, version 2.0
   * (the "License"). You may not use this work except in compliance with the License, which is
   * available at www.apache.org/licenses/LICENSE-2.0
   *
   * This software is distributed on an "AS IS" basis, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
   * either express or implied, as more fully set forth in the License.
   *
   * See the NOTICE file distributed with this work for information regarding copyright ownership.
   */
  
  package alluxio.collections;
  
  import java.util.function.Function;
  import javax.annotation.concurrent.ThreadSafe;
  
  /**
   * A class representing an index for an {@link IndexedSet}.
   *
   * For example, if an indexed set stores inodes, and we want to be able to look them up by their
   * "id" field, we would define an index like so:
   *
   * <pre>
   * public class IndexDefinition&lt;Inode, Long&gt;() {
   *   &commat;Override
   *   Long getFieldValue(Inode) {
   *     return Inode.getId();
   *   }
   * }
   * </pre>
   *
   * @param <T> the type that this is an index for
   * @param <V> the type of the value used for indexing
   */
  @ThreadSafe
  public abstract class IndexDefinition<T, V> {
    /** Whether it is a unique index. */
    private final boolean mIsUnique;
  
    /**
     * Constructs a new {@link IndexDefinition} instance.
     *
     * @param isUnique whether the index is unique. A unique index is an index where each index value
     *                 only maps to one object; A non-unique index is an index where an index value
     *                 can map to one or more objects.
     */
    public IndexDefinition(boolean isUnique) {
      mIsUnique = isUnique;
    }
  
    /**
     * @return whether the index requires all field values to be unique
     */
    public boolean isUnique() {
      return mIsUnique;
    }
  
    /**
     * Gets the value of the field that serves as index.
     *
     * @param o the instance to get the field value from
     * @return the field value
     */
    public abstract V getFieldValue(T o);
  
    /**
     * Creates a new, unique index definition.
     *
     * @param definition the mapping function that maps an indexed element in the set to its index
     * @return the unique definition
     * @param <T> the element type in the indexed set
     * @param <V> the index type
     */
    public static <T, V> IndexDefinition<T, V> ofUnique(Function<T, V> definition) {
      return new IndexDefinition<T, V>(true) {
        @Override
        public V getFieldValue(T o) {
          return definition.apply(o);
        }
      };
    }
  
    /**
     * Creates a new, non-unique index definition.
     *
     * @param definition the mapping function that maps an indexed element in the set to its index
     * @return the non-unique definition
     * @param <T> the element type in the indexed set
     * @param <V> the index type
     */
    public static <T, V> IndexDefinition<T, V> ofNonUnique(Function<T, V> definition) {
      return new IndexDefinition<T, V>(false) {
        @Override
        public V getFieldValue(T o) {
          return definition.apply(o);
        }
      };
    }
  }
  