/**
* Prunes the population to the specified size. This is similar to {@link #truncate(int)}, except
* the crowding distance is recalculated each time a solution is removed.
*
* @param size the target population size after pruning
*/
public void prune(int size) {
if (modified) {
update();
}
sort(new RankComparator());
// collect all solutions in the front which must be pruned
// note the use of super to prevent repeatedly triggering update()
int maxRank = (Integer) super.get(size - 1).getAttribute(RANK_ATTRIBUTE);
Population front = new Population();
for (int i = size() - 1; i >= 0; i--) {
Solution solution = super.get(i);
int rank = (Integer) solution.getAttribute(RANK_ATTRIBUTE);
if (rank >= maxRank) {
super.remove(i);
if (rank == maxRank) {
front.add(solution);
}
}
}
// prune front until correct size
while (size() + front.size() > size) {
nondominatedSorting.updateCrowdingDistance(front);
front.truncate(front.size() - 1, new CrowdingComparator());
}
addAll(front);
}
/**
* Updates the rank and crowding distance of all solutions in this population. This method will in
* general be called automatically when the population is modified. However, only changes made to
* this population can be tracked; changes made directly to the contained solutions will not be
* detected. Therefore, it may be necessary to invoke {@link #update()} manually.
*/
public void update() {
modified = false;
nondominatedSorting.evaluate(this);
}
