feat: add authoritative rating outcome scoring

server/domain/rating.go

@@ -30,6 +30,33 @@ type Opponent struct {
 	Score    float64
 }
 
+type MatchOutcome struct {
+	Team0Score int
+	Team1Score int
+	Overtime   bool
+	Abandoners map[string]bool
+}
+
+func ScoreForPlayer(outcome MatchOutcome, playerID string, team int) (float64, error) {
+	if playerID == "" || (team != 0 && team != 1) || outcome.Team0Score < 0 || outcome.Team1Score < 0 {
+		return 0, fmt.Errorf("invalid match outcome")
+	}
+	if outcome.Abandoners[playerID] {
+		return 0, nil
+	}
+	if outcome.Team0Score == outcome.Team1Score {
+		return 0.5, nil
+	}
+	winner := 0
+	if outcome.Team1Score > outcome.Team0Score {
+		winner = 1
+	}
+	if team == winner {
+		return 1, nil
+	}
+	return 0, nil
+}
+
 type RankedProfile struct {
 	Rating
 	RankedGames   int

server/domain/rating_test.go

@@ -6,6 +6,27 @@ import (
 	"time"
 )
 
+func TestScoreForPlayerHandlesDrawOvertimeAndAbandon(t *testing.T) {
+	draw := MatchOutcome{Team0Score: 2, Team1Score: 2}
+	if score, err := ScoreForPlayer(draw, "player-1", 0); err != nil || score != 0.5 {
+		t.Fatalf("draw score = %v, %v", score, err)
+	}
+	overtime := MatchOutcome{Team0Score: 2, Team1Score: 3, Overtime: true}
+	if score, err := ScoreForPlayer(overtime, "player-1", 0); err != nil || score != 0 {
+		t.Fatalf("overtime loser score = %v, %v", score, err)
+	}
+	if score, err := ScoreForPlayer(overtime, "player-2", 1); err != nil || score != 1 {
+		t.Fatalf("overtime winner score = %v, %v", score, err)
+	}
+	abandon := MatchOutcome{Team0Score: 0, Team1Score: 10, Abandoners: map[string]bool{"player-1": true}}
+	if score, err := ScoreForPlayer(abandon, "player-1", 0); err != nil || score != 0 {
+		t.Fatalf("abandoner score = %v, %v", score, err)
+	}
+	if _, err := ScoreForPlayer(draw, "", 0); err == nil {
+		t.Fatal("empty player accepted")
+	}
+}
+
 func TestUpdateRatingMatchesCanonicalGlicko2Example(t *testing.T) {
 	current := Rating{Value: 1500, RD: 200, Volatility: 0.06}
 	opponents := []Opponent{