AOC 2020 Day 12

day12.rs

pub mod day12 {

    // (+E/-W, +N/-S)
    #[derive(Debug, Copy, Clone)]
    pub struct Point(f32, f32);

    #[derive(Debug)]
    pub struct State {
        current: Point,
        heading: Point
    }

    impl State {
        fn rotate_heading(&mut self, angle_change: f32) {
            let rads = angle_change.to_radians();
            let last_heading = self.heading;
            self.heading.0 = (rads.sin() * last_heading.1 + rads.cos() * last_heading.0).round();
            self.heading.1 = (rads.cos() * last_heading.1 - rads.sin() * last_heading.0).round();
        }
    }

    pub fn run(input: &str, initial_heading: Point, part1: bool) -> f32 {
        let mut state = State { current: Point(0.0, 0.0), heading: initial_heading };
        for line in input.lines() {
            let cmd = line.chars().next().unwrap();
            let arg = line[1..].parse::<f32>().unwrap();
            match cmd {
                'E' => if part1 { state.current.0 += arg } else { state.heading.0 += arg },
                'W' => if part1 { state.current.0 -= arg } else { state.heading.0 -= arg },
                'N' => if part1 { state.current.1 += arg } else { state.heading.1 += arg },
                'S' => if part1 { state.current.1 -= arg } else { state.heading.1 -= arg },
                'F' => {
                    state.current.0 += arg * state.heading.0;
                    state.current.1 += arg * state.heading.1;
                },
                'L' => state.rotate_heading(-arg as f32),
                'R' => state.rotate_heading(arg as f32),
                _ => panic!("Unexpected cmd")
            }
            // println!("{} {:?}", line, state);
        }
        state.current.0.abs() + state.current.1.abs()
     }

     pub fn part1(input: &str) -> f32 {
         run(input, Point(1.0, 0.0), true)
     }

     pub fn part2(input: &str) -> f32 {
        run(input, Point(10.0, 1.0), false)
     }
}

fn main() {
    let input = std::fs::read_to_string("input.txt").expect("Reading input.txt");
    println!("Day 12: {} {}", day12::part1(&input), day12::part2(&input));
}