Multiwinner: Add Warren's Psi (digamma) voting method.

src/main/multiwinner.cc

@@ -22,6 +22,7 @@
 #include "multiwinner/exhaustive/birational.h"
 #include "multiwinner/exhaustive/isoelastic.h"
 #include "multiwinner/exhaustive/lpv.h"
+#include "multiwinner/exhaustive/psi.h"
 
 #include "multiwinner/auction.h"
 #include "multiwinner/compat_qbuck.h"
@@ -631,6 +632,10 @@ std::vector<multiwinner_stats> get_multiwinner_methods() {
 	e_methods.push_back(multiwinner_stats(std::make_shared<log_penalty>()));
 	e_methods.push_back(multiwinner_stats(std::make_shared<isoelastic>()));
 
+	e_methods.push_back(multiwinner_stats(std::make_shared<psi_voting>(0)));
+	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)));
+
 	// 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/psi.h

@@ -0,0 +1,105 @@
+#include "scored_method.h"
+#include "tools/tools.h"
+
+class psi_voting_eval : public scored_method {
+	private:
+		double evaluate(combo::it & start, combo::it & end,
+			const scored_ballot & this_ballot);
+
+		double delta;
+
+	public:
+		std::string name() const {
+			if (delta == 0.5) {
+				return "Cardinal: Psi (Sainte-Laguë)";
+			}
+			if (delta == 1) {
+				return "Cardinal: Psi (D'Hondt)";
+			}
+			return "Cardinal: Psi (delta = " + dtos(delta) + ")";
+		}
+
+		bool maximize() const {
+			return true;
+		}
+
+		psi_voting_eval() {
+			delta = 0.5;
+		}
+
+		psi_voting_eval(double delta_in) {
+			if (delta_in < 0) {
+				throw std::invalid_argument("Psi voting: delta can't be negative");
+			}
+
+			delta = delta_in;
+		}
+};
+
+// Lightly adapted from
+// https://github.com/lteacy/decBRL-cpp/blob/master/src/polygamma/digamma.cpp
+
+double digamma(double x) {
+	double c = 8.5;
+	double d1 = -0.5772156649;
+	double r;
+	double s = 0.00001;
+	double s3 = 0.08333333333;
+	double s4 = 0.0083333333333;
+	double s5 = 0.003968253968;
+	double value;
+	double y;
+
+	//  Check the input.
+
+	if (x <= 0.0) {
+		throw std::invalid_argument("digamma: x < 0 is not supported");
+	}
+
+	//  Initialize.
+
+	y = x;
+	value = 0.0;
+
+	//  Use approximation if argument <= S.
+
+	if (y <= s) {
+		value = d1 - 1.0 / y;
+		return value;
+	}
+
+	//  Reduce to DIGAMA(X + N) where (X + N) >= C.
+
+	while (y < c) {
+		value = value - 1.0 / y;
+		y = y + 1.0;
+	}
+
+	//  Use Stirling's (actually de Moivre's) expansion if argument > C.
+
+	r = 1.0 / y;
+	value = value + log(y) - 0.5 * r;
+	r = r * r;
+	value = value - r * (s3 - r * (s4 - r * s5));
+
+	return value;
+}
+
+double psi_voting_eval::evaluate(combo::it & start, combo::it & end,
+	const scored_ballot & this_ballot) {
+
+	// Sum over voters v: digamma( delta +
+	//		sum over elected candidates c: v's score of c)
+
+	// From https://rangevoting.org/QualityMulti.html
+
+	double norm_rating_sum = 0;
+
+	for (auto pos = start; pos != end; ++pos) {
+		norm_rating_sum += this_ballot.get_norm_score(*pos);
+	}
+
+	return digamma(delta + norm_rating_sum) * this_ballot.weight;
+}
+
+typedef exhaustive_method_runner<psi_voting_eval> psi_voting;

src/singlewinner/sets/unsuccessful/all.h

@@ -0,0 +1 @@
+#include "resistant_winner.h"