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"`
	// 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"`
}

// 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"`
	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
	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"`
	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:"-"`
}

// rollRockDie rolls one rock die: 0, 1, or 2 with equal probability.
func (g *Game) rollRockDie() int {
	if g.RollDie != nil {
		return g.RollDie()
	}
	return randInt(3)
}

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(),
		Phase:      PhaseLobby,
		WinnerSeat: -1,
	}
	g.buildShopDecks()
	for i := range g.ShopDecks {
		shuffle(g.ShopDecks[i])
	}
	return g
}

// AddPlayer seats a new player during the lobby phase and returns them (with
// their secret token). The game starts automatically once full.
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)
	if len(g.Players) == MaxPlayers {
		g.start()
	}
	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
}

// 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
	for _, p := range g.Players {
		p.Coins = CoinsPerRound
		p.Ready = false
		p.TripledThisRound = false
	}
	g.ShopRow = make([]Card, ShopRowSize)
	for i := range g.ShopRow {
		g.ShopRow[i] = g.drawFromTier(g.Round)
	}
	// 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)
	}
	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.
func (g *Game) Buy(playerID string, rowIdx int) error {
	p, err := g.requireShopTurn(playerID)
	if err != nil {
		return err
	}
	if p.Coins <= 0 {
		return ErrNoCoins
	}
	if rowIdx < 0 || rowIdx >= len(g.ShopRow) || g.ShopRow[rowIdx].ID == "" {
		return fmt.Errorf("%w: no card in that shop slot", ErrInvalidAction)
	}
	p.Coins--
	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 bought %s %s.", p.Name, article(bought.Name), bought.Name)})
	g.ShopRow[rowIdx] = g.drawFromTier(g.Round)
	g.applyShopTrigger(p, bought, TriggerBuy)
	g.advanceShopTurn()
	return nil
}

// 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()
			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 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)})
			}
		}
	}
}

// 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.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
}

// 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])
		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 traded in %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 trade 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 trade pick hidden.", p.Name)})
	}
	// Pets obtained via the Triple action trigger their Buy effects.
	g.applyShopTrigger(p, chosen, TriggerBuy)
	g.advanceShopTurn()
	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.resolveBattle()
	}
	return nil
}

// 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
	}
}

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
}