/*

  * 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 org.apache.commons.functor.Function;

 /**

  * Tail recursion for {@link Function functions}. If the {@link Function}

  * returns another function of type <code>functionType</code>, that function

  * is executed. Functions are executed until a non function value or a

  * function of a type other than that expected is returned.

  */

 public class RecursiveEvaluation implements Function<Object>, Serializable {

     /**

      * serialVersionUID declaration.

      */

     private static final long serialVersionUID = -7992078213921938619L;

     private final Class<?> functionType;

     private Function<?> function;

     /**

      * Create a new RecursiveEvaluation. Recursion will continue while the

      * returned value is of the same runtime class as <code>function</code>.

      * @param function initial, potentially recursive Function

      */

     public RecursiveEvaluation(Function<?> function) {

         this(function, getClass(function));

     }

     /**

      * Create a new RecursiveEvaluation.

      * @param function initial, potentially recursive Function

      * @param functionType as long as result is an instance, keep processing.

      */

     public RecursiveEvaluation(Function<?> function, Class<?> functionType) {

         if (function == null) {

             throw new IllegalArgumentException("Function argument was null");

         }

         if (functionType == null || !Function.class.isAssignableFrom(functionType)) {

             throw new IllegalArgumentException(Function.class + " is not assignable from " + functionType);

         }

         this.function = function;

         this.functionType = functionType;

     }

     /**

      * {@inheritDoc}

      */

     public final Object evaluate() {

         Object result = null;

         // if the function returns another function, execute it. stop executing

         // when the result is not of the expected type.

         while (true) {

             result = function.evaluate();

             if (functionType.isInstance(result)) {

                 function = (Function<?>) result;

                 continue;

             } else {

                 break;

             }

         }

         return result;

     }

     /**

      * {@inheritDoc}

      */

     public final boolean equals(Object obj) {

         if (obj == this) {

             return true;

         }

         if (!(obj instanceof RecursiveEvaluation)) {

             return false;

         }

         return ((RecursiveEvaluation) obj).function.equals(function);

     }

     /**

      * {@inheritDoc}

      */

     public int hashCode() {

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

     }

     /**

      * Get the class of the specified object, or <code>null</code> if <code>o</code> is <code>null</code>.

      * @param o Object to check

      * @return Class found

      */

     private static <T> Class<?> getClass(Function<?> f) {

         return f == null ? null : f.getClass();

     }

 }