use std::collections::HashSet;

use aoc_runner_derive::{aoc as aoc_run, aoc_generator};

type Position = (i32, i32);

#[aoc_generator(day9)]
fn parse_input(input: &str) -> Vec<Position> {
    let mut out = Vec::new();
    let (mut x, mut y) = (0, 0);
    out.push((x, y));
    for line in input.lines() {
        let mut split = line.split_ascii_whitespace();
        let dir = split.next().unwrap();
        let num: i32 = split.next().unwrap().parse().unwrap();
        for _ in 0..num {
            match dir {
                "U" => y += 1,
                "D" => y -= 1,
                "L" => x -= 1,
                "R" => x += 1,
                _ => unreachable!(),
            }
            out.push((x, y));
        }
    }
    out
}

#[aoc_run(day9, part1)]
fn part1(input: &[Position]) -> usize {
    let mut tail = (0, 0);
    let mut set = HashSet::new();
    set.insert(tail);
    for mv in input {
        tail = new_tail(mv, &tail);
        set.insert(tail);
    }

    set.len()
}

#[aoc_run(day9, part2)]
fn part2(input: &[Position]) -> u32 {
    0
}

fn new_tail(head: &Position, tail: &Position) -> Position {
    let dy = head.1 - tail.1;
    let dx = head.0 - tail.0;
    match (dx, dy) {
        (0, 0) => *tail,
        (n, m) if (n.abs() == 1) && (m.abs() == 1) => *tail,
        (n, 0) | (0, n) if n.abs() == 1 => *tail,
        (_, 2) | (_, -2) if dx.abs() == 1 => (tail.0 + dx, tail.1 + (dy / 2)),
        (2, y) | (-2, y) if y.abs() == 1 => {
            let sign = dx / dx.abs();
            (tail.0 + sign * (dx.abs() - 1), tail.1 + dy)
        }
        (n, _) if n.abs() == 2 => (tail.0 + n / 2, tail.1),
        (_, n) if n.abs() == 2 => (tail.0, tail.1 + n / 2),
        _ => unreachable!("got {:?}", (dx, dy)),
    }
}