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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.framework;
import java.util.List;
import java.util.Random;
/**
* An implementation of steady-state evolution, which is a type of evolutionary algorithm
* where a population is changed incrementally, with one individual evolved at a time. This
* differs from {@link GenerationalEvolutionEngine} in which the entire population is evolved in
* parallel.
*
* @param The type of entity that is to be evolved.
* @see GenerationalEvolutionEngine
* @see EvolutionStrategyEngine
* @author Daniel Dyer
*/
public class SteadyStateEvolutionEngine extends AbstractEvolutionEngine
{
private final EvolutionaryOperator evolutionScheme;
private final FitnessEvaluator fitnessEvaluator;
private final SelectionStrategy selectionStrategy;
private final int selectionSize;
private final boolean forceSingleCandidateUpdate;
/**
* Create a steady-state evolution strategy in which one or more (usually just one) evolved
* offspring replace randomly-chosen individuals.
* @param candidateFactory Factory used to create the initial population that is
* iteratively evolved.
* @param evolutionScheme The evolutionary operator that modifies the population. The
* number of candidates used as input is controlled by the {@code selectionSize} parameter.
* The number of candidates that will be outputted depends on the implementation. Typically
* it will be the same as the input size, but this is not necessary. In fact, for steady-state
* evolution, it is typical that the output size is always 1, regardless of the input size, so
* that only one member of the population is replaced at a time. To acheive this using cross-over
* requires a cross-over implementation that returns only one offspring, rather than the normal
* two.
* @param fitnessEvaluator The fitness function.
* @param selectionStrategy The strategy for selecting which candidate(s) will be
* the parent(s) when evolving individuals.
* @param selectionSize How many parent candidates are required by the evolution scheme.
* This controls how many individuals will be provided to the evolutionary operator at
* each iteration. If you are just using mutation, this will typically be 1. For
* cross-over, two separate parents are required, so this must be set to 2.
* @param forceSingleCandidateUpdate Some evolutionary operators, specifically cross-over
* operators, generate more than one evolved individual. A true steady-state algorithm will
* only replace one individual at a time. Setting this parameter to true forces the evolution
* to discard any additional generated offspring so that for each iteration of the algorithm
* there is only one updated individual. This allows cross-over operators that were designed
* for generational evolutionary algorithms to be reused for steady-state evolution. A more
* efficient, but less straightforward, alternative would be to implement a steady-state-specific
* cross-over operator that returns only a single evolved individual. Setting this parameter to
* false permits multiple candidates to be replaced per iteration, depending on the specifics of
* the evolutionary operator(s).
* @param rng The source of randomness used by all stochastic processes (including
* evolutionary operators and selection strategies).
*/
public SteadyStateEvolutionEngine(CandidateFactory candidateFactory,
EvolutionaryOperator evolutionScheme,
FitnessEvaluator fitnessEvaluator,
SelectionStrategy selectionStrategy,
int selectionSize,
boolean forceSingleCandidateUpdate,
Random rng)
{
super(candidateFactory, fitnessEvaluator, rng);
this.fitnessEvaluator = fitnessEvaluator;
this.evolutionScheme = evolutionScheme;
this.selectionStrategy = selectionStrategy;
this.selectionSize = selectionSize;
this.forceSingleCandidateUpdate = forceSingleCandidateUpdate;
}
/**
* {@inheritDoc}
*/
@Override
protected List> nextEvolutionStep(List> evaluatedPopulation,
int eliteCount,
Random rng)
{
EvolutionUtils.sortEvaluatedPopulation(evaluatedPopulation, fitnessEvaluator.isNatural());
List selectedCandidates = selectionStrategy.select(evaluatedPopulation,
fitnessEvaluator.isNatural(),
selectionSize,
rng);
List> offspring = evaluatePopulation(evolutionScheme.apply(selectedCandidates, rng));
doReplacement(evaluatedPopulation, offspring, eliteCount, rng);
return evaluatedPopulation;
}
/**
* Add the offspring to the population, removing the same number of existing individuals to make
* space for them.
* This method randomly chooses which individuals should be replaced, but it can be over-ridden
* in sub-classes if alternative behaviour is required.
* @param existingPopulation The full popultation, sorted in descending order of fitness.
* @param newCandidates The (unsorted) newly-created individual(s) that should replace existing members
* of the population.
* @param eliteCount The number of the fittest individuals that should be exempt from being replaced.
* @param rng A source of randomness.
*/
protected void doReplacement(List> existingPopulation,
List> newCandidates,
int eliteCount,
Random rng)
{
assert newCandidates.size() < existingPopulation.size() - eliteCount : "Too many new candidates for replacement.";
// If this is strictly steady-state (only one updated individual per iteration), then we can't keep multiple
// evolved individuals, so just pick one at random and use that.
if (newCandidates.size() > 1 && forceSingleCandidateUpdate)
{
// Replace a randomly selected individual, but not one of the "elite" individuals at the
// beginning of the sorted population.
existingPopulation.set(rng.nextInt(existingPopulation.size() - eliteCount) + eliteCount,
newCandidates.get(rng.nextInt(newCandidates.size())));
}
else
{
for (EvaluatedCandidate candidate : newCandidates)
{
// Replace a randomly selected individual, but not one of the "elite" individuals at the
// beginning of the sorted population.
existingPopulation.set(rng.nextInt(existingPopulation.size() - eliteCount) + eliteCount, candidate);
}
}
}
}