use std::collections::VecDeque;

use md5;

fn solve_puzzle(passwd: &str) -> Vec<String> {
    let mut paths = Vec::new();

    // Breadth-first search of all possible paths
    let start = (String::new(), (0, 0));
    let mut queue = VecDeque::from([start]);
    loop {
        let pp = queue.pop_front();
        // All paths have finished or got stuck at locked doors
        if pp.is_none() {
            break;
        }
        // Did we successfully reach the bottom-right corner?
        let (path, pos) = pp.unwrap();
        if pos == (3, 3) {
            paths.push(path.clone());
            continue;
        }

        // Doors open based on MD5 hash of `passw' + `path'
        let data = format!("{}{}", passwd, path);
        let hash: Vec<char> = format!("{:x}", md5::compute(data)).chars().collect();

        // Which doors are open, i.e. what are the valid next moves?
        let mut options = Vec::new();
        let unlock = ['b', 'c', 'd', 'e', 'f'];
        if unlock.contains(&hash[0]) && pos.1 > 0 {
            options.push((format!("{}U", path), (pos.0, pos.1 - 1)));
        }
        if unlock.contains(&hash[1]) && pos.1 < 3 {
            options.push((format!("{}D", path), (pos.0, pos.1 + 1)));
        }
        if unlock.contains(&hash[2]) && pos.0 > 0 {
            options.push((format!("{}L", path), (pos.0 - 1, pos.1)));
        }
        if unlock.contains(&hash[3]) && pos.0 < 3 {
            options.push((format!("{}R", path), (pos.0 + 1, pos.1)));
        }

        // This is a breadth-first search, but because `path' is encoded
        // into the state, we don't need to check that nodes are unexplored
        for (npath, npos) in options {
            queue.push_back((npath, npos));
        }
    }

    paths
}

fn main() {
    // My personal puzzle input
    let input = "edjrjqaa";

    let paths = solve_puzzle(input);

    // Part 1 gives "DUDRDLRRRD" for me
    println!("Part 1 solution: {}", paths.first().unwrap());

    // Part 2 gives 502 for me
    println!("Part 2 solution: {}", paths.last().unwrap().len());
}

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

    #[test]
    fn part1_example1() {
        let passwd = "ihgpwlah";
        assert_eq!(solve_puzzle(passwd).first().unwrap(), "DDRRRD");
    }

    #[test]
    fn part2_example1() {
        let passwd = "ihgpwlah";
        assert_eq!(solve_puzzle(passwd).last().unwrap().len(), 370);
    }

    #[test]
    fn part1_example2() {
        let passwd = "kglvqrro";
        assert_eq!(solve_puzzle(passwd).first().unwrap(), "DDUDRLRRUDRD");
    }

    #[test]
    fn part2_example2() {
        let passwd = "kglvqrro";
        assert_eq!(solve_puzzle(passwd).last().unwrap().len(), 492);
    }

    #[test]
    fn part1_example3() {
        let passwd = "ulqzkmiv";
        assert_eq!(
            solve_puzzle(passwd).first().unwrap(),
            "DRURDRUDDLLDLUURRDULRLDUUDDDRR"
        );
    }

    #[test]
    fn part2_example3() {
        let passwd = "ulqzkmiv";
        assert_eq!(solve_puzzle(passwd).last().unwrap().len(), 830);
    }
}