001 /*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements. See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License. You may obtain a copy of the License at
008 *
009 * http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017 package org.apache.commons.functor.core.algorithm;
018
019 import java.io.Serializable;
020
021 import org.apache.commons.functor.Function;
022
023 /**
024 * Tail recursion for {@link Function functions}. If the {@link Function}
025 * returns another function of type <code>functionType</code>, that function
026 * is executed. Functions are executed until a non function value or a
027 * function of a type other than that expected is returned.
028 */
029 public class RecursiveEvaluation implements Function<Object>, Serializable {
030 /**
031 * serialVersionUID declaration.
032 */
033 private static final long serialVersionUID = -7992078213921938619L;
034 private final Class<?> functionType;
035 private Function<?> function;
036
037 /**
038 * Create a new RecursiveEvaluation. Recursion will continue while the
039 * returned value is of the same runtime class as <code>function</code>.
040 * @param function initial, potentially recursive Function
041 */
042 public RecursiveEvaluation(Function<?> function) {
043 this(function, getClass(function));
044 }
045
046 /**
047 * Create a new RecursiveEvaluation.
048 * @param function initial, potentially recursive Function
049 * @param functionType as long as result is an instance, keep processing.
050 */
051 public RecursiveEvaluation(Function<?> function, Class<?> functionType) {
052 if (function == null) {
053 throw new IllegalArgumentException("Function argument was null");
054 }
055 if (functionType == null || !Function.class.isAssignableFrom(functionType)) {
056 throw new IllegalArgumentException(Function.class + " is not assignable from " + functionType);
057 }
058 this.function = function;
059 this.functionType = functionType;
060 }
061
062 /**
063 * {@inheritDoc}
064 */
065 public final Object evaluate() {
066 Object result = null;
067 // if the function returns another function, execute it. stop executing
068 // when the result is not of the expected type.
069 while (true) {
070 result = function.evaluate();
071 if (functionType.isInstance(result)) {
072 function = (Function<?>) result;
073 continue;
074 } else {
075 break;
076 }
077 }
078 return result;
079 }
080
081 /**
082 * {@inheritDoc}
083 */
084 public final boolean equals(Object obj) {
085 if (obj == this) {
086 return true;
087 }
088 if (!(obj instanceof RecursiveEvaluation)) {
089 return false;
090 }
091 return ((RecursiveEvaluation) obj).function.equals(function);
092 }
093
094 /**
095 * {@inheritDoc}
096 */
097 public int hashCode() {
098 return "RecursiveEvaluation".hashCode() << 2 ^ function.hashCode();
099 }
100
101 /**
102 * Get the class of the specified object, or <code>null</code> if <code>o</code> is <code>null</code>.
103 * @param o Object to check
104 * @return Class found
105 */
106 private static <T> Class<?> getClass(Function<?> f) {
107 return f == null ? null : f.getClass();
108 }
109 }