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+//=============================================================================
+// Copyright 2006-2010 Daniel W. Dyer
+//
+// Licensed 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.uncommons.watchmaker.examples.strings;
+
+import java.util.ArrayList;
+import java.util.List;
+import org.uncommons.maths.random.MersenneTwisterRNG;
+import org.uncommons.maths.random.Probability;
+import org.uncommons.watchmaker.framework.EvolutionEngine;
+import org.uncommons.watchmaker.framework.EvolutionObserver;
+import org.uncommons.watchmaker.framework.EvolutionaryOperator;
+import org.uncommons.watchmaker.framework.GenerationalEvolutionEngine;
+import org.uncommons.watchmaker.framework.PopulationData;
+import org.uncommons.watchmaker.framework.factories.StringFactory;
+import org.uncommons.watchmaker.framework.operators.EvolutionPipeline;
+import org.uncommons.watchmaker.framework.operators.StringCrossover;
+import org.uncommons.watchmaker.framework.operators.StringMutation;
+import org.uncommons.watchmaker.framework.selection.RouletteWheelSelection;
+import org.uncommons.watchmaker.framework.termination.TargetFitness;
+
+/**
+ * Simple evolutionary algorithm that evolves a population of randomly-generated
+ * strings until at least one matches a specified target string.
+ * @author Daniel Dyer
+ */
+public final class StringsExample
+{
+    private static final char[] ALPHABET = new char[27];
+    static
+    {
+        for (char c = 'A'; c <= 'Z'; c++)
+        {
+            ALPHABET[c - 'A'] = c;
+        }
+        ALPHABET[26] = ' ';
+    }
+
+
+    /**
+     * Entry point for the sample application.  Any data specified on the
+     * command line is considered to be the target String.  If no target is
+     * specified, a default of "HELLOW WORLD" is used instead.
+     * @param args The target String (as an array of words).
+     */
+    public static void main(String[] args)
+    {
+        String target = args.length == 0 ? "HELLO WORLD" : convertArgs(args);
+        String result = evolveString(target);
+        System.out.println("Evolution result: " + result);
+    }
+
+
+    public static String evolveString(String target)
+    {
+        StringFactory factory = new StringFactory(ALPHABET, target.length());
+        List<EvolutionaryOperator<String>> operators = new ArrayList<EvolutionaryOperator<String>>(2);
+        operators.add(new StringMutation(ALPHABET, new Probability(0.02d)));
+        operators.add(new StringCrossover());
+        EvolutionaryOperator<String> pipeline = new EvolutionPipeline<String>(operators);
+        EvolutionEngine<String> engine = new GenerationalEvolutionEngine<String>(factory,
+                                                                                 pipeline,
+                                                                                 new StringEvaluator(target),
+                                                                                 new RouletteWheelSelection(),
+                                                                                 new MersenneTwisterRNG());
+        engine.addEvolutionObserver(new EvolutionLogger());
+        return engine.evolve(100, // 100 individuals in the population.
+                             5, // 5% elitism.
+                             new TargetFitness(0, false));
+    }
+
+
+    /**
+     * Converts an arguments array into a single String of words
+     * separated by spaces.
+     * @param args The command-line arguments.
+     * @return A single String made from the command line data.
+     */
+    private static String convertArgs(String[] args)
+    {
+        StringBuilder result = new StringBuilder();
+        for (int i = 0; i < args.length; i++)
+        {
+            result.append(args[i]);
+            if (i < args.length - 1)
+            {
+                result.append(' ');
+            }
+        }
+        return result.toString().toUpperCase();
+    }
+
+
+    /**
+     * Trivial evolution observer for displaying information at the end
+     * of each generation.
+     */
+    private static class EvolutionLogger implements EvolutionObserver<String>
+    {
+        public void populationUpdate(PopulationData<? extends String> data)
+        {
+            System.out.printf("Generation %d: %s\n",
+                              data.getGenerationNumber(),
+                              data.getBestCandidate());
+        }
+    }
+}