content: /*
   *  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.causeway.applib.util;
  
  import java.util.List;
  import java.util.Objects;
  import java.util.function.Function;
  
  import org.apache.causeway.commons.internal.collections._Lists;
  
  /**
   * Fluent Object Hash Code Composition.
   *
   * @param <T>
   * @since 2.0 {@index}
   */
  public class Hashing<T> {
  
      public static <T> Hashing<T> hashing(Function<? super T, ?> getter){
          return new Hashing<>(getter);
      }
  
      private final List<Function<? super T, ?>> getters = _Lists.newArrayList();
  
      private Hashing(Function<? super T, ?> getter) {
          getters.add(getter);
      }
  
      public Hashing<T> thenHashing(Function<? super T, ?> getter){
          Objects.requireNonNull(getter);
          getters.add(getter);
          return this;
      }
  
      public int hashCode(T object){
          if(object==null) {
              return 0;
          }
          int result = 1;
          for(Function<? super T, ?> getter : getters) {
              final Object element = getter.apply(object);
              result = 31 * result + (element == null ? 0 : element.hashCode());
          }
          return result;
      }
  
  }
  