noitu/server/internal/game/engine_test.go

package game

import (
	"fmt"
	"iter"
	"slices"
	"strings"
	"testing"
	"time"
)

// fakeDict is a hand-built word graph. Tests state the exact edges they need,
// so a rule can be exercised on a board small enough to reason about — no
// SQLite, no real corpus.
type fakeDict struct {
	words   map[string][2]string // canonical -> {first, last}
	aliases map[string]string
}

func newDict(words ...string) *fakeDict {
	d := &fakeDict{words: map[string][2]string{}, aliases: map[string]string{}}
	for _, w := range words {
		parts := strings.Fields(w)
		d.words[w] = [2]string{parts[0], parts[len(parts)-1]}
	}
	return d
}

func (d *fakeDict) alias(variant, canonical string) *fakeDict {
	d.aliases[variant] = canonical
	return d
}

func (d *fakeDict) Resolve(word string) (string, bool) {
	if _, ok := d.words[word]; ok {
		return word, true
	}
	c, ok := d.aliases[word]
	return c, ok
}

func (d *fakeDict) FirstSyllable(word string) (string, bool) {
	e, ok := d.words[word]
	return e[0], ok
}

func (d *fakeDict) LastSyllable(word string) (string, bool) {
	e, ok := d.words[word]
	return e[1], ok
}

func (d *fakeDict) WordsStartingWith(syllable string) iter.Seq[string] {
	var out []string
	for w, e := range d.words {
		if e[0] == syllable {
			out = append(out, w)
		}
	}
	slices.Sort(out)
	return slices.Values(out)
}

func (d *fakeDict) OutDegree(syllable string) (int, error) {
	n := 0
	for _, e := range d.words {
		if e[0] == syllable {
			n++
		}
	}
	return n, nil
}

const (
	alice = PlayerID("alice")
	bob   = PlayerID("bob")
)

var t0 = time.Date(2026, 1, 1, 12, 0, 0, 0, time.UTC)

// standardDict gives both players room to move for several turns.
func standardDict() *fakeDict {
	return newDict(
		"ngôn ngữ",  // ngôn -> ngữ
		"ngữ pháp",  // ngữ  -> pháp
		"ngữ điệu",  // ngữ  -> điệu
		"pháp luật", // pháp -> luật
		"pháp lý",   // pháp -> lý
		"luật lệ",   // luật -> lệ
		"lý do",     // lý   -> do
		"vô tuyến điện",
	)
}

func newGame(t *testing.T, d Dictionary, opening string) *Engine {
	t.Helper()
	e, err := New(d, []PlayerID{alice, bob}, opening, 20*time.Second, t0)
	if err != nil {
		t.Fatalf("New: %v", err)
	}
	return e
}

func TestNewSeedsStateFromOpening(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")

	if got := e.Current(); got != "ngữ" {
		t.Errorf("Current = %q, want %q", got, "ngữ")
	}
	if got := e.Turn(); got != alice {
		t.Errorf("Turn = %q, want %q", got, alice)
	}
	if !e.Used("ngôn ngữ") {
		t.Error("opening word is not marked used")
	}
	if got := e.ChainLength(); got != 1 {
		t.Errorf("ChainLength = %d, want 1", got)
	}
}

func TestNewRejectsBadInput(t *testing.T) {
	d := standardDict()

	if _, err := New(nil, []PlayerID{alice, bob}, "ngôn ngữ", time.Second, t0); err == nil {
		t.Error("New accepted a nil dictionary")
	}
	if _, err := New(d, []PlayerID{alice}, "ngôn ngữ", time.Second, t0); err == nil {
		t.Error("New accepted a single player")
	}
	if _, err := New(d, []PlayerID{alice, bob}, "ngôn ngữ", 0, t0); err == nil {
		t.Error("New accepted a zero turn limit")
	}
	if _, err := New(d, []PlayerID{alice, bob}, "không tồn tại", time.Second, t0); err == nil {
		t.Error("New accepted an opening word outside the dictionary")
	}
}

func TestSubmitAcceptsLegalMove(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")

	move, reason := e.Submit(alice, "ngữ pháp", t0)
	if reason != ReasonNone {
		t.Fatalf("Submit rejected a legal move: %s", reason)
	}
	if move.Word != "ngữ pháp" || move.First != "ngữ" || move.Last != "pháp" {
		t.Errorf("move = %+v, want ngữ pháp / ngữ / pháp", move)
	}
	if got := e.Current(); got != "pháp" {
		t.Errorf("Current = %q, want %q", got, "pháp")
	}
	if got := e.Turn(); got != bob {
		t.Errorf("Turn = %q, want %q", got, bob)
	}
	if !e.Used("ngữ pháp") {
		t.Error("accepted word is not marked used")
	}
}

// One test per rejection reason: a boolean would not tell a player what to fix.
func TestSubmitRejectionReasons(t *testing.T) {
	tests := []struct {
		name  string
		setup func(*Engine)
		who   PlayerID
		word  string
		when  time.Time
		want  RejectReason
	}{
		{
			name: "not your turn",
			who:  bob, word: "ngữ pháp", when: t0,
			want: ReasonNotYourTurn,
		},
		{
			name: "single syllable",
			who:  alice, word: "ngữ", when: t0,
			want: ReasonTooFewSyllables,
		},
		{
			name: "empty input",
			who:  alice, word: "   ", when: t0,
			want: ReasonTooFewSyllables,
		},
		{
			name: "unknown word",
			who:  alice, word: "ngữ xyzzy", when: t0,
			want: ReasonNotInDictionary,
		},
		{
			name: "wrong link",
			who:  alice, word: "luật lệ", when: t0,
			want: ReasonWrongLink,
		},
		{
			name: "turn expired",
			who:  alice, word: "ngữ pháp", when: t0.Add(21 * time.Second),
			want: ReasonTimeout,
		},
	}

	// ReasonAlreadyUsed needs a played-out board, so it has its own tests:
	// TestSubmitBlocksReuseAcrossPlayers and TestSubmitBlocksReuseViaAlias.
	for _, tc := range tests {
		t.Run(tc.name, func(t *testing.T) {
			e := newGame(t, standardDict(), "ngôn ngữ")
			if tc.setup != nil {
				tc.setup(e)
			}
			if _, got := e.Submit(tc.who, tc.word, tc.when); got != tc.want {
				t.Errorf("Submit = %s, want %s", got, tc.want)
			}
		})
	}
}

// A word is spent for the whole game, not per player.
func TestSubmitBlocksReuseAcrossPlayers(t *testing.T) {
	d := newDict(
		"ngôn ngữ",
		"ngữ pháp",
		"pháp ngữ", // lets play return to "ngữ"
		"ngữ điệu",
	)
	e := newGame(t, d, "ngôn ngữ")

	if _, r := e.Submit(alice, "ngữ pháp", t0); r != ReasonNone {
		t.Fatalf("alice's move rejected: %s", r)
	}
	if _, r := e.Submit(bob, "pháp ngữ", t0); r != ReasonNone {
		t.Fatalf("bob's move rejected: %s", r)
	}
	// Back on "ngữ": alice cannot replay the word she already used.
	if _, r := e.Submit(alice, "ngữ pháp", t0); r != ReasonAlreadyUsed {
		t.Errorf("replaying a used word = %s, want %s", r, ReasonAlreadyUsed)
	}
}

// Canonicalization can move the first syllable, so the link must be checked
// against the canonical form. Checking the typed form rejects a legal move.
func TestSubmitAcceptsAliasWithFirstSyllableDrift(t *testing.T) {
	d := newDict("ngôn ngữ", "ngữ pháp").alias("ngử pháp", "ngữ pháp")
	e := newGame(t, d, "ngôn ngữ")

	move, reason := e.Submit(alice, "ngử pháp", t0)
	if reason != ReasonNone {
		t.Fatalf("alias with first-syllable drift rejected: %s", reason)
	}
	if move.Word != "ngữ pháp" {
		t.Errorf("move.Word = %q, want the canonical %q", move.Word, "ngữ pháp")
	}
	if move.Typed != "ngử pháp" {
		t.Errorf("move.Typed = %q, want what the player typed", move.Typed)
	}
	// The chain must continue from the canonical's last syllable.
	if got := e.Current(); got != "pháp" {
		t.Errorf("Current = %q, want %q", got, "pháp")
	}
}

// An alias must be spent along with its canonical: the same word cannot be
// played twice under two spellings.
func TestSubmitBlocksReuseViaAlias(t *testing.T) {
	// "ngữ điệu" keeps a move available after play returns to "ngữ", so the
	// game does not end on a dead end before the reuse can be attempted.
	d := newDict("ngôn ngữ", "ngữ pháp", "pháp ngữ", "ngữ điệu").alias("ngử pháp", "ngữ pháp")
	e := newGame(t, d, "ngôn ngữ")

	e.Submit(alice, "ngữ pháp", t0)
	e.Submit(bob, "pháp ngữ", t0)

	if _, r := e.Submit(alice, "ngử pháp", t0); r != ReasonAlreadyUsed {
		t.Errorf("replaying via an alias = %s, want %s", r, ReasonAlreadyUsed)
	}
}

func TestSubmitScoring(t *testing.T) {
	d := newDict("ngôn ngữ", "ngữ pháp", "pháp vô tuyến điện")
	e := newGame(t, d, "ngôn ngữ")

	// Spec: 10 + 2*chainLength + 5*(syllables-2) + speed + rarity, where
	// chainLength counts the words already down, opening word included, speed
	// is 10 for an answer with the whole turn still on the clock, and rarity
	// is 15 for a syllable this dictionary answers with one word. Asserted as
	// literals so the test pins the specified formula rather than whatever the
	// code computes.
	two, _ := e.Submit(alice, "ngữ pháp", t0)
	if want := 37; two.Points != want { // 10 + 2*1 + 5*0 + 10 + 15
		t.Errorf("two-syllable word at chain 1 scored %d, want %d", two.Points, want)
	}

	four, r := e.Submit(bob, "pháp vô tuyến điện", t0)
	if r != ReasonNone {
		t.Fatalf("four-syllable word rejected: %s", r)
	}
	if want := 49; four.Points != want { // 10 + 2*2 + 5*2 + 10 + 15
		t.Errorf("four-syllable word at chain 2 scored %d, want %d", four.Points, want)
	}
	if four.Points <= two.Points {
		t.Error("longer word did not score more than a shorter one")
	}
}

// The speed term falls off linearly across the turn and is gone by the
// deadline, which is the last instant a submission is still in time.
func TestSubmitScoringPaysForTimeLeft(t *testing.T) {
	half := t0.Add(10 * time.Second) // half of the 20-second turn newGame gives
	buzzer := t0.Add(20 * time.Second)

	instant, _ := newGame(t, newDict("ngôn ngữ", "ngữ pháp"), "ngôn ngữ").Submit(alice, "ngữ pháp", t0)
	halfway, _ := newGame(t, newDict("ngôn ngữ", "ngữ pháp"), "ngôn ngữ").Submit(alice, "ngữ pháp", half)
	late, r := newGame(t, newDict("ngôn ngữ", "ngữ pháp"), "ngôn ngữ").Submit(alice, "ngữ pháp", buzzer)
	if r != ReasonNone {
		t.Fatalf("word played on the deadline rejected: %s", r)
	}

	if want := 37; instant.Points != want { // ... + speed 10
		t.Errorf("instant answer scored %d, want %d", instant.Points, want)
	}
	if want := 32; halfway.Points != want { // ... + speed 5
		t.Errorf("answer with half the turn left scored %d, want %d", halfway.Points, want)
	}
	if want := 27; late.Points != want { // ... + speed 0
		t.Errorf("answer on the buzzer scored %d, want %d", late.Points, want)
	}
}

// The rarity term pays for the syllable being answered, not for the word: the
// same word is worth more when the dictionary offers little else on that link,
// and it loses 3 points per doubling of the answers rather than falling in a
// straight line, so a crowded link pays nothing at all.
func TestSubmitScoringPaysForARareLink(t *testing.T) {
	// A dictionary where "ngữ" is answered by options words, one of which is
	// the word the test plays.
	link := func(options int) *fakeDict {
		words := []string{"ngôn ngữ", "ngữ pháp"}
		for i := 1; i < options; i++ {
			words = append(words, fmt.Sprintf("ngữ từ%d", i))
		}
		return newDict(words...)
	}

	// 10 base + 2 chain + 0 syllables + 10 speed = 22 before the rarity term.
	tests := []struct {
		options int
		want    int
	}{
		{1, 22 + 15},
		{2, 22 + 12},
		{3, 22 + 12},
		{4, 22 + 9},
		{8, 22 + 6},
		{16, 22 + 3},
		{32, 22},
		{64, 22},
	}
	for _, tc := range tests {
		e := newGame(t, link(tc.options), "ngôn ngữ")
		m, r := e.Submit(alice, "ngữ pháp", t0)
		if r != ReasonNone {
			t.Fatalf("word on a %d-answer link rejected: %s", tc.options, r)
		}
		if m.Points != tc.want {
			t.Errorf("word on a %d-answer link scored %d, want %d", tc.options, m.Points, tc.want)
		}
	}
}

// The chain term stops growing at chainBonusWords. Beyond it a word is priced
// by the move — its length, its speed, its link — and not by how long the game
// happened to have been running.
func TestSubmitScoringStopsPayingForChainLength(t *testing.T) {
	// A single line of two-syllable words, "a0 a1" -> "a1 a2" -> ..., so every
	// link has exactly one answer and only the chain term varies.
	const words = chainBonusWords + 3
	var line []string
	for i := range words {
		line = append(line, fmt.Sprintf("a%d a%d", i, i+1))
	}
	e := newGame(t, newDict(line...), line[0])

	// 10 base + 0 syllables + 10 speed + 15 rarity, plus the chain term.
	var points []int
	for i := 1; i < words; i++ {
		m, r := e.Submit(e.Turn(), line[i], t0)
		if r != ReasonNone {
			t.Fatalf("word %d of the line rejected: %s", i, r)
		}
		points = append(points, m.Points)
	}

	for i, got := range points {
		chain := i + 1 // the words already down when this one was played
		want := 35 + 2*min(chain, chainBonusWords)
		if got != want {
			t.Errorf("word at chain %d scored %d, want %d", chain, got, want)
		}
	}
	if first, last := points[chainBonusWords-1], points[len(points)-1]; first != last {
		t.Errorf("chain term kept growing past %d words: %d then %d", chainBonusWords, first, last)
	}
}

// Parts must name every term the total is made of, with nothing left over and
// nothing zero: a term that did not contribute has nothing to show a player.
func TestSubmitScoringPartsMatchTheTotal(t *testing.T) {
	d := newDict("ngôn ngữ", "ngữ pháp", "pháp vô tuyến điện")
	e := newGame(t, d, "ngôn ngữ")

	two, _ := e.Submit(alice, "ngữ pháp", t0)
	if want := []PointPart{
		{Kind: PointKindBase, Value: 10},
		{Kind: PointKindChain, Value: 2},
		{Kind: PointKindSpeed, Value: 10},
		{Kind: PointKindRarity, Value: 15},
	}; !slices.Equal(two.Parts, want) {
		t.Errorf("two-syllable word parts = %+v, want %+v", two.Parts, want)
	}
	if sum := sumParts(two.Parts); sum != two.Points {
		t.Errorf("parts sum to %d, want Points %d", sum, two.Points)
	}

	// Four syllables is two past the minimum, so this word's syllable term is
	// the one difference from the case above and must appear rather than be
	// silently folded into the total.
	four, r := e.Submit(bob, "pháp vô tuyến điện", t0)
	if r != ReasonNone {
		t.Fatalf("four-syllable word rejected: %s", r)
	}
	if want := []PointPart{
		{Kind: PointKindBase, Value: 10},
		{Kind: PointKindChain, Value: 4},
		{Kind: PointKindSyllables, Value: 10},
		{Kind: PointKindSpeed, Value: 10},
		{Kind: PointKindRarity, Value: 15},
	}; !slices.Equal(four.Parts, want) {
		t.Errorf("four-syllable word parts = %+v, want %+v", four.Parts, want)
	}
	if sum := sumParts(four.Parts); sum != four.Points {
		t.Errorf("parts sum to %d, want Points %d", sum, four.Points)
	}
}

// When the naive sum overruns maxPointsPerWord, the parts must still sum to
// the capped total exactly — trimmed from the end (rarity, then speed, then
// syllables) rather than the total being clamped while the breakdown still
// claims the uncapped numbers.
func TestSubmitScoringPartsAreTrimmedAtTheCap(t *testing.T) {
	// A 20-syllable word answering "ngữ", the only continuation the dictionary
	// offers there, so rarity pays its maximum: base 10 + chain 2 + syllables
	// 5*18=90 + speed 10 + rarity 15 = 127 raw, 27 over the cap.
	tokens := []string{"ngữ"}
	for i := range 18 {
		tokens = append(tokens, fmt.Sprintf("s%d", i))
	}
	tokens = append(tokens, "z")
	long := strings.Join(tokens, " ")

	e := newGame(t, newDict("ngôn ngữ", long), "ngôn ngữ")
	move, r := e.Submit(alice, long, t0)
	if r != ReasonNone {
		t.Fatalf("long word rejected: %s", r)
	}
	if move.Points != maxPointsPerWord {
		t.Fatalf("Points = %d, want the cap %d", move.Points, maxPointsPerWord)
	}
	if sum := sumParts(move.Parts); sum != maxPointsPerWord {
		t.Errorf("parts sum to %d, want the cap %d", sum, maxPointsPerWord)
	}
	// Rarity and speed are trimmed away entirely (15 + 10 = 25 of the 27
	// overflow) before syllables gives up the remaining 2, so base and chain
	// are untouched and syllables lands at 88 rather than its raw 90.
	if want := []PointPart{
		{Kind: PointKindBase, Value: 10},
		{Kind: PointKindChain, Value: 2},
		{Kind: PointKindSyllables, Value: 88},
	}; !slices.Equal(move.Parts, want) {
		t.Errorf("trimmed parts = %+v, want %+v", move.Parts, want)
	}
}

func TestPointKindStrings(t *testing.T) {
	seen := map[string]bool{}
	for k := PointKind(0); k < NumPointKinds; k++ {
		s := k.String()
		if s == "" || s == "unknown" {
			t.Errorf("PointKind(%d).String() = %q", k, s)
		}
		if seen[s] {
			t.Errorf("duplicate description %q", s)
		}
		seen[s] = true
	}
	if got := PointKind(99).String(); got != "unknown" {
		t.Errorf("unknown PointKind string = %q", got)
	}
}

func TestLegalMovesAndHasLegalMove(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")

	got := e.LegalMoves()
	want := []string{"ngữ pháp", "ngữ điệu"}
	slices.Sort(got)
	slices.Sort(want)
	if !slices.Equal(got, want) {
		t.Errorf("LegalMoves = %v, want %v", got, want)
	}
	if !e.HasLegalMove() {
		t.Error("HasLegalMove = false with moves available")
	}

	// Once both continuations are spent there is nothing left from "ngữ".
	e.Submit(alice, "ngữ pháp", t0)
	if got := e.Current(); got != "pháp" {
		t.Fatalf("Current = %q, want pháp", got)
	}
}

// A dead end does not end the game by itself. The player who walked into one
// keeps the turn they were given, and it is the clock that takes it from them.
func TestDeadEndLeavesTheTurnWithThePlayer(t *testing.T) {
	e := deadEndGame(t)

	if e.Over() {
		t.Fatal("game ended on the move into a dead end instead of leaving the turn to be played")
	}
	if e.Turn() != alice {
		t.Errorf("Turn = %q, want %q", e.Turn(), alice)
	}
	if e.HasLegalMove() {
		t.Error("HasLegalMove = true in a position with nothing to play")
	}
}

// Losing a turn nobody could have answered is reported as the dead end it was,
// not as time spent thinking.
func TestDeadEndEndsOnTheClockAsNoLegalMove(t *testing.T) {
	e := deadEndGame(t)

	if !e.Timeout(e.Deadline().Add(time.Nanosecond)) {
		t.Fatal("Timeout did not end an expired turn")
	}
	if e.Winner() != bob {
		t.Errorf("Winner = %q, want %q (alice had no move)", e.Winner(), bob)
	}
	if got := e.Snapshot().EndReason; got != EndNoLegalMove {
		t.Errorf("EndReason = %s, want %s", got, EndNoLegalMove)
	}
}

// NoMove settles a dead end without a clock, which is what the room does for
// the bot: it has nothing to wait for.
func TestNoMoveEndsADeadEndImmediately(t *testing.T) {
	e := deadEndGame(t)

	if !e.NoMove(t0) {
		t.Fatal("NoMove = false in a position with nothing to play")
	}
	if e.Winner() != bob {
		t.Errorf("Winner = %q, want %q", e.Winner(), bob)
	}
	if got := e.Snapshot().EndReason; got != EndNoLegalMove {
		t.Errorf("EndReason = %s, want %s", got, EndNoLegalMove)
	}

	if e.NoMove(t0) {
		t.Error("NoMove ended an already finished game a second time")
	}
}

// NoMove is not a resignation button: a player who still has words to play
// cannot use it to end the game.
func TestNoMoveRefusesAPlayablePosition(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")

	if e.NoMove(t0) {
		t.Error("NoMove = true with legal moves available")
	}
	if e.Over() {
		t.Error("game ended from a playable position")
	}
}

// deadEndGame leaves alice on turn with nothing to play: "lệ" starts no word.
func deadEndGame(t *testing.T) *Engine {
	t.Helper()

	d := newDict("ngôn ngữ", "ngữ luật", "luật lệ")
	e := newGame(t, d, "ngôn ngữ")

	if _, r := e.Submit(alice, "ngữ luật", t0); r != ReasonNone {
		t.Fatalf("alice's move rejected: %s", r)
	}
	if _, r := e.Submit(bob, "luật lệ", t0); r != ReasonNone {
		t.Fatalf("bob's move rejected: %s", r)
	}
	return e
}

// What a losing player is shown: a few of the words the position still had,
// and nothing at all when it had none.
func TestSuggestions(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")

	if got, want := e.Suggestions(3), []string{"ngữ pháp", "ngữ điệu"}; !slices.Equal(got, want) {
		t.Errorf("Suggestions(3) = %v, want %v", got, want)
	}
	// Capped, and stable: the same position answers the same way every time.
	if got := e.Suggestions(1); !slices.Equal(got, []string{"ngữ pháp"}) {
		t.Errorf("Suggestions(1) = %v, want [ngữ pháp]", got)
	}
	if got := e.Suggestions(0); got != nil {
		t.Errorf("Suggestions(0) = %v, want nil", got)
	}

	// A played word is no longer a suggestion.
	e.Submit(alice, "ngữ pháp", t0)
	for _, word := range e.Suggestions(3) {
		if word == "ngữ pháp" {
			t.Error("Suggestions offered a word that had already been played")
		}
	}

	if got := deadEndGame(t).Suggestions(3); len(got) != 0 {
		t.Errorf("Suggestions in a dead end = %v, want none", got)
	}
}

func TestIsExpiredBoundary(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")
	deadline := e.Deadline()

	if e.IsExpired(deadline.Add(-time.Nanosecond)) {
		t.Error("expired just before the deadline")
	}
	if e.IsExpired(deadline) {
		t.Error("expired exactly on the deadline")
	}
	if !e.IsExpired(deadline.Add(time.Nanosecond)) {
		t.Error("not expired just after the deadline")
	}
}

func TestTimeoutAwardsOpponent(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")

	if e.Timeout(t0) {
		t.Error("Timeout fired before the deadline")
	}
	if !e.Timeout(t0.Add(21 * time.Second)) {
		t.Fatal("Timeout did not fire after the deadline")
	}
	if e.Winner() != bob {
		t.Errorf("Winner = %q, want %q (alice ran out of time)", e.Winner(), bob)
	}
	if e.Timeout(t0.Add(30 * time.Second)) {
		t.Error("Timeout fired twice")
	}
}

func TestResign(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")

	if !e.Resign(alice, t0) {
		t.Fatal("Resign returned false")
	}
	if e.Winner() != bob {
		t.Errorf("Winner = %q, want %q", e.Winner(), bob)
	}
	if e.Snapshot().EndReason != EndResigned {
		t.Errorf("EndReason = %s, want %s", e.Snapshot().EndReason, EndResigned)
	}
	if e.Resign(bob, t0) {
		t.Error("Resign succeeded on a finished game")
	}
}

// A finished game is not a turn-order problem, and the distinction reaches the
// player as copy.
func TestSubmitAfterGameOver(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")
	e.Resign(alice, t0)

	if _, r := e.Submit(bob, "ngữ pháp", t0); r != ReasonGameOver {
		t.Errorf("Submit after the game ended = %s, want %s", r, ReasonGameOver)
	}
}

// An opening whose last syllable starts nothing hands the first player a game
// they have already lost, with no move and no reason -- it would resolve only
// on the turn timer, reported as a timeout. New must refuse it.
func TestNewRejectsDeadEndOpening(t *testing.T) {
	// "lệ" starts no word.
	d := newDict("luật lệ", "ngôn ngữ", "ngữ pháp")

	_, err := New(d, []PlayerID{alice, bob}, "luật lệ", 20*time.Second, t0)
	if err == nil {
		t.Fatal("New accepted an opening that leaves the first player no move")
	}
	if !strings.Contains(err.Error(), "starts no other word") {
		t.Errorf("error %q does not explain the dead end", err)
	}
}

func TestNewRejectsDuplicatePlayers(t *testing.T) {
	if _, err := New(standardDict(), []PlayerID{alice, alice}, "ngôn ngữ", time.Second, t0); err == nil {
		t.Error("New accepted the same player twice")
	}
}

func TestEndReasonStrings(t *testing.T) {
	seen := map[string]bool{}
	for _, r := range []EndReason{EndNone, EndTimeout, EndNoLegalMove, EndResigned} {
		s := r.String()
		if s == "" || s == "unknown" {
			t.Errorf("EndReason(%d).String() = %q", r, s)
		}
		if seen[s] {
			t.Errorf("duplicate description %q", s)
		}
		seen[s] = true
	}
	if got := EndReason(99).String(); got != "unknown" {
		t.Errorf("unknown EndReason string = %q", got)
	}
}

// Move.Typed must carry what the player actually sent, so the UI can show that
// a correction happened.
func TestMoveRecordsRawInput(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")

	const raw = "  NGỮ   Pháp  "
	move, r := e.Submit(alice, raw, t0)
	if r != ReasonNone {
		t.Fatalf("Submit rejected %q: %s", raw, r)
	}
	if move.Typed != raw {
		t.Errorf("move.Typed = %q, want the raw input %q", move.Typed, raw)
	}
	if move.Word != "ngữ pháp" {
		t.Errorf("move.Word = %q, want the canonical form", move.Word)
	}
}

// Snapshot must not hand out engine state.
func TestSnapshotIsACopy(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")
	e.Submit(alice, "ngữ pháp", t0)

	snap := e.Snapshot()
	snap.Scores[alice] = 9999

	fresh := e.Snapshot()
	if fresh.Scores[alice] == 9999 {
		t.Error("mutating a snapshot's scores changed engine state")
	}
}

// LastMove is the resume replay's whole reason to exist: the tail of the
// chain without copying the rest of it.
func TestLastMove(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")

	if _, ok := e.LastMove(); ok {
		t.Fatal("LastMove reported a move before any word was played")
	}

	if _, r := e.Submit(alice, "ngữ pháp", t0); r != ReasonNone {
		t.Fatalf("Submit: %s", r)
	}
	move, ok := e.LastMove()
	if !ok {
		t.Fatal("LastMove reported none after a word was played")
	}
	if move.Word != "ngữ pháp" {
		t.Errorf("LastMove = %q, want %q", move.Word, "ngữ pháp")
	}

	if _, r := e.Submit(bob, "pháp luật", t0); r != ReasonNone {
		t.Fatalf("Submit: %s", r)
	}
	move, ok = e.LastMove()
	if !ok || move.Word != "pháp luật" {
		t.Errorf("LastMove after a second word = %+v, %v, want %q", move, ok, "pháp luật")
	}
}

// UsedWords is the bot's search boundary: the opening word and everything
// played since, with nothing else in it.
func TestUsedWords(t *testing.T) {
	e := newGame(t, standardDict(), "ngôn ngữ")
	if _, r := e.Submit(alice, "ngữ pháp", t0); r != ReasonNone {
		t.Fatalf("Submit: %s", r)
	}

	used := map[string]bool{}
	for w := range e.UsedWords() {
		used[w] = true
	}
	if !used["ngôn ngữ"] || !used["ngữ pháp"] || len(used) != 2 {
		t.Errorf("UsedWords = %v, want exactly the opening word and the move played", used)
	}
}

func TestRejectReasonStrings(t *testing.T) {
	// Every reason needs a distinct, non-empty description: these become
	// player-facing messages.
	seen := map[string]bool{}
	for _, r := range []RejectReason{
		ReasonNone, ReasonNotYourTurn, ReasonTooFewSyllables,
		ReasonNotInDictionary, ReasonWrongLink, ReasonAlreadyUsed, ReasonTimeout,
	} {
		s := r.String()
		if s == "" || s == "unknown" {
			t.Errorf("RejectReason(%d).String() = %q", r, s)
		}
		if seen[s] {
			t.Errorf("duplicate description %q", s)
		}
		seen[s] = true
	}
}