CosmicClash/server/domain/backfill_test.go

package domain

import (
	"errors"
	"testing"
	"time"
)

func backfillTarget() BackfillTarget {
	return BackfillTarget{
		MatchID: "match-1", ServerID: "server-1", Region: "EU",
		Anchor:       Candidate{Playlist: Casual, ClientBuild: "build-1", ProtocolVersion: 1},
		Slot:         CasualSlot{Slot: 2, Team: 0, PlayerID: "bot-slot-2", IsBot: true},
		Phase:        CasualKickoff,
		AnchorRating: 1500,
		VacatedAt:    time.Unix(1000, 0).UTC(),
	}
}

func backfillCandidate(ticketID string, enqueuedAt time.Time) Candidate {
	return Candidate{
		TicketID: ticketID, PlayerID: "player-" + ticketID, Playlist: Casual,
		ClientBuild: "build-1", ProtocolVersion: 1, Rating: 1500,
		EnqueuedAt: enqueuedAt, PredictedRTT: map[string]float64{"EU": 40},
	}
}

// docs/MATCHMAKING.md: "Choose the oldest ordinary casual ticket ... ties use
// ticket ID."
func TestBackfillPicksTheOldestTicketAndBreaksTiesByID(t *testing.T) {
	now := time.Unix(1000, 0).UTC()
	base := now.Add(-time.Minute)
	candidates := []Candidate{
		backfillCandidate("ticket-c", base.Add(2*time.Second)),
		backfillCandidate("ticket-b", base), // tie with ticket-a, loses on ID
		backfillCandidate("ticket-a", base), // oldest, lowest ID
		backfillCandidate("ticket-d", base.Add(time.Second)),
	}
	chosen, err := SelectCasualBackfillCandidate(backfillTarget(), candidates, now)
	if err != nil {
		t.Fatalf("select: %v", err)
	}
	if chosen.TicketID != "ticket-a" {
		t.Fatalf("chose %q, want the oldest ticket with the lowest ID", chosen.TicketID)
	}

	// Determinism: the result must not depend on scan order, or two replicas
	// could offer the same slot to different players.
	reversed := []Candidate{candidates[2], candidates[1], candidates[3], candidates[0]}
	again, err := SelectCasualBackfillCandidate(backfillTarget(), reversed, now)
	if err != nil || again.TicketID != chosen.TicketID {
		t.Fatalf("selection depends on input order: %q vs %q (err=%v)", again.TicketID, chosen.TicketID, err)
	}
}

func TestBackfillRejectsIncompatibleCandidates(t *testing.T) {
	now := time.Unix(1000, 0).UTC()
	base := now.Add(-time.Minute)
	for name, mutate := range map[string]func(*Candidate){
		"wrong build":    func(c *Candidate) { c.ClientBuild = "build-2" },
		"wrong protocol": func(c *Candidate) { c.ProtocolVersion = 2 },
		"ranked ticket":  func(c *Candidate) { c.Playlist = Ranked },
		// The server already exists in one region; sharing some other region
		// is not enough.
		"no RTT for the match region":  func(c *Candidate) { c.PredictedRTT = map[string]float64{"NA": 20} },
		"over the placement ceiling":   func(c *Candidate) { c.PredictedRTT = map[string]float64{"EU": MaxPlacementRTT + 1} },
		"no RTT evidence at all":       func(c *Candidate) { c.PredictedRTT = nil },
		"rating far outside tolerance": func(c *Candidate) { c.Rating = 1500 + MaxRatingTolerance + 1 },
	} {
		t.Run(name, func(t *testing.T) {
			candidate := backfillCandidate("ticket-a", base)
			mutate(&candidate)
			if _, err := SelectCasualBackfillCandidate(backfillTarget(), []Candidate{candidate}, now); !errors.Is(err, ErrNoBackfillCandidate) {
				t.Fatalf("err = %v, want ErrNoBackfillCandidate", err)
			}
		})
	}
}

// Backfill replaces a bot slot at a kickoff boundary only, never a live human
// slot and never mid-play.
func TestBackfillOnlyFillsBotSlotsAtKickoff(t *testing.T) {
	now := time.Unix(1000, 0).UTC()
	candidates := []Candidate{backfillCandidate("ticket-a", now.Add(-time.Minute))}
	for name, mutate := range map[string]func(*BackfillTarget){
		"mid-play":            func(target *BackfillTarget) { target.Phase = CasualLive },
		"occupied by a human": func(target *BackfillTarget) { target.Slot.IsBot = false },
		"ranked match":        func(target *BackfillTarget) { target.Anchor.Playlist = Ranked },
	} {
		t.Run(name, func(t *testing.T) {
			target := backfillTarget()
			mutate(&target)
			if _, err := SelectCasualBackfillCandidate(target, candidates, now); !errors.Is(err, ErrNoBackfillCandidate) {
				t.Fatalf("err = %v, want ErrNoBackfillCandidate", err)
			}
		})
	}
}

// Tolerance widens with the wait, exactly as it does for an ordinary queue, so
// a slot that has sat vacant longer accepts a wider rating spread.
func TestBackfillToleranceWidensWithTheVacancy(t *testing.T) {
	base := time.Unix(1000, 0).UTC()
	target := backfillTarget()
	target.VacatedAt = base
	distant := backfillCandidate("ticket-a", base.Add(-time.Minute))
	distant.Rating = target.AnchorRating + MinRatingTolerance + 1

	if _, err := SelectCasualBackfillCandidate(target, []Candidate{distant}, base); !errors.Is(err, ErrNoBackfillCandidate) {
		t.Fatalf("a candidate outside the initial tolerance was accepted: %v", err)
	}
	widened := base.Add(10 * time.Minute)
	if _, err := SelectCasualBackfillCandidate(target, []Candidate{distant}, widened); err != nil {
		t.Fatalf("tolerance did not widen with the vacancy: %v", err)
	}
}

func TestBackfillRejectsInvalidTargets(t *testing.T) {
	now := time.Unix(1000, 0).UTC()
	candidates := []Candidate{backfillCandidate("ticket-a", now.Add(-time.Minute))}
	for name, mutate := range map[string]func(*BackfillTarget){
		"no match":  func(target *BackfillTarget) { target.MatchID = "" },
		"no server": func(target *BackfillTarget) { target.ServerID = "" },
		"no region": func(target *BackfillTarget) { target.Region = "" },
	} {
		t.Run(name, func(t *testing.T) {
			target := backfillTarget()
			mutate(&target)
			if _, err := SelectCasualBackfillCandidate(target, candidates, now); err == nil {
				t.Fatalf("invalid target %s was accepted", name)
			}
		})
	}
	if _, err := SelectCasualBackfillCandidate(backfillTarget(), nil, time.Time{}); err == nil {
		t.Fatal("zero time was accepted")
	}
}

// Declining or ignoring a backfill offer costs nothing: the player asked for
// an ordinary match and is being offered a partly-played one.
func TestBackfillCarriesNoDeclinePenaltyAndAShortWindow(t *testing.T) {
	if BackfillDeclinePenalty() != 0 || CasualBackfillPenalty() != 0 {
		t.Fatal("backfill must not carry a cooldown")
	}
	if BackfillProposalWindow != 10*time.Second {
		t.Fatalf("backfill window = %v, want the documented 10s", BackfillProposalWindow)
	}
	// Same duration as an ordinary proposal. What makes a backfill offer
	// "separate" is its payload and the absent penalty, not its timing.
	if BackfillProposalWindow != ProposalWindow {
		t.Fatalf("backfill window %v diverged from the ordinary proposal window %v", BackfillProposalWindow, ProposalWindow)
	}
}