Multiwinner: Add Warren's Harmonic voting method.

src/main/multiwinner.cc

@@ -20,6 +20,7 @@
 
 #include "multiwinner/methods.h"
 #include "multiwinner/exhaustive/birational.h"
+#include "multiwinner/exhaustive/harmonic.h"
 #include "multiwinner/exhaustive/isoelastic.h"
 #include "multiwinner/exhaustive/lpv.h"
 #include "multiwinner/exhaustive/psi.h"
@@ -643,6 +644,14 @@ std::vector<multiwinner_stats> get_multiwinner_methods() {
 	e_methods.push_back(multiwinner_stats(std::make_shared<psi_voting>(0.5)));
 	e_methods.push_back(multiwinner_stats(std::make_shared<psi_voting>(1)));
 
+	e_methods.push_back(multiwinner_stats(std::make_shared<harmonic_voting>
+			(0)));
+	e_methods.push_back(multiwinner_stats(std::make_shared<harmonic_voting>
+			(0.5)));
+	e_methods.push_back(multiwinner_stats(std::make_shared<harmonic_voting>
+			(1)));
+
+
 	// It's possible to parameterize, but this way of passing parameters is
 	// not very elegant.
 	e_methods.push_back(multiwinner_stats(std::make_shared<isoelastic>(

src/multiwinner/exhaustive/harmonic.h

@@ -0,0 +1,76 @@
+#include "scored_method.h"
+#include "tools/tools.h"
+
+class harmonic_voting_eval : public scored_method {
+	private:
+		double evaluate(combo::it & start, combo::it & end,
+			const scored_ballot & this_ballot);
+
+		double delta;
+
+		std::vector<double> elected_ratings;
+
+	public:
+		std::string name() const {
+			if (delta == 0.5) {
+				return "Cardinal: Harmonic (Sainte-Laguë)";
+			}
+			if (delta == 1) {
+				return "Cardinal: Harmonic (D'Hondt)";
+			}
+			return "Cardinal: Harmonic (delta = " + dtos(delta) + ")";
+		}
+
+		bool maximize() const {
+			return true;
+		}
+
+		harmonic_voting_eval() {
+			delta = 0.5;
+		}
+
+		harmonic_voting_eval(double delta_in) {
+			if (delta_in < 0) {
+				throw std::invalid_argument("Harmonic voting: "
+					"delta can't be negative");
+			}
+
+			delta = delta_in;
+		}
+};
+
+
+double harmonic_voting_eval::evaluate(combo::it & start, combo::it & end,
+	const scored_ballot & this_ballot) {
+
+	// Sort the ratings given to the candidates proposed for election
+	// by this voter in descending order. Then the quality contributed
+	// to the proposed seat assignment by the voter is
+
+	// sum i=1..|S| : (ith greatest rating of candidate
+	//	for someone to be elected)/ (i-1 + delta).
+
+	// From https://rangevoting.org/QualityMulti.html
+
+	size_t council_size = end - start;
+
+	elected_ratings.resize(council_size);
+
+	double quality = 0;
+	size_t idx = 0;
+
+	for (auto pos = start; pos != end; ++pos) {
+		elected_ratings[idx++] = this_ballot.get_norm_score(*pos);
+	}
+
+	std::sort(elected_ratings.begin(), elected_ratings.begin()+idx,
+		std::greater<double>());
+
+	for (size_t i = 0; i < council_size; ++i) {
+		quality += elected_ratings[i] / (i + delta);
+	}
+
+	return quality;
+}
+
+typedef exhaustive_method_runner<harmonic_voting_eval> harmonic_voting;