// Package game implements a controller for Buckshot Roulette matches.
//
// The terminology of this package is as follows:
//   - A match is a pairing of two players, the dealer and challenger.
//     It ends when the challenger wins three rounds or the dealer wins once.
//   - A round is an instance of gameplay. Both players start each round with
//     the same HP, and it continues until either player reaches zero.
//   - A game is a set of 2-8 shells and 1-4 items delivered to each player.
//     The challenger always moves first at the start of each game.
package game

import (
	"errors"

	"git.sunturtle.xyz/studio/shotgun/player"
	"git.sunturtle.xyz/studio/shotgun/serve"
)

type Match struct {
	// rng is the PRNG state used for this match.
	rng RNG
	// players are the players in the match. The first element is the dealer
	// and the second is the challenger.
	players [2]Player
	// shells is the list of remaining shells in the current game. It always
	// uses shellArray as its backing array.
	shells []bool
	// round is the round number. Once it reaches 3, the match is over.
	round int8
	// turn is the turn number. When even, it is the dealer's turn.
	turn int8
	// hp is the starting and max HP of both players in the current round.
	hp int8
	// damage is the amount of damage a live shell will deal this turn.
	damage int8
	// reveal indicates whether the current player has revealed the shell that
	// will fire next.
	reveal bool
	// prev is a pointer to the element of shellArray which was fired last this
	// game, or nil if none has been.
	prev *bool

	// shellArray is the backing storage for shells and prev.
	shellArray [8]bool
}

// New creates a new match started at round 1.
func New(dealer, challenger player.ID) *Match {
	g := &Match{
		rng: NewRNG(),
		players: [2]Player{
			{id: dealer},
			{id: challenger},
		},
	}
	g.NextRound()
	return g
}

// NextRound starts the next round of a match.
func (g *Match) NextRound() {
	g.hp = int8(g.rng.Intn(3) + 2)
	g.players[0].StartRound(g.hp)
	g.players[1].StartRound(g.hp)
	g.round++
	g.NextGame()
}

// NextGame starts the next game of a round.
func (g *Match) NextGame() {
	items := g.rng.Intn(4) + 1
	g.players[0].StartGroup(&g.rng, items)
	g.players[1].StartGroup(&g.rng, items)
	shells := g.rng.Intn(6) + 2
	for i := 0; i < shells/2; i++ {
		g.shellArray[i] = true
	}
	for i := shells / 2; i < shells; i++ {
		g.shellArray[i] = false
	}
	g.shells = g.shellArray[:shells]
	ShuffleSlice(&g.rng, g.shells)
	g.turn = 0
	g.prev = nil
	g.NextTurn()
}

// NextTurn advances the turn but not the match or round state.
func (g *Match) NextTurn() {
	g.turn++
	g.damage = 1
	g.reveal = false
	cur := g.CurrentPlayer()
	skip := cur.cuffs == CuffedSkip
	cur.cuffs = cur.cuffs.NextState()
	if skip {
		g.NextTurn()
	}
}

// CurrentPlayer gets the current player.
func (g *Match) CurrentPlayer() *Player {
	return &g.players[g.turn&1]
}

// Opponent returns the player who is not the current player.
func (g *Match) Opponent() *Player {
	return &g.players[g.turn&1^1]
}

// Apply uses an item by index for the current player.
// If this causes the current game to end (a beer discharges the last shell),
// returns ErrGameEnded but does not start the next game.
// Returns ErrWrongTurn if id does not correspond to the current player.
func (g *Match) Apply(id player.ID, item int) error {
	cur := g.CurrentPlayer()
	if cur.id != id {
		return ErrWrongTurn
	}
	if item < 0 || item >= len(cur.items) {
		return errors.New("item index out of bounds")
	}
	if cur.items[item].Apply(g) {
		cur.items[item] = ItemNone
	}
	if g.Empty() {
		return ErrGameEnded
	}
	return nil
}

// popShell removes a shell from the shotgun.
// This may cause the shotgun to be empty, but does not start the next game
// if so.
func (g *Match) popShell() bool {
	g.prev = &g.shells[0]
	g.shells = g.shells[1:]
	return *g.prev
}

// Peek returns the current turn's shell if it is revealed for the player with
// the given ID, or nil otherwise.
func (g *Match) Peek(id player.ID) *bool {
	if len(g.shells) == 0 || id != g.CurrentPlayer().id || !g.reveal {
		return nil
	}
	return &g.shells[0]
}

// Empty returns whether the shotgun is empty.
func (g *Match) Empty() bool {
	return len(g.shells) == 0
}

// RoundWinner returns the player who won the current round, or nil if the
// round is not over.
func (g *Match) RoundWinner() *Player {
	if g.players[0].hp <= 0 {
		return &g.players[1]
	}
	if g.players[1].hp <= 0 {
		return &g.players[0]
	}
	return nil
}

// MatchWinner returns the player who has won the match, or nil if the match
// is not over.
func (g *Match) MatchWinner() *Player {
	if g.round < 3 {
		return nil
	}
	return g.RoundWinner()
}

// Shoot fires the shotgun, at the opponent if self is false and at the current
// player if self is true. Advances the turn if appropriate.
// Returns ErrRoundEnded if this causes the round to end.
// Returns ErrGameEnded if this causes the game but not the round to end.
// Returns ErrWrongTurn if id does not correspond to the current player.
func (g *Match) Shoot(id player.ID, self bool) error {
	cur := g.CurrentPlayer()
	if cur.id != id {
		return ErrWrongTurn
	}
	target := g.Opponent()
	if self {
		target = g.CurrentPlayer()
	}
	live := g.popShell()
	if live {
		target.hp -= g.damage
		if target.hp <= 0 {
			target.hp = 0 // in case it goes negative
			// If the target is the challenger, the match is over as well.
			if target == &g.players[1] {
				g.round = 3
			}
			return ErrRoundEnded
		}
	}
	if g.Empty() {
		return ErrGameEnded
	}
	if !self || live {
		g.NextTurn()
	}
	return nil
}

// Concede sets the player with the given ID to zero health and ends the match.
// The returned error is ErrRoundEnded if id corresponds to either player and
// ErrWrongTurn otherwise.
func (g *Match) Concede(id player.ID) error {
	switch id {
	case g.players[0].id:
		g.players[0].hp = 0
	case g.players[1].id:
		g.players[1].hp = 0
	default:
		return ErrWrongTurn
	}
	g.round = 3
	return ErrRoundEnded
}

// DTO returns the current match state as viewed by the given player.
func (g *Match) DTO(id player.ID) serve.Game {
	var live, blank int
	if g.turn == 1 {
		live = len(g.shells) / 2
		blank = len(g.shells) - live
	}
	return serve.Game{
		Players: [2]serve.Player{
			g.players[0].DTO(),
			g.players[1].DTO(),
		},
		Round:    g.round,
		Dealer:   g.CurrentPlayer() == &g.players[0],
		Damage:   g.damage,
		Shell:    g.Peek(id),
		Previous: g.prev,
		Live:     live,
		Blank:    blank,
	}
}

var (
	ErrWrongTurn  = errors.New("not your turn")
	ErrGameEnded  = errors.New("the shotgun is empty")
	ErrRoundEnded = errors.New("someone h*ckin died")
)