/**
* Determine the fitness of the given Chromosome instance. The higher the return value, the more
* fit the instance. This method should always return the same fitness value for two equivalent
* Chromosome instances.
*
* @param a_subject the Chromosome instance to evaluate
* @return positive double reflecting the fitness rating of the given Chromosome
* @since 2.0 (until 1.1: return type int)
* @author Neil Rotstan, Klaus Meffert, John Serri
*/
public double evaluate(IChromosome a_subject) {
// Take care of the fitness evaluator. It could either be weighting higher
// fitness values higher (e.g.DefaultFitnessEvaluator). Or it could weight
// lower fitness values higher, because the fitness value is seen as a
// defect rate (e.g. DeltaFitnessEvaluator)
boolean defaultComparation = a_subject.getConfiguration().getFitnessEvaluator().isFitter(2, 1);
// The fitness value measures both how close the value is to the
// target amount supplied by the user and the total number of coins
// represented by the solution. We do this in two steps: first,
// we consider only the represented amount of change vs. the target
// amount of change and return higher fitness values for amounts
// closer to the target, and lower fitness values for amounts further
// away from the target. Then we go to step 2, which returns a higher
// fitness value for solutions representing fewer total coins, and
// lower fitness values for solutions representing more total coins.
// ------------------------------------------------------------------
int changeAmount = amountOfChange(a_subject);
int totalCoins = getTotalNumberOfCoins(a_subject);
int changeDifference = Math.abs(m_targetAmount - changeAmount);
double fitness;
if (defaultComparation) {
fitness = 0.0d;
} else {
fitness = MAX_BOUND / 2;
}
// Step 1: Determine distance of amount represented by solution from
// the target amount. If the change difference is greater than zero we
// will divide one by the difference in change between the
// solution amount and the target amount. That will give the desired
// effect of returning higher values for amounts closer to the target
// amount and lower values for amounts further away from the target
// amount.
// In the case where the change difference is zero it means that we have
// the correct amount and we assign a higher fitness value.
// ---------------------------------------------------------------------
if (defaultComparation) {
fitness += changeDifferenceBonus(MAX_BOUND / 2, changeDifference);
} else {
fitness -= changeDifferenceBonus(MAX_BOUND / 2, changeDifference);
}
// Step 2: We divide the fitness value by a penalty based on the number of
// coins. The higher the number of coins the higher the penalty and the
// smaller the fitness value.
// And inversely the smaller number of coins in the solution the higher
// the resulting fitness value.
// -----------------------------------------------------------------------
if (defaultComparation) {
fitness -= computeCoinNumberPenalty(MAX_BOUND / 2, totalCoins);
} else {
fitness += computeCoinNumberPenalty(MAX_BOUND / 2, totalCoins);
}
// Make sure fitness value is always positive.
// -------------------------------------------
return Math.max(1.0d, fitness);
}
