//=============================================================================
// 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.framework.operators;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import org.uncommons.maths.binary.BitString;
import org.uncommons.maths.number.ConstantGenerator;
import org.uncommons.maths.number.NumberGenerator;
import org.uncommons.maths.random.Probability;
import org.uncommons.watchmaker.framework.EvolutionaryOperator;
/**
* Mutation of individual bits in a {@link BitString} according to some
* probability.
* @see org.uncommons.maths.binary.BitString
* @author Daniel Dyer
*/
public class BitStringMutation implements EvolutionaryOperator<BitString>
{
private final NumberGenerator<Probability> mutationProbability;
private final NumberGenerator<Integer> mutationCount;
/**
* Creates a mutation operator for bit strings with the specified probability that a given
* bit string will be mutated, with exactly one bit being flipped.
* @param mutationProbability The probability of a candidate being mutated.
*/
public BitStringMutation(Probability mutationProbability)
{
this(new ConstantGenerator<Probability>(mutationProbability),
new ConstantGenerator<Integer>(1));
}
/**
* Creates a mutation operator for bit strings, with the probability that any
* given bit will be flipped governed by the specified number generator.
* @param mutationProbability The (possibly variable) probability of a candidate
* bit string being mutated at all.
* @param mutationCount The (possibly variable) number of bits that will be flipped
* on any candidate bit string that is selected for mutation.
*/
public BitStringMutation(NumberGenerator<Probability> mutationProbability,
NumberGenerator<Integer> mutationCount)
{
this.mutationProbability = mutationProbability;
this.mutationCount = mutationCount;
}
public List<BitString> apply(List<BitString> selectedCandidates, Random rng)
{
List<BitString> mutatedPopulation = new ArrayList<BitString>(selectedCandidates.size());
for (BitString b : selectedCandidates)
{
mutatedPopulation.add(mutateBitString(b, rng));
}
return mutatedPopulation;
}
/**
* Mutate a single bit string. Zero or more bits may be flipped. The
* probability of any given bit being flipped is governed by the probability
* generator configured for this mutation operator.
* @param bitString The bit string to mutate.
* @param rng A source of randomness.
* @return The mutated bit string.
*/
private BitString mutateBitString(BitString bitString, Random rng)
{
if (mutationProbability.nextValue().nextEvent(rng))
{
BitString mutatedBitString = bitString.clone();
int mutations = mutationCount.nextValue();
for (int i = 0; i < mutations; i++)
{
mutatedBitString.flipBit(rng.nextInt(mutatedBitString.getLength()));
}
return mutatedBitString;
}
return bitString;
}
}