/*

  * 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.commons.functor.core.algorithm;

 import java.io.Serializable;

 import java.util.Stack;

 import org.apache.commons.functor.BinaryFunction;

 import org.apache.commons.functor.UnaryFunction;

 import org.apache.commons.functor.UnaryProcedure;

 import org.apache.commons.functor.generator.Generator;

 /**

  * Functional right-fold algorithm against the elements of a {@link Generator}.

  * Uses the seed object (if supplied) as the initial right-side argument to the {@link BinaryFunction},

  * then uses the result of that evaluation as the next right-side argument, until the {@link Generator}'s

  * elements have been expended.

  * @version $Revision: 1171154 $ $Date: 2011-09-15 17:58:38 +0200 (Thu, 15 Sep 2011) $

  */

 public class FoldRight<T> implements UnaryFunction<Generator<T>, T>, BinaryFunction<Generator<T>, T, T>, Serializable {

     /**

      * serialVersionUID declaration.

      */

     private static final long serialVersionUID = 4671387086700391506L;

     /**

      * Helper class

      */

     private static class FoldRightHelper<T> implements UnaryProcedure<T> {

         private final Stack<T> stk = new Stack<T>();

         private final BinaryFunction<? super T, ? super T, ? extends T> function;

         private final T seed;

         private final boolean hasSeed;

         /**

          * Create a seedless FoldRightHelper.

          */

         public FoldRightHelper(BinaryFunction<? super T, ? super T, ? extends T> function) {

             this(null, function);

         }

         /**

          * Create a new FoldRightHelper.

          * @param seed initial right-side argument

          */

         FoldRightHelper(T seed, BinaryFunction<? super T, ? super T, ? extends T> function) {

             this.seed = seed;

             hasSeed = seed != null ? true : false;

             this.function = function;

         }

         /**

          * {@inheritDoc}

          */

         public void run(T obj) {

             stk.push(obj);

         }

         /**

          * Get result after processing.

          * Get current result.

          * @return Object

          */

         T getResult() {

             T right = seed;

             if (!hasSeed) {

                 if (stk.isEmpty()) {

                     return null;

                 }

                 right = stk.pop();

             }

             while (!stk.isEmpty()) {

                 right = function.evaluate(stk.pop(), right);

             }

             return right;

         }

     }

     private final BinaryFunction<? super T, ? super T, ? extends T> function;

     /**

      * Create a new FoldRight.

      * @param function {@link BinaryFunction} to apply to each (seed, next)

      */

     public FoldRight(BinaryFunction<? super T, ? super T, ? extends T> function) {

         this.function = function;

     }

     /**

      * {@inheritDoc}

      * @param obj {@link Generator} to transform

      */

     public final T evaluate(Generator<T> obj) {

         FoldRightHelper<T> helper = new FoldRightHelper<T>(function);

         obj.run(helper);

         return helper.getResult();

     }

     /**

      * {@inheritDoc}

      * @param left {@link Generator} to transform

      * @param right initial right-side seed object

      */

     public final T evaluate(Generator<T> left, T right) {

         FoldRightHelper<T> helper = new FoldRightHelper<T>(right, function);

         left.run(helper);

         return helper.getResult();

     }

     /**

      * {@inheritDoc}

      */

     public final boolean equals(Object obj) {

         if (obj == this) {

             return true;

         }

         if (!(obj instanceof FoldRight<?>)) {

             return false;

         }

         return ((FoldRight<?>) obj).function.equals(function);

     }

     /**

      * {@inheritDoc}

      */

     public int hashCode() {

         return "FoldRight".hashCode() << 2 ^ function.hashCode();

     }

 }