// Package selfplay drives greedy AI-vs-AI Scrabble games used to validate the move
// generators (the same position is offered to both) and to benchmark them.
package selfplay

import (
	"math/rand"
	"sort"
	"time"

	"gitea.iliadenisov.ru/developer/scrabble-solver/board"
	"gitea.iliadenisov.ru/developer/scrabble-solver/rack"
	"gitea.iliadenisov.ru/developer/scrabble-solver/rules"
	"gitea.iliadenisov.ru/developer/scrabble-solver/scrabble"
)

// blankTile marks a blank in the bag and in a player's hand.
const blankTile byte = 0xff

// Bag is a shuffled draw pile of tiles.
type Bag struct {
	tiles []byte
}

// NewBag fills a bag from the ruleset's tile counts and blanks and shuffles it with the
// given seed (so games are reproducible).
func NewBag(rs *rules.Ruleset, seed int64) *Bag {
	var tiles []byte
	for i, n := range rs.Counts {
		for range n {
			tiles = append(tiles, byte(i))
		}
	}
	for range rs.Blanks {
		tiles = append(tiles, blankTile)
	}
	rng := rand.New(rand.NewSource(seed))
	rng.Shuffle(len(tiles), func(i, j int) { tiles[i], tiles[j] = tiles[j], tiles[i] })
	return &Bag{tiles: tiles}
}

// Len returns the number of tiles left in the bag.
func (b *Bag) Len() int { return len(b.tiles) }

// Draw removes up to n tiles from the bag and returns them.
func (b *Bag) Draw(n int) []byte {
	if n > len(b.tiles) {
		n = len(b.tiles)
	}
	out := b.tiles[len(b.tiles)-n:]
	b.tiles = b.tiles[:len(b.tiles)-n]
	return out
}

// rackOf builds a generation rack from a hand of tiles.
func rackOf(tiles []byte, size int) rack.Rack {
	r := rack.New(size)
	for _, t := range tiles {
		if t == blankTile {
			r.AddBlank()
		} else {
			r.Add(t)
		}
	}
	return r
}

// removeUsed returns the hand with the tiles consumed by m removed.
func removeUsed(tiles []byte, m scrabble.Move) []byte {
	out := append([]byte(nil), tiles...)
	for _, p := range m.Tiles {
		want := p.Letter
		if p.Blank {
			want = blankTile
		}
		for i, t := range out {
			if t == want {
				out = append(out[:i], out[i+1:]...)
				break
			}
		}
	}
	return out
}

// greedy returns the highest-scoring move (ties broken by canonical key for
// reproducibility), or ok=false if there is no legal move.
func greedy(gen scrabble.Generator, b *board.Board, rk rack.Rack, mode scrabble.Mode) (scrabble.Move, int, bool) {
	moves := gen.GenerateMoves(b, rk, mode)
	if len(moves) == 0 {
		return scrabble.Move{}, 0, false
	}
	sort.Slice(moves, func(i, j int) bool {
		if moves[i].Score != moves[j].Score {
			return moves[i].Score > moves[j].Score
		}
		return moves[i].Key() < moves[j].Key()
	})
	return moves[0], len(moves), true
}

// Result summarizes a finished game.
type Result struct {
	Turns          int           // turns taken (plays plus passes)
	Plays          int           // scoring plays made
	Scores         [2]int        // final score per player
	MovesGenerated int           // total legal moves generated across all turns
	GenTime        time.Duration // time spent generating moves
}

// PlayGame plays one greedy AI-vs-AI game with generator gen and returns its result. If
// observe is non-nil it is called before each turn with a clone of the board and the
// player's rack, so a caller can compare generators on identical positions.
func PlayGame(rs *rules.Ruleset, gen scrabble.Generator, mode scrabble.Mode, seed int64,
	observe func(b *board.Board, rk rack.Rack)) Result {

	const maxTurns = 300
	bag := NewBag(rs, seed)
	b := board.New(rs.Rows, rs.Cols)
	hands := [2][]byte{bag.Draw(rs.RackSize), bag.Draw(rs.RackSize)}

	var res Result
	passes := 0
	for turn := range maxTurns {
		p := turn % 2
		rk := rackOf(hands[p], rs.Size())
		if observe != nil {
			observe(b.Clone(), rk.Clone())
		}
		res.Turns++

		t0 := time.Now()
		m, n, ok := greedy(gen, b, rk, mode)
		res.GenTime += time.Since(t0)
		res.MovesGenerated += n
		if !ok {
			if passes++; passes >= 4 {
				break
			}
			continue
		}
		passes = 0
		scrabble.Apply(b, m)
		res.Scores[p] += m.Score
		res.Plays++
		hands[p] = removeUsed(hands[p], m)
		if need := rs.RackSize - len(hands[p]); need > 0 {
			hands[p] = append(hands[p], bag.Draw(need)...)
		}
		if len(hands[p]) == 0 && bag.Len() == 0 {
			break
		}
	}
	return res
}