Bowling

lib.rs

#[derive(Debug)]
enum State {
    STRIKE,
    STRIKE_PLUS_1,
    STRIKE_PLUS_2,
    SPARE,
    SECOND,
    FIRST
}

pub struct BowlingGame {
    state: State,
    score: usize,
    frames: usize,
    prev: usize,
}

impl BowlingGame {
    pub fn new() -> BowlingGame {
        BowlingGame { state: State::FIRST, score: 0, frames: 0, prev: 0 }
    }

    pub fn roll(&mut self, pins: usize) -> Result<(), ()> {
        if pins > 10 || self.frames == 10 {
            Err(())
        } else {
            match self.state {
                State::FIRST => {
                    if pins == 10 {
                        self.state = State::STRIKE;
                    } else {
                        self.state = State::SECOND;
                    }
                }
                State::SECOND => {
                    if pins == 10 {
                        self.state = State::STRIKE;
                    } else if self.prev + pins == 10 {
                        self.state = State::SPARE;
                    } else if self.prev + pins <= 10 {
                        self.score += self.prev + pins;
                        self.frames += 1;
                        self.state = State::FIRST;
                    } else {
                        return Err(())
                    }
                }
                State::SPARE => {
                    self.score += 10 + pins;
                    self.frames += 1;
                    if pins == 10 {
                        self.state = State::STRIKE;
                    } else {
                        self.state = State::SECOND;
                    }
                }
                State::STRIKE => {
                    self.state = State::STRIKE_PLUS_2;
                }
                State::STRIKE_PLUS_1 => {
                    self.score += 10 + self.prev + pins;
                    self.frames += 1;
                    self.state = State::STRIKE_PLUS_2;
                }
                State::STRIKE_PLUS_2 => {
                    if self.prev < 10 && self.prev + pins > 10 {
                        return Err(());
                    }

                    self.score += 10 + self.prev + pins;
                    self.frames += 1;

                    if self.prev == 10 {
                        self.state = State::STRIKE_PLUS_1;
                    } else if pins == 10 {
                        self.state = State::STRIKE;
                    } else if self.prev + pins == 10 {
                        self.state = State::SPARE;
                    } else {
                        if self.frames < 10 { // not the last roll
                            self.frames += 1;
                            self.score += self.prev + pins;
                        }
                        self.state = State::FIRST;
                    }
                }
            }

            self.prev = pins;
            Ok(())
        }
    }

    pub fn score(&self) -> Result<usize, String> {
        if self.frames == 10 {
            Ok(self.score)
        } else {
            Err(format!("incomplete game, {} frames, score {}", self.frames, self.score))
        }
    }
}