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.commons.internal.base;
  
  import java.util.Optional;
  import java.util.function.BinaryOperator;
  import java.util.function.Consumer;
  import java.util.stream.Stream;
  
  import org.jspecify.annotations.Nullable;
  
  import org.jspecify.annotations.NonNull;
  
  /**
   * <h1>- internal use only -</h1>
   * <p>
   * Provides a generic (left-fold) reduction class.
   * <p>
   * Most intuitive example of a reduction is finding the
   * minimum value from a list of values.
   * See {@code org.apache.causeway.commons.internal.base.ReductionTest} for examples.
   * <p>
   * <b>WARNING</b>: Do <b>NOT</b> use any of the classes provided by this package! <br/>
   * These may be changed or removed without notice!
   * </p>
   *
   * @since 2.0
   *
   */
  public final class _Reduction<T> implements Consumer<T> {
  
      private final BinaryOperator<T> accumulator;
      private T result;
      private boolean initialized = false;
  
      /**
       * Inspired by {@link Stream#reduce(Object, BinaryOperator)}
       * @param identity
       * @param accumulator
       */
      public static <T> _Reduction<T> of(final @Nullable T identity, final BinaryOperator<T> accumulator){
          return new _Reduction<T>(identity, accumulator, true);
      }
  
      /**
       * Inspired by {@link Stream#reduce(BinaryOperator)}
       * @param accumulator
       */
      public static <T> _Reduction<T> of(final BinaryOperator<T> accumulator){
          return new _Reduction<T>(null, accumulator, false);
      }
  
      private _Reduction(final @Nullable T identity, final @NonNull BinaryOperator<T> accumulator, final boolean initialized) {
          this.initialized = initialized;
          this.result = identity;
          this.accumulator = accumulator;
      }
  
      @Override
      public void accept(final @Nullable T next) {
          if(!initialized) {
              result = next;
              initialized = true;
              return;
          }
          result = accumulator.apply(result, next);
      }
  
      /**
       * Returns the reduction result if ever initialized, Optional.empty() otherwise.
       * @return non-null
       */
      public Optional<T> getResult() {
          if(!initialized) {
              return Optional.empty();
          }
          return Optional.ofNullable(result);
      }
  
  }
  