/// A Robot *aka droid* is represented here.
/// Each robot must have a unique id.
pub struct Robot {
    id: i32,
    o: Orientation,
    p: Position,
    i: Vec<char>,
}

impl Robot {
    /// Create new `Robot` with given id, `Orientation`, `Position` and instructions.
    fn new(id: i32, o: Orientation, p: Position, i: Vec<char>) -> Robot {
        Robot { id, o, p, i }
    }
    /// Apply given instruction to a `Robot`.
    fn execute_instruction(&mut self) -> Result<(), &'static str> {
        match self.i.pop() {
            Some(instruction) => match instruction {
                'L' => Ok(match self.o {
                    Orientation::N => self.o = Orientation::W,
                    Orientation::E => self.o = Orientation::N,
                    Orientation::S => self.o = Orientation::E,
                    Orientation::W => self.o = Orientation::S,
                }),
                'R' => Ok(match self.o {
                    Orientation::N => self.o = Orientation::E,
                    Orientation::E => self.o = Orientation::S,
                    Orientation::S => self.o = Orientation::W,
                    Orientation::W => self.o = Orientation::N,
                }),
                'F' => Ok(match self.o {
                    Orientation::N => self.p.y += 1,
                    Orientation::E => self.p.x += 1,
                    Orientation::S => self.p.y -= 1,
                    Orientation::W => self.p.x -= 1,
                }),
                _ => Err("Invalid instruction."),
            },
            None => Ok(()),
        }
    }
}

/// Enum to store all possible orientations.
enum Orientation {
    N,
    E,
    S,
    W,
}

/// Struct to store robot position.
struct Position {
    x: i32,
    y: i32,
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_new_robot() {
        let r: Robot = Robot::new(
            0,
            Orientation::N,
            Position { x: 1, y: 2 },
            vec!['L', 'L', 'F', 'R'],
        );
        assert_eq!(r.id, 0);
        assert!(matches!(r.o, Orientation::N));
        assert_eq!(r.p.x, 1);
        assert_eq!(r.p.y, 2);
        assert_eq!(r.i, vec!['L', 'L', 'F', 'R']);
    }

    #[test]
    fn test_execute_instruction() {
        let mut r: Robot = Robot::new(
            0,
            Orientation::N,
            Position { x: 1, y: 2 },
            vec!['R', 'L', 'F', 'F'],
        );
        assert!(r.execute_instruction().is_ok());
        assert_eq!(r.p.x, 1);
        assert_eq!(r.p.y, 3);
    }
}