[yul-phaser] Population: Extract a function for comparing fitness of individuals

- Mostly for readability and convenience. This significantly shortens calls to sort().
- I could define it as Individual::operator< instead but it would be inconsistent with operator== because it does not compare the chromosomes, only fitness. It could result in an unintuitive situation where (a <= b <= a) does not necessarily imply (a == b).

test/yulPhaser/Population.cpp

@@ -92,6 +92,24 @@ protected:
 BOOST_AUTO_TEST_SUITE(Phaser)
 BOOST_AUTO_TEST_SUITE(PopulationTest)
 
+BOOST_AUTO_TEST_CASE(isFitter_should_use_fitness_as_the_main_criterion)
+{
+	BOOST_TEST(isFitter(Individual{Chromosome("a"), 5}, Individual{Chromosome("a"), 10}));
+	BOOST_TEST(!isFitter(Individual{Chromosome("a"), 10}, Individual{Chromosome("a"), 5}));
+
+	BOOST_TEST(isFitter(Individual{Chromosome("aaa"), 5}, Individual{Chromosome("aaaaa"), 10}));
+	BOOST_TEST(!isFitter(Individual{Chromosome("aaaaa"), 10}, Individual{Chromosome("aaa"), 5}));
+
+	BOOST_TEST(isFitter(Individual{Chromosome("aaaaa"), 5}, Individual{Chromosome("aaa"), 10}));
+	BOOST_TEST(!isFitter(Individual{Chromosome("aaa"), 10}, Individual{Chromosome("aaaaa"), 5}));
+}
+
+BOOST_AUTO_TEST_CASE(isFitter_should_return_false_for_identical_individuals)
+{
+	BOOST_TEST(!isFitter(Individual{Chromosome("a"), 3}, Individual{Chromosome("a"), 3}));
+	BOOST_TEST(!isFitter(Individual{Chromosome("acT"), 0}, Individual{Chromosome("acT"), 0}));
+}
+
 BOOST_FIXTURE_TEST_CASE(constructor_should_copy_chromosomes_and_not_compute_fitness, PopulationFixture)
 {
 	vector<Chromosome> chromosomes = {

tools/yulPhaser/Population.cpp

@@ -48,6 +48,13 @@ ostream& phaser::operator<<(ostream& _stream, Individual const& _individual)
 	return _stream;
 }
 
+bool phaser::isFitter(Individual const& a, Individual const& b)
+{
+	assert(a.fitness.has_value() && b.fitness.has_value());
+
+	return a.fitness.value() < b.fitness.value();
+}
+
 Population::Population(Program _program, vector<Chromosome> const& _chromosomes):
 	m_program{move(_program)}
 {
@@ -128,12 +135,7 @@ void Population::doSelection()
 {
 	assert(all_of(m_individuals.begin(), m_individuals.end(), [](auto& i){ return i.fitness.has_value(); }));
 
-	sort(
+	sort(m_individuals.begin(), m_individuals.end(), isFitter);
-		m_individuals.begin(),
-		m_individuals.end(),
-		[](auto const& a, auto const& b){ return a.fitness.value() < b.fitness.value(); }
-	);
-
 	randomizeWorstChromosomes(m_individuals, m_individuals.size() / 2);
 }
 

tools/yulPhaser/Population.h

@@ -40,6 +40,9 @@ struct Individual
 	friend std::ostream& operator<<(std::ostream& _stream, Individual const& _individual);
 };
 
+/// Determines which individual is better by comparing fitness values.
+bool isFitter(Individual const& a, Individual const& b);
+
 /**
  * Represents a changing set of individuals undergoing a genetic algorithm.
  * Each round of the algorithm involves mutating existing individuals, evaluating their fitness