[yul-phaser] Mutations: Add symmetricRandomPointCrossover()

test/yulPhaser/Mutations.cpp

@@ -225,6 +225,20 @@ BOOST_AUTO_TEST_CASE(randomPointCrossover_should_swap_chromosome_parts_at_random
 	BOOST_TEST(result2 == Chromosome("cccaaaaaaa"));
 }
 
+BOOST_AUTO_TEST_CASE(symmetricRandomPointCrossover_should_swap_chromosome_parts_at_random_point)
+{
+	function<SymmetricCrossover> crossover = symmetricRandomPointCrossover();
+
+	SimulationRNG::reset(1);
+	tuple<Chromosome, Chromosome> result1 = crossover(Chromosome("aaaaaaaaaa"), Chromosome("cccccc"));
+	tuple<Chromosome, Chromosome> expectedPair1 = {Chromosome("aaaccc"), Chromosome("cccaaaaaaa")};
+	BOOST_TEST(result1 == expectedPair1);
+
+	tuple<Chromosome, Chromosome> result2 = crossover(Chromosome("cccccc"), Chromosome("aaaaaaaaaa"));
+	tuple<Chromosome, Chromosome> expectedPair2 = {Chromosome("ccccccaaaa"), Chromosome("aaaaaa")};
+	BOOST_TEST(result2 == expectedPair2);
+}
+
 BOOST_AUTO_TEST_CASE(randomPointCrossover_should_only_consider_points_available_on_both_chromosomes)
 {
 	SimulationRNG::reset(1);

tools/yulPhaser/Mutations.cpp

@@ -98,7 +98,7 @@ function<Mutation> phaser::alternativeMutations(
 namespace
 {
 
-Chromosome buildChromosomesBySwappingParts(
+ChromosomePair fixedPointSwap(
 	Chromosome const& _chromosome1,
 	Chromosome const& _chromosome2,
 	size_t _crossoverPoint
@@ -109,11 +109,19 @@ Chromosome buildChromosomesBySwappingParts(
 
 	auto begin1 = _chromosome1.optimisationSteps().begin();
 	auto begin2 = _chromosome2.optimisationSteps().begin();
+	auto end1 = _chromosome1.optimisationSteps().end();
+	auto end2 = _chromosome2.optimisationSteps().end();
 
-	return Chromosome(
-		vector<string>(begin1, begin1 + _crossoverPoint) +
-		vector<string>(begin2 + _crossoverPoint, _chromosome2.optimisationSteps().end())
-	);
+	return {
+		Chromosome(
+			vector<string>(begin1, begin1 + _crossoverPoint) +
+			vector<string>(begin2 + _crossoverPoint, end2)
+		),
+		Chromosome(
+			vector<string>(begin2, begin2 + _crossoverPoint) +
+			vector<string>(begin1 + _crossoverPoint, end1)
+		),
+	};
 }
 
 }
@@ -129,7 +137,22 @@ function<Crossover> phaser::randomPointCrossover()
 		assert(minPoint <= minLength);
 
 		size_t randomPoint = SimulationRNG::uniformInt(minPoint, minLength);
-		return buildChromosomesBySwappingParts(_chromosome1, _chromosome2, randomPoint);
+		return get<0>(fixedPointSwap(_chromosome1, _chromosome2, randomPoint));
+	};
+}
+
+function<SymmetricCrossover> phaser::symmetricRandomPointCrossover()
+{
+	return [=](Chromosome const& _chromosome1, Chromosome const& _chromosome2)
+	{
+		size_t minLength = min(_chromosome1.length(), _chromosome2.length());
+
+		// Don't use position 0 (because this just swaps the values) unless it's the only choice.
+		size_t minPoint = (minLength > 0? 1 : 0);
+		assert(minPoint <= minLength);
+
+		size_t randomPoint = SimulationRNG::uniformInt(minPoint, minLength);
+		return fixedPointSwap(_chromosome1, _chromosome2, randomPoint);
 	};
 }
 
@@ -142,6 +165,6 @@ function<Crossover> phaser::fixedPointCrossover(double _crossoverPoint)
 		size_t minLength = min(_chromosome1.length(), _chromosome2.length());
 		size_t concretePoint = static_cast<size_t>(round(minLength * _crossoverPoint));
 
-		return buildChromosomesBySwappingParts(_chromosome1, _chromosome2, concretePoint);
+		return get<0>(fixedPointSwap(_chromosome1, _chromosome2, concretePoint));
 	};
 }

tools/yulPhaser/Mutations.h

@@ -28,8 +28,11 @@
 namespace solidity::phaser
 {
 
+using ChromosomePair = std::tuple<Chromosome, Chromosome>;
+
 using Mutation = Chromosome(Chromosome const&);
 using Crossover = Chromosome(Chromosome const&, Chromosome const&);
+using SymmetricCrossover = ChromosomePair(Chromosome const&, Chromosome const&);
 
 // MUTATIONS
 
@@ -61,6 +64,10 @@ std::function<Mutation> alternativeMutations(
 /// position at which to perform perform @a fixedPointCrossover.
 std::function<Crossover> randomPointCrossover();
 
+/// Symmetric version of @a randomPointCrossover(). Creates an operator that returns a pair
+/// containing both possible results for the same crossover point.
+std::function<SymmetricCrossover> symmetricRandomPointCrossover();
+
 /// 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 new chromosome by
 /// splitting both inputs at the crossover point and stitching output from the first half or first