D'Hondt method

dhondt.go

package electoral

import (
	"sort"
)

// CandidatureResult is all the votes tallied for a candidature.
type CandidatureResult struct {
	Name  string
	Votes int
}

// Seat is an assigned seat.
type Seat struct {
	Candidature CandidatureResult
	Quotient    int
}

func quotient(votes, allocatedSeats int) int {
	return votes / (allocatedSeats + 1)
}

// DHondt applies D'Hondt method on results for provided seats.
func DHondt(tally []CandidatureResult, seats int) []Seat {
	result := make([]Seat, 0)
	for _, r := range tally {
		for i := 0; i < seats; i++ {
			s := Seat{
				r,
				quotient(r.Votes, i),
			}
			result = append(result, s)
		}
	}
	sort.Slice(result[:], func(i, j int) bool {
		return result[i].Quotient >= result[j].Quotient
	})
	return result[:seats]
}

dhondt_test.go

package electoral

import (
	"testing"
)

func TestDHondt(t *testing.T) {
	seats := 54
	tally := []CandidatureResult{
		CandidatureResult{"PP", 4074363},
		CandidatureResult{"PSOE", 3596324},
		CandidatureResult{"LA IZQUIERDA PLURAL", 1562567},
		CandidatureResult{"PODEMOS", 1245948},
		CandidatureResult{"UPyD", 1015994},
		CandidatureResult{"CEU", 850690},
		CandidatureResult{"EPDD", 629071},
		CandidatureResult{"C's", 495114},
		CandidatureResult{"LPD", 324534},
		CandidatureResult{"PRIMAVERA EUROPEA", 299884},
		CandidatureResult{"VOX", 244929},
		CandidatureResult{"PACMA", 176237},
		CandidatureResult{"EB", 115308},
		CandidatureResult{"MOVIMIENTO RED", 105183},
		CandidatureResult{"PARTIDO X", 100115},
		CandidatureResult{"PARTIDO ANDALUCISTA", 49251},
		CandidatureResult{"PIRATAS", 38422},
		CandidatureResult{"F.A.C.", 33826},
		CandidatureResult{"DISCAPACITADOS Y ENFERMEDADES RARAS", 32531},
		CandidatureResult{"RECORTES CERO", 30958},
		CandidatureResult{"P.C.P.E.", 29000},
		CandidatureResult{"I.Fem", 25199},
		CandidatureResult{"FE de las JONS", 21577},
		CandidatureResult{"CILUS", 18225},
		CandidatureResult{"ImpulsoSocial", 17774},
		CandidatureResult{"LEM", 16879},
		CandidatureResult{"PH", 15278},
		CandidatureResult{"D.N.", 12904},
		CandidatureResult{"ACNV-BAR-PRAO-R.E.P.O-UNIO", 11427},
		CandidatureResult{"RRUE", 10076},
		CandidatureResult{"PT", 9825},
		CandidatureResult{"ALTER", 9654},
		CandidatureResult{"P-LIB", 9644},
		CandidatureResult{"M.S.R.", 8875},
		CandidatureResult{"EXTREMADURA UNIDA", 8830},
		CandidatureResult{"PREPAL", 8783},
		CandidatureResult{"SAIn", 6894},
		CandidatureResult{"IPEX-PREX-CREX", 6110},
		CandidatureResult{"M.C.R.", 5084},
	}
	result := DHondt(tally, seats)
	if len(result) != seats {
		t.Fatalf("expected %d seats, got %d", seats, len(result))
	}
	tests := []struct {
		name     string
		expected int
	}{
		{"PP", 16},
		{"PSOE", 14},
		{"LA IZQUIERDA PLURAL", 6},
		{"PODEMOS", 5},
		{"UPyD", 4},
		{"CEU", 3},
		{"EPDD", 2},
		{"C's", 2},
		{"LPD", 1},
		{"PRIMAVERA EUROPEA", 1},
		{"VOX", 0},
		{"PACMA", 0},
		{"EB", 0},
		{"MOVIMIENTO RED", 0},
		{"PARTIDO X", 0},
		{"PARTIDO ANDALUCISTA", 0},
		{"PIRATAS", 0},
		{"F.A.C.", 0},
		{"DISCAPACITADOS Y ENFERMEDADES RARAS", 0},
		{"RECORTES CERO", 0},
		{"P.C.P.E.", 0},
		{"I.Fem", 0},
		{"FE de las JONS", 0},
		{"CILUS", 0},
		{"ImpulsoSocial", 0},
		{"LEM", 0},
		{"PH", 0},
		{"D.N.", 0},
		{"ACNV-BAR-PRAO-R.E.P.O-UNIO", 0},
		{"RRUE", 0},
		{"PT", 0},
		{"ALTER", 0},
		{"P-LIB", 0},
		{"M.S.R.", 0},
		{"EXTREMADURA UNIDA", 0},
		{"PREPAL", 0},
		{"SAIn", 0},
		{"IPEX-PREX-CREX", 0},
		{"M.C.R.", 0},
	}
	for _, test := range tests {
		assignedSeats := 0
		for _, r := range result {
			if r.Candidature.Name == test.name {
				assignedSeats++
			}
		}
		if assignedSeats != test.expected {
			t.Fatalf("expected %d seats for %s, got %d", assignedSeats, test.name, test.expected)
		}
	}
}