[yul-phaser] Change the design of crossover operators so that they produce a single chromosome rather than a pair

tools/yulPhaser/GeneticAlgorithms.cpp

@@ -71,7 +71,7 @@ void GenerationalElitistWithExclusivePools::runNextRound()
 			)
 		) +
 		m_population.select(elite).crossover(
-			RandomPairSelection(m_options.crossoverPoolSize / elitePoolSize / 2),
+			RandomPairSelection(m_options.crossoverPoolSize / elitePoolSize),
 			randomPointCrossover()
 		);
 }

tools/yulPhaser/Mutations.cpp

@@ -98,7 +98,7 @@ function<Mutation> phaser::alternativeMutations(
 namespace
 {
 
-ChromosomePair buildChromosomesBySwappingParts(
+Chromosome buildChromosomesBySwappingParts(
 	Chromosome const& _chromosome1,
 	Chromosome const& _chromosome2,
 	size_t _crossoverPoint
@@ -110,15 +110,9 @@ ChromosomePair buildChromosomesBySwappingParts(
 	auto begin1 = _chromosome1.optimisationSteps().begin();
 	auto begin2 = _chromosome2.optimisationSteps().begin();
 
-	return ChromosomePair(
-		Chromosome(
-			vector<string>(begin1, begin1 + _crossoverPoint) +
-			vector<string>(begin2 + _crossoverPoint, _chromosome2.optimisationSteps().end())
-		),
-		Chromosome(
-			vector<string>(begin2, begin2 + _crossoverPoint) +
-			vector<string>(begin1 + _crossoverPoint, _chromosome1.optimisationSteps().end())
-		)
+	return Chromosome(
+		vector<string>(begin1, begin1 + _crossoverPoint) +
+		vector<string>(begin2 + _crossoverPoint, _chromosome2.optimisationSteps().end())
 	);
 }
 

tools/yulPhaser/Mutations.h

@@ -28,10 +28,8 @@
 namespace solidity::phaser
 {
 
-using ChromosomePair = std::tuple<Chromosome, Chromosome>;
-
 using Mutation = Chromosome(Chromosome const&);
-using Crossover = ChromosomePair(Chromosome const&, Chromosome const&);
+using Crossover = Chromosome(Chromosome const&, Chromosome const&);
 
 // MUTATIONS
 
@@ -64,10 +62,9 @@ std::function<Mutation> alternativeMutations(
 std::function<Crossover> randomPointCrossover();
 
 /// Creates a crossover operator that always chooses a point that lies at @a _crossoverPoint
-/// percent of the length of the shorter chromosome. Then creates a pair of chromosomes by
-/// splitting both inputs at the crossover point and stitching the resulting parts. The first
-/// output is created from the first half or first input and the second half of the second input
-/// The second output from the remaining two halves.
+/// percent of the length of the shorter chromosome. Then creates a new chromosome by
+/// splitting both inputs at the crossover point and stitching output from the first half or first
+/// input and the second half of the second input.
 ///
 /// Avoids selecting position 0 (since this just produces a chromosome identical to the second one)
 /// unless there is no other choice (i.e. one of the chromosomes is empty).

tools/yulPhaser/Population.cpp

@@ -108,12 +108,11 @@ Population Population::crossover(PairSelection const& _selection, function<Cross
 	vector<Individual> crossedIndividuals;
 	for (auto const& [i, j]: _selection.materialise(m_individuals.size()))
 	{
-		auto [childChromosome1, childChromosome2] = _crossover(
+		auto childChromosome = _crossover(
 			m_individuals[i].chromosome,
 			m_individuals[j].chromosome
 		);
-		crossedIndividuals.emplace_back(move(childChromosome1), *m_fitnessMetric);
-		crossedIndividuals.emplace_back(move(childChromosome2), *m_fitnessMetric);
+		crossedIndividuals.emplace_back(move(childChromosome), *m_fitnessMetric);
 	}
 
 	return Population(m_fitnessMetric, crossedIndividuals);