[yul-phaser] Add RelativeProgramSize metric

tools/yulPhaser/FitnessMetrics.cpp

@@ -17,6 +17,8 @@
 
 #include <tools/yulPhaser/FitnessMetrics.h>
 
+#include <cmath>
+
 using namespace std;
 using namespace solidity::phaser;
 
@@ -33,3 +35,18 @@ size_t ProgramSize::evaluate(Chromosome const& _chromosome)
 {
 	return optimisedProgram(_chromosome).codeSize();
 }
+
+size_t RelativeProgramSize::evaluate(Chromosome const& _chromosome)
+{
+	size_t const scalingFactor = pow(10, m_fixedPointPrecision);
+
+	size_t unoptimisedSize = optimisedProgram(Chromosome("")).codeSize();
+	if (unoptimisedSize == 0)
+		return scalingFactor;
+
+	size_t optimisedSize = optimisedProgram(_chromosome).codeSize();
+
+	return static_cast<size_t>(round(
+		static_cast<double>(optimisedSize) / unoptimisedSize * scalingFactor
+	));
+}

tools/yulPhaser/FitnessMetrics.h

@@ -87,4 +87,30 @@ public:
 	size_t evaluate(Chromosome const& _chromosome) override;
 };
 
+/**
+ * Fitness metric based on the size of a specific program after applying the optimisations from the
+ * chromosome to it in relation to the original, unoptimised program.
+ *
+ * Since metric values are integers, the class multiplies the ratio by 10^@a _fixedPointPrecision
+ * before rounding it.
+ */
+class RelativeProgramSize: public ProgramBasedMetric
+{
+public:
+	explicit RelativeProgramSize(
+		Program _program,
+		size_t _fixedPointPrecision,
+		size_t _repetitionCount = 1
+	):
+		ProgramBasedMetric(std::move(_program), _repetitionCount),
+		m_fixedPointPrecision(_fixedPointPrecision) {}
+
+	size_t fixedPointPrecision() const { return m_fixedPointPrecision; }
+
+	size_t evaluate(Chromosome const& _chromosome) override;
+
+private:
+	size_t m_fixedPointPrecision;
+};
+
 }