feat: add deterministic matchmaking candidate selection

server/domain/matcher.go

@@ -0,0 +1,190 @@
+package domain
+
+import (
+	"fmt"
+	"sort"
+	"time"
+)
+
+const (
+	MaxPlacementRTT = 100.0
+	MinRatingTolerance = 100.0
+	MaxRatingTolerance = 400.0
+	RatingWidenStep = 25.0
+	RatingWidenPeriod = 30.0
+)
+
+// Candidate is the server-side projection of a verified, live queue ticket.
+// RTT values come from backend probes, never from the client request body.
+type Candidate struct {
+	TicketID   string
+	PlayerID   string
+	Rating     float64
+	EnqueuedAt time.Time
+	PredictedRTT map[string]float64
+}
+
+type Selection struct {
+	Players []Candidate
+	Region string
+	WorstRTT float64
+	TotalRTT float64
+	RatingRange float64
+	TotalWaitSeconds float64
+}
+
+func RatingTolerance(waitSeconds float64) float64 {
+	if waitSeconds < 0 {
+		waitSeconds = 0
+	}
+	value := MinRatingTolerance + RatingWidenStep*float64(int(waitSeconds/RatingWidenPeriod))
+	if value > MaxRatingTolerance {
+		return MaxRatingTolerance
+	}
+	return value
+}
+
+func SelectCandidates(anchor Candidate, candidates []Candidate, size int, now time.Time) (Selection, error) {
+	if size < 1 {
+		return Selection{}, fmt.Errorf("candidate size must be positive")
+	}
+	pool := make([]Candidate, 0, len(candidates)+1)
+	seen := map[string]bool{}
+	add := func(candidate Candidate) {
+		if candidate.TicketID != "" && !seen[candidate.TicketID] {
+			seen[candidate.TicketID] = true
+			pool = append(pool, candidate)
+		}
+	}
+	add(anchor)
+	for _, candidate := range candidates {
+		add(candidate)
+	}
+	if len(pool) < size {
+		return Selection{}, fmt.Errorf("only %d compatible candidates available for size %d", len(pool), size)
+	}
+
+	best := Selection{}
+	found := false
+	chosen := make([]Candidate, 0, size)
+	var visit func(int)
+	visit = func(start int) {
+		if len(chosen) == size {
+			if !containsTicket(chosen, anchor.TicketID) || !compatibleSet(chosen, now) {
+				return
+			}
+			selection, ok := scoreSelection(chosen, now)
+			if !ok {
+				return
+			}
+			if !found || betterSelection(selection, best) {
+				best = selection
+				found = true
+			}
+			return
+		}
+		for i := start; i < len(pool); i++ {
+			chosen = append(chosen, pool[i])
+			visit(i + 1)
+			chosen = chosen[:len(chosen)-1]
+		}
+	}
+	visit(0)
+	if !found {
+		return Selection{}, fmt.Errorf("no candidate set satisfies latency and mutual rating limits")
+	}
+	sort.Slice(best.Players, func(i, j int) bool { return best.Players[i].TicketID < best.Players[j].TicketID })
+	return best, nil
+}
+
+func compatibleSet(players []Candidate, now time.Time) bool {
+	regions := commonRegions(players)
+	if len(regions) == 0 {
+		return false
+	}
+	for i := range players {
+		for j := i + 1; j < len(players); j++ {
+			waitI := now.Sub(players[i].EnqueuedAt).Seconds()
+			waitJ := now.Sub(players[j].EnqueuedAt).Seconds()
+			delta := abs(players[i].Rating - players[j].Rating)
+			if delta > RatingTolerance(waitI) || delta > RatingTolerance(waitJ) {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+func commonRegions(players []Candidate) []string {
+	if len(players) == 0 {
+		return nil
+	}
+	regions := make(map[string]bool)
+	for region, rtt := range players[0].PredictedRTT {
+		if rtt <= MaxPlacementRTT {
+			regions[region] = true
+		}
+	}
+	for _, player := range players[1:] {
+		for region := range regions {
+			rtt, ok := player.PredictedRTT[region]
+			if !ok || rtt > MaxPlacementRTT {
+				delete(regions, region)
+			}
+		}
+	}
+	out := make([]string, 0, len(regions))
+	for region := range regions {
+		out = append(out, region)
+	}
+	sort.Strings(out)
+	return out
+}
+
+func scoreSelection(players []Candidate, now time.Time) (Selection, bool) {
+	regions := commonRegions(players)
+	if len(regions) == 0 {
+		return Selection{}, false
+	}
+	best := Selection{}
+	for _, region := range regions {
+		worst, total := 0.0, 0.0
+		minRating, maxRating := players[0].Rating, players[0].Rating
+		wait := 0.0
+		for _, player := range players {
+			rtt := player.PredictedRTT[region]
+			if rtt > worst { worst = rtt }
+			total += rtt
+			if player.Rating < minRating { minRating = player.Rating }
+			if player.Rating > maxRating { maxRating = player.Rating }
+			if seconds := now.Sub(player.EnqueuedAt).Seconds(); seconds > 0 { wait += seconds }
+		}
+		candidate := Selection{Players: append([]Candidate(nil), players...), Region: region, WorstRTT: worst, TotalRTT: total, RatingRange: maxRating-minRating, TotalWaitSeconds: wait}
+		if best.Players == nil || betterSelection(candidate, best) { best = candidate }
+	}
+	return best, true
+}
+
+func betterSelection(a, b Selection) bool {
+	if a.WorstRTT != b.WorstRTT { return a.WorstRTT < b.WorstRTT }
+	if a.TotalRTT != b.TotalRTT { return a.TotalRTT < b.TotalRTT }
+	if a.RatingRange != b.RatingRange { return a.RatingRange < b.RatingRange }
+	if a.TotalWaitSeconds != b.TotalWaitSeconds { return a.TotalWaitSeconds > b.TotalWaitSeconds }
+	return ticketIDs(a.Players) < ticketIDs(b.Players)
+}
+
+func containsTicket(players []Candidate, ticketID string) bool {
+	for _, player := range players { if player.TicketID == ticketID { return true } }
+	return false
+}
+
+func ticketIDs(players []Candidate) string {
+	ids := make([]string, 0, len(players))
+	for _, player := range players { ids = append(ids, player.TicketID) }
+	sort.Strings(ids)
+	result := ""
+	for _, id := range ids { result += id + "\x00" }
+	return result
+}
+
+func abs(value float64) float64 { if value < 0 { return -value }; return value }