package game

import (
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"math/big"
	"slices"
	"strings"
)

// Tunable rules. The engine supports any player count >= 2; MinPlayers /
// MaxPlayers gate when a lobby can start (2 for now, more later).
const (
	MaxRounds     = 6
	CoinsPerRound = 3
	ShopRowSize   = 4
	MaxPets       = 5
	TradeInCount  = 3
	MinPlayers    = 2
	MaxPlayers    = 2
)

// Phase is the game's top-level state.
type Phase string

const (
	PhaseLobby    Phase = "lobby"    // waiting for players
	PhaseShop     Phase = "shop"     // players take turns acting until all pass
	PhaseArrange  Phase = "arrange"  // players order their decks for battle
	PhaseBattle   Phase = "battle"   // battle resolved; players review the log
	PhaseGameOver Phase = "gameover" // all rounds played
)

// Player holds everything about one seat. All fields are exported so a Game
// serializes to JSON for persistence.
type Player struct {
	ID        string `json:"id"`
	Token     string `json:"token"` // secret; never sent in views
	Name      string `json:"name"`
	Seat      int    `json:"seat"`
	Coins     int    `json:"coins"`
	Deck      []Card `json:"deck"`
	Trophies  int    `json:"trophies"`
	Ready     bool   `json:"ready"` // shop passed / arrange submitted / battle acknowledged
	Connected bool   `json:"connected"`
	// TripledThisRound records whether the player used the Triple (trade-in)
	// action during the current round's shop (Bison's Battle Prep).
	TripledThisRound bool `json:"tripledThisRound"`
	// Avocados (Golden pack) is the player's stash of set-aside Avocado tokens,
	// each discardable in place of a coin to buy a card. It persists across
	// rounds and is not part of the deck.
	Avocados int `json:"avocados,omitempty"`
	// PendingApplesInPlay (Golden pack) is apples banked this round by a sold
	// Hercules Beetle; the next battle starts the player's first pet with that
	// many apples, then it resets.
	PendingApplesInPlay int `json:"pendingApplesInPlay,omitempty"`
	// FirstBuyFree (Golden pack: Manta Ray) waives the gold cost of the
	// player's first Buy this round; set at shop start, cleared when used.
	FirstBuyFree bool `json:"firstBuyFree,omitempty"`
	// BuysThisRound (Golden pack: Blue-Ringed Octopus) counts Buy actions the
	// player has taken in the current shop round.
	BuysThisRound int `json:"buysThisRound,omitempty"`
	// PendingTrumpets (Golden pack: Bird of Paradise) is Trumpets the next
	// battle starts with in the pool; reset after that battle.
	PendingTrumpets int `json:"pendingTrumpets,omitempty"`
	// Mana (Unicorn pack) is a persistent resource, added to the play area as a
	// counter. It is gained shop-time (Cuddle Toad, Thunderbird) and battle-time
	// (Alchemedes, Fur-Bearing Trout), spent to power abilities (see
	// Effect.CostMana), and — unlike Trumpets — kept between rounds.
	Mana int `json:"mana,omitempty"`
	// NextRoundApples (Unicorn pack: Skeleton Dog) is apples banked in battle
	// that land in the player's hand at the start of the next round, then reset.
	NextRoundApples int `json:"nextRoundApples,omitempty"`
	// ShopPeekedRound (Unicorn pack: Bigfoot) is the round the player last used
	// Bigfoot's reveal (once per round); ShopPeek is the card they saw — a
	// snapshot of the shop deck's top, shown only in that player's own view.
	ShopPeekedRound int  `json:"shopPeekedRound,omitempty"`
	ShopPeek        *Card `json:"shopPeek,omitempty"`
	// IsBot marks a computer-controlled seat. The engine treats bots exactly
	// like humans; the server drives their actions. BotLevel is the bot's
	// skill in [0, 1]; BotMemory is the bot's private notebook, opaque to the
	// engine and persisted with the game so knowledge survives restarts.
	IsBot     bool            `json:"isBot,omitempty"`
	BotLevel  float64         `json:"botLevel,omitempty"`
	BotMemory json.RawMessage `json:"botMemory,omitempty"`
}

// PetCount counts pet cards in the player's deck.
func (p *Player) PetCount() int {
	n := 0
	for _, c := range p.Deck {
		if c.IsPet() {
			n++
		}
	}
	return n
}

func (p *Player) cardIndex(cardID string) int {
	return slices.IndexFunc(p.Deck, func(c Card) bool { return c.ID == cardID })
}

// PendingTrade is an in-progress trade-in: the trading player has paid and
// must now pick one of two revealed cards from the next tier's deck.
type PendingTrade struct {
	PlayerID string  `json:"playerId"`
	Tier     int     `json:"tier"` // 1-based tier the options came from
	Options  [2]Card `json:"options"`
}

// PendingReveal is an in-progress Cockatoo buy (Golden pack): the buyer must
// reveal another pet in their hand, gaining apples equal to its power. Options
// lists the card ids they may reveal.
type PendingReveal struct {
	PlayerID string   `json:"playerId"`
	Source   string   `json:"source"`  // the Cockatoo's card id
	Options  []string `json:"options"` // eligible pet card ids in the buyer's deck
	// Apples, when > 0, is a fixed apple reward (Unicorn pack: Quetzalcoatl gives
	// 3). When 0 the reward is the revealed pet's Power (Golden pack: Cockatoo).
	Apples int `json:"apples,omitempty"`
}

// PendingSacrifice is an in-progress Water of Youth buy (Unicorn pack): the
// buyer must discard one of their pets; that pet and the Water of Youth food
// are consumed to take the top card of the next tier's deck for free. Tier is
// the tier the reward comes from.
type PendingSacrifice struct {
	PlayerID string   `json:"playerId"`
	Source   string   `json:"source"`  // the Water of Youth card id (also discarded)
	Tier     int      `json:"tier"`    // 1-based tier the reward is drawn from
	Options  []string `json:"options"` // eligible pet card ids to sacrifice
}

// Game is the complete authoritative state. It is a pure state machine: no
// goroutines, no clocks, no I/O. Callers are responsible for locking.
type Game struct {
	ID   string `json:"id"`
	Code string `json:"code"`
	// Pack is the selected card pack (see packs.go). Chosen in the lobby by
	// the host; determines which cards fill the shop decks.
	Pack      string    `json:"pack"`
	Phase     Phase     `json:"phase"`
	Round     int       `json:"round"` // 1-based
	Players   []*Player `json:"players"`
	ShopDecks [][]Card  `json:"shopDecks"` // index 0 = tier 1
	ShopRow   []Card    `json:"shopRow"`   // empty ID = empty slot
	// Discards (Unicorn pack) is the pile of real cards that came from the shop
	// decks and later left a player's deck (sold, traded, sacrificed), keyed by
	// tier. Chimera and Abomination draw from it. Temporary cards never enter.
	Discards map[int][]Card `json:"discards,omitempty"`
	Turn      int       `json:"turn"`      // seat with the current shop turn
	// PrioritySeat holds the priority token: that seat shops first each round
	// and wins simultaneity races in battle. Assigned randomly at game start;
	// a battle winner hands it to the loser, a loser keeps it, a draw leaves
	// it put.
	PrioritySeat int           `json:"prioritySeat"`
	Pending      *PendingTrade `json:"pending,omitempty"`
	// PendingReveal is an in-progress Cockatoo reveal (Golden pack); it blocks
	// other shop actions on that seat until resolved, like Pending.
	PendingReveal *PendingReveal `json:"pendingReveal,omitempty"`
	// PendingSacrifice is an in-progress Water of Youth choice (Unicorn pack);
	// it blocks other shop actions on that seat until resolved, like Pending.
	PendingSacrifice *PendingSacrifice `json:"pendingSacrifice,omitempty"`
	Battle           *BattleResult     `json:"battle,omitempty"` // most recent battle
	NextCardID   int           `json:"nextCardId"`
	WinnerSeat   int           `json:"winnerSeat"` // set at gameover; -1 = tie
	// Log is the running, human-readable event log shown across every phase.
	Log    []LogEntry `json:"log,omitempty"`
	LogSeq int        `json:"logSeq"` // last assigned entry sequence number

	// RollDie overrides the rock die (faces 0,0,1,1,2,2) for tests. Nil
	// (including after loading from storage) means a fair random roll.
	RollDie func() int `json:"-"`
}

// battleDraw returns a random value in [0, n) for a battle's randomness — rock
// dice and Komodo's apple shuffle alike. The RollDie test override applies to
// rock dice (n == 3).
func (g *Game) battleDraw(n int) int {
	switch {
	case n <= 0:
		return 0
	case n == 3 && g.RollDie != nil:
		return g.RollDie() // test override applies to rock dice
	default:
		return randInt(n)
	}
}

// rollRockDie rolls one rock die: 0, 1, or 2 with equal probability.
func (g *Game) rollRockDie() int { return g.battleDraw(3) }

// battleShuffle does an in-place Fisher-Yates over a battle deck, routing its
// randomness through battleDraw like every other in-battle draw (Komodo).
func (g *Game) battleShuffle(s []Card) {
	for i := len(s) - 1; i > 0; i-- {
		j := g.battleDraw(i + 1)
		s[i], s[j] = s[j], s[i]
	}
}

var (
	ErrNotYourTurn   = errors.New("not your turn")
	ErrWrongPhase    = errors.New("action not allowed in this phase")
	ErrNoCoins       = errors.New("no coins remaining")
	ErrInvalidAction = errors.New("invalid action")
)

func randomID(n int) string {
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		panic(err)
	}
	return hex.EncodeToString(b)
}

func randomCode() string {
	const letters = "ABCDEFGHJKMNPQRSTUVWXYZ23456789" // no easily-confused chars
	code := make([]byte, 5)
	for i := range code {
		n, err := rand.Int(rand.Reader, big.NewInt(int64(len(letters))))
		if err != nil {
			panic(err)
		}
		code[i] = letters[n.Int64()]
	}
	return string(code)
}

func randInt(n int) int {
	v, err := rand.Int(rand.Reader, big.NewInt(int64(n)))
	if err != nil {
		panic(err)
	}
	return int(v.Int64())
}

func shuffle[T any](s []T) {
	for i := len(s) - 1; i > 0; i-- {
		j := randInt(i + 1)
		s[i], s[j] = s[j], s[i]
	}
}

// New creates a game in the lobby phase with its shop decks built and
// shuffled. All later "randomness" is just drawing from these decks, so the
// state is fully deterministic (and serializable) after this point.
func New() *Game {
	g := &Game{
		ID:         randomID(16),
		Code:       randomCode(),
		Pack:       DefaultPack,
		Phase:      PhaseLobby,
		WinnerSeat: -1,
	}
	g.buildDecks()
	return g
}

// buildDecks (re)creates and shuffles the shop decks for the current pack.
// Called on creation and whenever the pack changes, so ShopDecks always match
// g.Pack and are ready the moment the game starts.
func (g *Game) buildDecks() {
	g.buildShopDecks()
	for i := range g.ShopDecks {
		shuffle(g.ShopDecks[i])
	}
}

// AddPlayer seats a new player during the lobby phase and returns them (with
// their secret token). The host starts the game explicitly once the lobby is
// ready (see StartGame).
func (g *Game) AddPlayer(name string) (*Player, error) {
	if g.Phase != PhaseLobby {
		return nil, fmt.Errorf("%w: game already started", ErrWrongPhase)
	}
	if len(g.Players) >= MaxPlayers {
		return nil, errors.New("game is full")
	}
	if name == "" {
		name = fmt.Sprintf("Player %d", len(g.Players)+1)
	}
	p := &Player{
		ID:    randomID(8),
		Token: randomID(16),
		Name:  name,
		Seat:  len(g.Players),
	}
	g.Players = append(g.Players, p)
	return p, nil
}

// AddBot seats a computer-controlled player. Bots count as connected from
// the start; the server is responsible for driving their actions.
func (g *Game) AddBot(name string, level float64) (*Player, error) {
	p, err := g.AddPlayer(name)
	if err != nil {
		return nil, err
	}
	p.IsBot = true
	p.BotLevel = min(max(level, 0), 1)
	p.Connected = true
	return p, nil
}

// SetPack changes the game's card pack during the lobby and rebuilds the shop
// decks to match. Only playable packs may be selected.
func (g *Game) SetPack(packID string) error {
	if g.Phase != PhaseLobby {
		return fmt.Errorf("%w: game already started", ErrWrongPhase)
	}
	pack, ok := packByID(packID)
	if !ok {
		return fmt.Errorf("%w: unknown pack", ErrInvalidAction)
	}
	if !pack.Playable {
		return fmt.Errorf("%w: that pack isn't available yet", ErrInvalidAction)
	}
	g.Pack = pack.ID
	g.buildDecks()
	return nil
}

// RemovePlayer drops a seat from the lobby. The host (seat 0) can't be
// removed. Remaining players are re-seated so seats stay contiguous.
func (g *Game) RemovePlayer(targetID string) error {
	if g.Phase != PhaseLobby {
		return fmt.Errorf("%w: game already started", ErrWrongPhase)
	}
	idx := slices.IndexFunc(g.Players, func(p *Player) bool { return p.ID == targetID })
	if idx < 0 {
		return fmt.Errorf("%w: no such player", ErrInvalidAction)
	}
	if idx == 0 {
		return fmt.Errorf("%w: the host can't be removed", ErrInvalidAction)
	}
	g.Players = slices.Delete(g.Players, idx, idx+1)
	for i, p := range g.Players {
		p.Seat = i
	}
	return nil
}

// StartGame begins the match from the lobby once enough players are seated.
// The shop decks are already built for g.Pack (see buildDecks); this just
// validates and makes the transition.
func (g *Game) StartGame() error {
	if g.Phase != PhaseLobby {
		return fmt.Errorf("%w: game already started", ErrWrongPhase)
	}
	if len(g.Players) < MinPlayers {
		return fmt.Errorf("%w: need at least %d players to start", ErrInvalidAction, MinPlayers)
	}
	if pack, ok := packByID(g.Pack); !ok || !pack.Playable {
		return fmt.Errorf("%w: that pack isn't available yet", ErrInvalidAction)
	}
	g.start()
	return nil
}

// PlayerByID returns the player, or nil.
func (g *Game) PlayerByID(id string) *Player {
	for _, p := range g.Players {
		if p.ID == id {
			return p
		}
	}
	return nil
}

func (g *Game) start() {
	g.Round = 1
	// The priority token starts with a random seat.
	g.PrioritySeat = randInt(len(g.Players))
	g.startShopRound()
}

// startShopRound resets coins, deals the shop row from this round's tier
// deck, and rotates the starting player.
func (g *Game) startShopRound() {
	g.Phase = PhaseShop
	g.Pending = nil
	g.PendingReveal = nil
	g.PendingSacrifice = nil
	for _, p := range g.Players {
		p.Coins = CoinsPerRound
		p.Ready = false
		p.TripledThisRound = false
		p.FirstBuyFree = false
		p.BuysThisRound = 0
		p.ShopPeek = nil
		// Unicorn pack: apples banked in battle (Skeleton Dog) arrive in hand now.
		for range p.NextRoundApples {
			p.Deck = append(p.Deck, g.newApple())
		}
		if p.NextRoundApples > 0 {
			g.logf(p.Seat, "🍎", "%s starts the round with %d banked apple%s.", p.Name, p.NextRoundApples, plural(p.NextRoundApples))
			p.NextRoundApples = 0
		}
	}
	g.ShopRow = make([]Card, ShopRowSize)
	for i := range g.ShopRow {
		g.ShopRow[i] = g.drawFromTier(g.Round)
	}
	// Shop-start deck effects fire now (Golden pack: Manta Ray's free first buy).
	for _, p := range g.Players {
		for _, c := range slices.Clone(p.Deck) {
			g.applyShopTrigger(p, c, TriggerShopStart)
		}
	}
	// The priority-token holder shops first.
	g.Turn = g.PrioritySeat
	g.logf(-1, "🛒", "Round %d — shop opens (%s goes first).", g.Round, g.Players[g.PrioritySeat].Name)
}

// drawFromTier pops the top card of the given tier's deck (1-based tier).
// Returns a zero Card if the deck is empty.
func (g *Game) drawFromTier(tier int) Card {
	deck := g.ShopDecks[tier-1]
	if len(deck) == 0 {
		return Card{}
	}
	top := deck[0]
	g.ShopDecks[tier-1] = deck[1:]
	return top
}

// requireShopTurn validates that playerID may act in the shop right now.
func (g *Game) requireShopTurn(playerID string) (*Player, error) {
	if g.Phase != PhaseShop {
		return nil, ErrWrongPhase
	}
	p := g.PlayerByID(playerID)
	if p == nil {
		return nil, errors.New("unknown player")
	}
	if g.Players[g.Turn].ID != playerID {
		return nil, ErrNotYourTurn
	}
	if g.Pending != nil {
		return nil, fmt.Errorf("%w: finish your trade first", ErrInvalidAction)
	}
	if g.PendingReveal != nil {
		return nil, fmt.Errorf("%w: finish your reveal first", ErrInvalidAction)
	}
	if g.PendingSacrifice != nil {
		return nil, fmt.Errorf("%w: finish your choice first", ErrInvalidAction)
	}
	return p, nil
}

// Buy spends one coin to take the card at rowIdx into the player's deck.
// The slot refills from the current round's tier deck. Buying is the only
// action that costs gold (or, in the Golden pack, an Avocado — see BuyAvocado).
func (g *Game) Buy(playerID string, rowIdx int) error {
	p, err := g.requireShopTurn(playerID)
	if err != nil {
		return err
	}
	// Golden pack: Manta Ray waives the cost of the first buy this round.
	free := p.FirstBuyFree
	if !free && p.Coins <= 0 {
		return ErrNoCoins
	}
	if err := g.validRow(rowIdx); err != nil {
		return err
	}
	if free {
		p.FirstBuyFree = false
	} else {
		p.Coins--
	}
	p.BuysThisRound++
	verb := "bought"
	if free {
		verb = "bought (free)"
	}
	g.finishBuy(p, rowIdx, verb)
	g.advanceAfterBuy()
	return nil
}

// BuyAvocado (Golden pack) takes the card at rowIdx into the player's deck by
// discarding a set-aside Avocado instead of spending a coin.
func (g *Game) BuyAvocado(playerID string, rowIdx int) error {
	p, err := g.requireShopTurn(playerID)
	if err != nil {
		return err
	}
	if p.Avocados <= 0 {
		return fmt.Errorf("%w: no Avocado to discard", ErrInvalidAction)
	}
	if err := g.validRow(rowIdx); err != nil {
		return err
	}
	p.Avocados--
	p.BuysThisRound++
	g.finishBuy(p, rowIdx, "bought (with an Avocado)")
	g.advanceAfterBuy()
	return nil
}

// advanceAfterBuy hands off the turn after a buy, unless the buy opened a
// Cockatoo reveal that the same player must resolve first.
func (g *Game) advanceAfterBuy() {
	if g.PendingReveal != nil || g.PendingSacrifice != nil {
		return
	}
	g.advanceShopTurn()
}

// validRow checks that a shop slot holds a card.
func (g *Game) validRow(rowIdx int) error {
	if rowIdx < 0 || rowIdx >= len(g.ShopRow) || g.ShopRow[rowIdx].ID == "" {
		return fmt.Errorf("%w: no card in that shop slot", ErrInvalidAction)
	}
	return nil
}

// finishBuy takes the row card into the deck, logs the (public) buy, refills
// the slot, and fires the card's Buy effect — shared by every buy path. The
// how phrase describes how it was paid for. Payment is the caller's job.
func (g *Game) finishBuy(p *Player, rowIdx int, how string) {
	bought := g.ShopRow[rowIdx]
	p.Deck = append(p.Deck, bought)
	g.addLog(LogEntry{Seat: p.Seat, Icon: "🛒", Kind: LogBuy, Source: bought.ID, CardName: bought.Name,
		Text: fmt.Sprintf("%s %s %s %s.", p.Name, how, article(bought.Name), bought.Name)})
	g.ShopRow[rowIdx] = g.drawFromTier(g.Round)
	g.applyShopTrigger(p, bought, TriggerBuy)
}

// Sell converts any number (>=1) of the player's cards: each becomes an
// apple, and Sell effects on the sold cards fire. Selling is free.
func (g *Game) Sell(playerID string, cardIDs []string) error {
	p, err := g.requireShopTurn(playerID)
	if err != nil {
		return err
	}
	if len(cardIDs) == 0 {
		return fmt.Errorf("%w: choose at least one card to sell", ErrInvalidAction)
	}
	if err := g.sellCards(p, cardIDs); err != nil {
		return err
	}
	g.advanceShopTurn()
	return nil
}

// applyShopTrigger fires a shop-time trigger (buy/sell/triple/battle prep)
// on one card. Battle-time actions on the same trigger (Monkey's
// applesInPlay) are ignored here and handled by the battle resolver.
func (g *Game) applyShopTrigger(p *Player, c Card, trigger EffectTrigger) {
	for _, e := range c.Effects {
		if e.Trigger != trigger || g.Round < e.MinRound {
			continue
		}
		if e.Condition == ConditionTripled && !p.TripledThisRound {
			continue
		}
		switch e.Action {
		case ActionGainApple:
			n := e.count()
			switch e.Per {
			case PerShopFaintPets:
				n *= g.countShopFaintPets()
			case PerBuysThisRound:
				n *= p.BuysThisRound
			}
			if n <= 0 {
				continue
			}
			for range n {
				p.Deck = append(p.Deck, g.newApple())
			}
			g.addLog(LogEntry{Seat: p.Seat, Icon: "🍎", Source: c.ID, Spawn: "apple", Count: n,
				Text: fmt.Sprintf("%s adds %d apple%s to %s's deck.", c.Name, n, plural(n), p.Name)})
		case ActionFirstBuyFree:
			// Golden pack: Manta Ray, at shop start.
			if trigger == TriggerShopStart && p.PetCount() <= e.count() {
				p.FirstBuyFree = true
				g.logf(p.Seat, "🛒", "%s readies %s's first buy for free.", c.Name, p.Name)
			}
		case ActionRevealForApples:
			// Golden pack: Cockatoo — reveal another pet in hand for apples. Only
			// during a real shop turn (skipped when Catfish reactivates buys at
			// battle prep), and needs at least one other pet.
			if g.Phase != PhaseShop {
				continue
			}
			var opts []string
			for _, dc := range p.Deck {
				if dc.IsPet() && dc.ID != c.ID {
					opts = append(opts, dc.ID)
				}
			}
			if len(opts) > 0 {
				g.PendingReveal = &PendingReveal{PlayerID: p.ID, Source: c.ID, Options: opts}
			}
		case ActionRevealTierForApples:
			// Unicorn pack: Quetzalcoatl — reveal a hand pet at tier <= Cap for a
			// fixed number of apples.
			if g.Phase != PhaseShop {
				continue
			}
			maxTier := e.Cap
			var opts []string
			for _, dc := range p.Deck {
				if dc.IsPet() && dc.ID != c.ID && (maxTier <= 0 || dc.Tier <= maxTier) {
					opts = append(opts, dc.ID)
				}
			}
			if len(opts) > 0 {
				g.PendingReveal = &PendingReveal{PlayerID: p.ID, Source: c.ID, Options: opts, Apples: e.count()}
			}
		case ActionApplesInPlay:
			// Golden pack: apples-in-play banked when sold (Hercules Beetle) or
			// bought (Bird of Paradise). Monkey's battle-prep version is resolved
			// inside resolveBattle, so battlePrep is excluded here.
			if trigger == TriggerSell || trigger == TriggerBuy {
				p.PendingApplesInPlay += e.count()
				g.logf(p.Seat, "🍎", "%s — %s will start the next battle with %d apple%s in play.", c.Name, p.Name, e.count(), plural(e.count()))
			}
		case ActionStartTrumpets:
			// Golden pack: Bird of Paradise banks Trumpets for the next battle.
			p.PendingTrumpets += e.count()
			g.logf(p.Seat, "🎺", "%s — %s will start the next battle with %d Trumpet%s.", c.Name, p.Name, e.count(), plural(e.count()))
		case ActionReactivateBuys:
			// Golden pack: Catfish re-fires every pet's Buy ability at battle prep.
			if trigger == TriggerBattlePrep {
				for _, pet := range slices.Clone(p.Deck) {
					if pet.IsPet() && pet.ID != c.ID {
						g.applyShopTrigger(p, pet, TriggerBuy)
					}
				}
			}
		case ActionBuyTopFree:
			// Golden pack: Stoat pulls the top of the current tier deck for free.
			top := g.drawFromTier(g.Round)
			if top.ID == "" {
				continue
			}
			p.Deck = append(p.Deck, top)
			if hasBuyEffect(top) {
				g.addLog(LogEntry{Seat: p.Seat, Icon: "🛒", Kind: LogBuy, Source: top.ID, CardName: top.Name,
					Text: fmt.Sprintf("%s grabs %s %s from the shop deck for free — its buy ability triggers.", c.Name, article(top.Name), top.Name)})
			} else {
				g.logf(p.Seat, "🛒", "%s grabs a card from the shop deck for free.", c.Name)
			}
			g.applyShopTrigger(p, top, TriggerBuy)
		case ActionSetAside:
			// Golden pack: Avocado is diverted out of the deck into the
			// persistent set-aside stash (the caller already appended it).
			if idx := p.cardIndex(c.ID); idx >= 0 {
				p.Deck = slices.Delete(p.Deck, idx, idx+1)
			}
			p.Avocados++
			g.logf(p.Seat, "🥑", "%s sets %s aside.", p.Name, c.Name)
		case ActionRefreshGold:
			p.Coins = min(p.Coins+e.count(), CoinsPerRound)
			g.logf(p.Seat, "🪙", "%s refreshes %s's coins.", c.Name, p.Name)
		case ActionDoubleApples:
			apples := 0
			for _, dc := range p.Deck {
				if dc.Food == FoodApple {
					apples++
				}
			}
			for range apples {
				p.Deck = append(p.Deck, g.newApple())
			}
			if apples > 0 {
				g.addLog(LogEntry{Seat: p.Seat, Icon: "🍎", Source: c.ID, Spawn: "apple", Count: apples,
					Text: fmt.Sprintf("%s doubles %s's apples (+%d).", c.Name, p.Name, apples)})
			}
		case ActionGainMana:
			// Unicorn pack: shop-time mana (Cuddle Toad, Thunderbird). Battle-time
			// mana is handled by the resolver; skip it here.
			if trigger == TriggerBuy || trigger == TriggerSell || trigger == TriggerTriple {
				p.Mana += e.count()
				g.addLog(LogEntry{Seat: p.Seat, Icon: "🔮", Kind: LogMana, Source: c.ID, Count: e.count(),
					Text: fmt.Sprintf("%s gains %d Mana (now %d).", c.Name, e.count(), p.Mana)})
			}
		case ActionUpgradeNextTier:
			// Unicorn pack: Water of Youth — sacrifice a pet + this food to buy the
			// top of the next tier for free. Only a real shop buy opens the choice.
			if trigger != TriggerBuy || g.Phase != PhaseShop {
				continue
			}
			nextTier := g.Round + 1
			if nextTier > MaxRounds || len(g.ShopDecks[nextTier-1]) == 0 {
				continue // no higher tier to upgrade into; the food is wasted
			}
			var opts []string
			for _, dc := range p.Deck {
				if dc.IsPet() {
					opts = append(opts, dc.ID)
				}
			}
			if len(opts) > 0 {
				g.PendingSacrifice = &PendingSacrifice{PlayerID: p.ID, Source: c.ID, Tier: nextTier, Options: opts}
			}
		}
	}
}

// sellCards removes the given cards from p's deck, adds one apple per
// removed card, and fires the sold cards' Sell effects. It validates before
// mutating.
func (g *Game) sellCards(p *Player, cardIDs []string) error {
	if hasDuplicates(cardIDs) {
		return fmt.Errorf("%w: duplicate card", ErrInvalidAction)
	}
	for _, id := range cardIDs {
		if p.cardIndex(id) < 0 {
			return fmt.Errorf("%w: card not in your deck", ErrInvalidAction)
		}
	}
	sold := make([]Card, 0, len(cardIDs))
	for _, id := range cardIDs {
		idx := p.cardIndex(id)
		sold = append(sold, p.Deck[idx])
		p.Deck = slices.Delete(p.Deck, idx, idx+1)
	}
	for _, c := range sold {
		p.Deck = append(p.Deck, g.newApple())
		g.discardCard(c)
		g.addLog(LogEntry{Seat: p.Seat, Icon: "🍎", Kind: LogSell, Source: c.ID, CardName: c.Name, Spawn: "apple",
			Text: fmt.Sprintf("%s sold %s — it becomes an apple.", p.Name, c.Name)})
		g.applyShopTrigger(p, c, TriggerSell)
	}
	return nil
}

// hasBuyEffect reports whether a card carries any buy-triggered ability, which
// means acquiring it does something visible to everyone.
func hasBuyEffect(c Card) bool {
	for _, e := range c.Effects {
		if e.Trigger == TriggerBuy {
			return true
		}
	}
	return false
}

// countShopFaintPets counts pets currently in the shop row that carry a Faint
// effect (Opossum's Sell payout, Golden pack).
func (g *Game) countShopFaintPets() int {
	n := 0
	for _, c := range g.ShopRow {
		if c.ID == "" || !c.IsPet() {
			continue
		}
		for _, e := range c.Effects {
			if e.Trigger == TriggerFaint {
				n++
				break
			}
		}
	}
	return n
}

// TradeStart trades three same-suit pets from the player's deck to reveal
// the top two cards of the next tier's deck — a free action. The player must
// then call TradeChoose before anything else happens.
func (g *Game) TradeStart(playerID string, cardIDs []string) error {
	p, err := g.requireShopTurn(playerID)
	if err != nil {
		return err
	}
	if g.Round >= MaxRounds {
		return fmt.Errorf("%w: no higher tier to trade into", ErrInvalidAction)
	}
	if len(cardIDs) != TradeInCount || hasDuplicates(cardIDs) {
		return fmt.Errorf("%w: trade in exactly %d cards", ErrInvalidAction, TradeInCount)
	}
	var suit Suit
	for i, id := range cardIDs {
		idx := p.cardIndex(id)
		if idx < 0 {
			return fmt.Errorf("%w: card not in your deck", ErrInvalidAction)
		}
		c := p.Deck[idx]
		if !c.IsPet() {
			return fmt.Errorf("%w: only pets have suits", ErrInvalidAction)
		}
		if i == 0 {
			suit = c.Suit
		} else if c.Suit != suit {
			return fmt.Errorf("%w: cards must share a suit", ErrInvalidAction)
		}
	}
	nextTier := g.Round + 1
	if len(g.ShopDecks[nextTier-1]) < 2 {
		return fmt.Errorf("%w: next tier deck is exhausted", ErrInvalidAction)
	}
	// Validated; commit.
	traded := make([]Card, 0, TradeInCount)
	for _, id := range cardIDs {
		idx := p.cardIndex(id)
		traded = append(traded, p.Deck[idx])
		g.discardCard(p.Deck[idx])
		p.Deck = slices.Delete(p.Deck, idx, idx+1)
	}
	p.TripledThisRound = true
	// The discarded trio is public — everyone sees what was given up — even
	// though the pet ultimately chosen stays secret (see TradeChoose).
	names := make([]string, len(traded))
	ids := make([]string, len(traded))
	for i, c := range traded {
		names[i] = c.Name
		ids[i] = c.ID
	}
	g.addLog(LogEntry{Seat: p.Seat, Icon: "🔄", Kind: LogTrade, Cards: ids,
		Text: fmt.Sprintf("%s tripled %s (%s) for a tier %d pick.",
			p.Name, strings.Join(names, ", "), suit, nextTier)})
	g.Pending = &PendingTrade{
		PlayerID: playerID,
		Tier:     nextTier,
		Options:  [2]Card{g.drawFromTier(nextTier), g.drawFromTier(nextTier)},
	}
	// Triple effects fire on the traded-in cards themselves (e.g. Fish).
	for _, c := range traded {
		g.applyShopTrigger(p, c, TriggerTriple)
	}
	return nil
}

// TradeChoose resolves a pending trade: pick (0 or 1) joins the player's
// deck, the other goes to the bottom of its tier deck.
func (g *Game) TradeChoose(playerID string, pick int) error {
	if g.Phase != PhaseShop || g.Pending == nil || g.Pending.PlayerID != playerID {
		return fmt.Errorf("%w: no trade waiting on you", ErrInvalidAction)
	}
	if pick != 0 && pick != 1 {
		return fmt.Errorf("%w: pick 0 or 1", ErrInvalidAction)
	}
	p := g.PlayerByID(playerID)
	chosen, other := g.Pending.Options[pick], g.Pending.Options[1-pick]
	p.Deck = append(p.Deck, chosen)
	tierIdx := g.Pending.Tier - 1
	g.ShopDecks[tierIdx] = append(g.ShopDecks[tierIdx], other)
	g.Pending = nil
	// The pick is secret — opponents never saw the two revealed options and
	// can't see the deck. But a pet with a Buy ability performs it publicly, so
	// we have to reveal that pet (its effect log names it anyway).
	if hasBuyEffect(chosen) {
		g.addLog(LogEntry{Seat: p.Seat, Icon: "🔄", Kind: LogTradePick, CardName: chosen.Name,
			Text: fmt.Sprintf("%s's Triple pick is %s %s — its buy ability triggers.",
				p.Name, article(chosen.Name), chosen.Name)})
	} else {
		g.addLog(LogEntry{Seat: p.Seat, Icon: "🔄", Kind: LogTradePick,
			Text: fmt.Sprintf("%s keeps their Triple pick hidden.", p.Name)})
	}
	// Pets obtained via the Triple action trigger their Buy effects.
	g.applyShopTrigger(p, chosen, TriggerBuy)
	g.advanceAfterBuy()
	return nil
}

// RevealChoose resolves a pending Cockatoo reveal (Golden pack): the buyer
// reveals one of their other pets and gains apples equal to its power.
func (g *Game) RevealChoose(playerID, cardID string) error {
	if g.Phase != PhaseShop || g.PendingReveal == nil || g.PendingReveal.PlayerID != playerID {
		return fmt.Errorf("%w: no reveal waiting on you", ErrInvalidAction)
	}
	if !slices.Contains(g.PendingReveal.Options, cardID) {
		return fmt.Errorf("%w: reveal one of your other pets", ErrInvalidAction)
	}
	p := g.PlayerByID(playerID)
	idx := p.cardIndex(cardID)
	if idx < 0 {
		return fmt.Errorf("%w: card not in your deck", ErrInvalidAction)
	}
	revealed := p.Deck[idx]
	n := revealed.Power
	if g.PendingReveal.Apples > 0 {
		n = g.PendingReveal.Apples // Quetzalcoatl: fixed reward
	}
	for range n {
		p.Deck = append(p.Deck, g.newApple())
	}
	// The reveal is public (the opponent learns the pet); the apple spawn rides
	// on this entry so observers count it (Kind "" + Spawn "apple").
	g.addLog(LogEntry{Seat: p.Seat, Icon: "🦜", Source: revealed.ID, Spawn: "apple", Count: n, CardName: revealed.Name,
		Text: fmt.Sprintf("%s reveals %s (power %d) — adds %d apple%s to hand.", p.Name, revealed.Name, revealed.Power, n, plural(n))})
	g.PendingReveal = nil
	g.advanceShopTurn()
	return nil
}

// PeekShopDeck resolves Bigfoot's optional reveal (Unicorn pack): on the
// player's shop turn, once per round, they look at the top card of the current
// tier's shop deck. It doesn't spend gold or end the turn — it only stores a
// private snapshot served in that player's own view.
func (g *Game) PeekShopDeck(playerID string) error {
	p, err := g.requireShopTurn(playerID)
	if err != nil {
		return err
	}
	if p.ShopPeekedRound == g.Round {
		return fmt.Errorf("%w: already peeked this round", ErrInvalidAction)
	}
	hasBigfoot := false
	for _, c := range p.Deck {
		for _, e := range c.Effects {
			if e.Action == ActionPeekShop {
				hasBigfoot = true
			}
		}
	}
	if !hasBigfoot {
		return fmt.Errorf("%w: no pet can peek the shop deck", ErrInvalidAction)
	}
	p.ShopPeekedRound = g.Round
	deck := g.ShopDecks[g.Round-1]
	if len(deck) > 0 {
		top := deck[0]
		p.ShopPeek = &top
	}
	g.logf(p.Seat, "👁", "%s reveals a pet to peek at the shop deck.", p.Name)
	return nil
}

// SacrificeChoose resolves a pending Water of Youth choice (Unicorn pack): the
// chosen pet and the Water of Youth food are discarded, and the top of the next
// tier's deck joins the player's deck for free (its Buy effect fires).
func (g *Game) SacrificeChoose(playerID, cardID string) error {
	if g.Phase != PhaseShop || g.PendingSacrifice == nil || g.PendingSacrifice.PlayerID != playerID {
		return fmt.Errorf("%w: no choice waiting on you", ErrInvalidAction)
	}
	if !slices.Contains(g.PendingSacrifice.Options, cardID) {
		return fmt.Errorf("%w: choose one of your pets", ErrInvalidAction)
	}
	p := g.PlayerByID(playerID)
	ps := g.PendingSacrifice
	g.PendingSacrifice = nil
	// Discard the sacrificed pet and the Water of Youth food.
	var sacrificed Card
	if idx := p.cardIndex(cardID); idx >= 0 {
		sacrificed = p.Deck[idx]
		g.discardCard(sacrificed)
		p.Deck = slices.Delete(p.Deck, idx, idx+1)
	}
	if idx := p.cardIndex(ps.Source); idx >= 0 {
		g.discardCard(p.Deck[idx])
		p.Deck = slices.Delete(p.Deck, idx, idx+1)
	}
	top := g.drawFromTier(ps.Tier)
	if top.ID == "" {
		// Tier emptied out between the buy and the choice; nothing to grant.
		g.logf(p.Seat, "⏳", "%s's Water of Youth fizzles — the next tier is empty.", p.Name)
		g.advanceShopTurn()
		return nil
	}
	p.Deck = append(p.Deck, top)
	if hasBuyEffect(top) {
		g.addLog(LogEntry{Seat: p.Seat, Icon: "⏳", Kind: LogBuy, Source: top.ID, CardName: top.Name,
			Text: fmt.Sprintf("%s trades %s for %s %s from tier %d — its buy ability triggers.",
				p.Name, sacrificed.Name, article(top.Name), top.Name, ps.Tier)})
	} else {
		g.logf(p.Seat, "⏳", "%s trades %s for a fresh tier %d pet.", p.Name, sacrificed.Name, ps.Tier)
	}
	g.applyShopTrigger(p, top, TriggerBuy)
	g.advanceAfterBuy()
	return nil
}

// DebugGrant drops any card straight into a player's deck during the shop,
// free and off-turn — a testing aid gated behind the server's DEBUG flag, not
// a normal action. No buy effects fire.
func (g *Game) DebugGrant(playerID, name string) error {
	if g.Phase != PhaseShop {
		return ErrWrongPhase
	}
	p := g.PlayerByID(playerID)
	if p == nil {
		return errors.New("unknown player")
	}
	c, ok := g.cardByName(name)
	if !ok {
		return fmt.Errorf("%w: no card named %q", ErrInvalidAction, name)
	}
	p.Deck = append(p.Deck, c)
	return nil
}

// Pass is a player's final shop action: it ends their shopping for the rest
// of the round, forfeiting any remaining coins. It is only legal at or under
// the pet limit — a player holding too many pets must sell down first.
func (g *Game) Pass(playerID string) error {
	p, err := g.requireShopTurn(playerID)
	if err != nil {
		return err
	}
	if p.PetCount() > MaxPets {
		return fmt.Errorf("%w: sell down to %d pets before passing", ErrInvalidAction, MaxPets)
	}
	p.Coins = 0
	p.Ready = true
	g.logf(p.Seat, "✋", "%s passed — done shopping this round.", p.Name)
	g.advanceShopTurn()
	return nil
}

// advanceShopTurn hands the turn to the next player still shopping, or moves
// on to arranging once everyone has passed. Passing is the only way out of
// the shop, and it requires being at the pet limit, so no cleanup step is
// needed here.
func (g *Game) advanceShopTurn() {
	for i := 1; i <= len(g.Players); i++ {
		seat := (g.Turn + i) % len(g.Players)
		if !g.Players[seat].Ready {
			g.Turn = seat
			return
		}
	}
	g.beginArrange()
}

func (g *Game) allReady() bool {
	for _, p := range g.Players {
		if !p.Ready {
			return false
		}
	}
	return true
}

func (g *Game) beginArrange() {
	g.Phase = PhaseArrange
	for _, p := range g.Players {
		p.Ready = false
		// Battle Prep effects fire now, before players order their cards
		// (e.g. Giraffe hands out apples that can go into the deck order).
		for _, c := range slices.Clone(p.Deck) {
			g.applyShopTrigger(p, c, TriggerBattlePrep)
		}
	}
}

// SubmitOrder records the player's battle ordering (a permutation of their
// deck's card IDs, top of deck first). When everyone has submitted, the
// battle resolves.
func (g *Game) SubmitOrder(playerID string, orderedIDs []string) error {
	if g.Phase != PhaseArrange {
		return ErrWrongPhase
	}
	p := g.PlayerByID(playerID)
	if p == nil {
		return errors.New("unknown player")
	}
	if p.Ready {
		return fmt.Errorf("%w: order already submitted", ErrInvalidAction)
	}
	if len(orderedIDs) != len(p.Deck) || hasDuplicates(orderedIDs) {
		return fmt.Errorf("%w: order must include each of your cards exactly once", ErrInvalidAction)
	}
	ordered := make([]Card, 0, len(p.Deck))
	for _, id := range orderedIDs {
		idx := p.cardIndex(id)
		if idx < 0 {
			return fmt.Errorf("%w: card not in your deck", ErrInvalidAction)
		}
		ordered = append(ordered, p.Deck[idx])
	}
	p.Deck = ordered
	p.Ready = true
	if g.allReady() {
		g.startBattle()
	}
	return nil
}

// startBattle enters the battle phase and resolves it.
func (g *Game) startBattle() {
	for _, p := range g.Players {
		p.Ready = false
	}
	g.Phase = PhaseBattle
	g.resolveBattle()
}

// AcknowledgeBattle marks the player done reviewing the battle. When all
// players acknowledge, the next round starts (or the game ends).
func (g *Game) AcknowledgeBattle(playerID string) error {
	if g.Phase != PhaseBattle {
		return ErrWrongPhase
	}
	p := g.PlayerByID(playerID)
	if p == nil {
		return errors.New("unknown player")
	}
	p.Ready = true
	if !g.allReady() {
		return nil
	}
	// Temporary cards (apples, bees) expire once their battle has happened.
	for _, pl := range g.Players {
		pl.Deck = slices.DeleteFunc(pl.Deck, func(c Card) bool { return c.Temporary })
	}
	if g.Round >= MaxRounds {
		g.finish()
		return nil
	}
	g.Round++
	g.startShopRound()
	return nil
}

func (g *Game) finish() {
	g.Phase = PhaseGameOver
	best, bestSeat, tie := -1, -1, false
	for _, p := range g.Players {
		switch {
		case p.Trophies > best:
			best, bestSeat, tie = p.Trophies, p.Seat, false
		case p.Trophies == best:
			tie = true
		}
	}
	if tie {
		g.WinnerSeat = -1
	} else {
		g.WinnerSeat = bestSeat
	}
}

// discardCard records a real (non-temporary) card leaving a player's deck into
// the tier's discard pile (Unicorn pack: Chimera / Abomination source). Apples,
// bees, and ailments never enter the pile.
func (g *Game) discardCard(c Card) {
	if c.Temporary || c.Tier <= 0 {
		return
	}
	if g.Discards == nil {
		g.Discards = map[int][]Card{}
	}
	g.Discards[c.Tier] = append(g.Discards[c.Tier], c)
}

func hasDuplicates(ids []string) bool {
	seen := make(map[string]struct{}, len(ids))
	for _, id := range ids {
		if _, ok := seen[id]; ok {
			return true
		}
		seen[id] = struct{}{}
	}
	return false
}