// Package bowling is for counting bowling scores.
package bowling
import (
	"fmt"
)
const maxRolls = 21
type Game struct {
	currRoll int // current Roll
	rolls    []int
}
// NewGame creates a new bowling game.
func NewGame() *Game {
	return &Game{rolls: make([]int, maxRolls), currRoll: 0}
}
// Score calculates the score.
func (g *Game) Score() (int, error) {
	if g.currRoll < maxRolls-1 {
		return 0, fmt.Errorf("not enough rolls to score")
	}
	if g.currRoll == maxRolls-1 && (g.rolls[maxRolls-2] == 10 ||
		g.rolls[maxRolls-2]+g.rolls[maxRolls-3] == 10) {
		return 0, fmt.Errorf("last one is a strike, waiting for fill ball to score")
	}
	var ret int
	for i, v := range g.rolls {
		ret += v
		// strike handling
		if i%2 == 0 && v == 10 {
			// this is a strike
			if i < 18 {
				ret += g.rolls[i+2]
				if g.rolls[i+2] == 10 {
					// strike after strike
					ret += g.rolls[i+4]
				} else {
					ret += g.rolls[i+3]
				}
			} else if i == 18 {
				// ret += g.rolls[i+1] + g.rolls[i+2]
			}
		}
		// spare handling
		if i%2 != 0 && v+g.rolls[i-1] == 10 && g.rolls[i-1] != 10 {
			// spare counts twice
			if i == 19 {
				// unless this is the last frame
				continue
			}
			ret += g.rolls[i+1]
		}
	}
	return ret, nil
}
// Roll handles a roll in the game.
func (g *Game) Roll(pins int) error {
	if pins < 0 || pins > 10 {
		return fmt.Errorf("pins out of range")
	}
	if g.currRoll == maxRolls {
		return fmt.Errorf("to much rolls")
	}
	if g.currRoll == maxRolls-1 && g.rolls[maxRolls-3]+g.rolls[maxRolls-2] < 10 {
		return fmt.Errorf("21st roll is only valid when previous is strike/spare")
	}
	g.rolls[g.currRoll] = pins
	if g.currRoll%2 != 0 && g.currRoll != maxRolls-2 {
		if g.rolls[g.currRoll]+g.rolls[g.currRoll-1] > 10 {
			return fmt.Errorf("two rolls sum in frame > 10")
		}
	}
	// 20 is fill ball
	if g.currRoll == maxRolls-1 {
		if g.rolls[18]+g.rolls[19] != 10 && g.rolls[18]+g.rolls[19] != 20 {
			if g.rolls[g.currRoll]+g.rolls[g.currRoll-1] > 10 {
				return fmt.Errorf("fill ball problem")
			}
		}
	}
	// strike and !last frame closes the frame
	if pins == 10 && g.currRoll < 18 {
		g.currRoll++
	}
	g.currRoll++
	return nil
}