@Override
public GRCandidateProgram[] crossover(final CandidateProgram p1, final CandidateProgram p2) {
final GRCandidateProgram child1 = (GRCandidateProgram) p1;
final GRCandidateProgram child2 = (GRCandidateProgram) p2;
final NonTerminalSymbol parseTree1 = child1.getParseTree();
final NonTerminalSymbol parseTree2 = child2.getParseTree();
final List<NonTerminalSymbol> nonTerminals1 = parseTree1.getNonTerminalSymbols();
final List<NonTerminalSymbol> nonTerminals2 = parseTree2.getNonTerminalSymbols();
final int point1 = rng.nextInt(nonTerminals1.size());
final NonTerminalSymbol subtree1 = nonTerminals1.get(point1);
// Generate a list of matching non-terminals from the second program.
final List<NonTerminalSymbol> matchingNonTerminals = new ArrayList<NonTerminalSymbol>();
for (final NonTerminalSymbol nt : nonTerminals2) {
if (nt.getGrammarRule().equals(subtree1.getGrammarRule())) {
matchingNonTerminals.add(nt);
}
}
if (matchingNonTerminals.isEmpty()) {
// No valid points in second program, cancel crossover.
return null;
} else {
// Randomly choose a second point out of the matching non-terminals.
final int point2 = rng.nextInt(matchingNonTerminals.size());
final NonTerminalSymbol subtree2 = matchingNonTerminals.get(point2);
// Add crossover points to the stats manager.
Stats.get().addData(XO_POINT1, point1);
Stats.get().addData(XO_POINT2, point2);
// Swap the non-terminals' children.
final List<Symbol> temp = subtree1.getChildren();
subtree1.setChildren(subtree2.getChildren());
subtree2.setChildren(temp);
// Add subtrees into the stats manager.
Stats.get().addData(XO_SUBTREE1, subtree1);
Stats.get().addData(XO_SUBTREE2, subtree2);
}
return new GRCandidateProgram[] {child1, child2};
}