aoc2023/8/src/main.rs

use std::collections::HashMap;
use std::fs::File;
use std::io::{BufRead, BufReader, Lines};

use lazy_regex::{lazy_regex, Lazy, Regex};

// BOILERPLATE
type InputIter = Lines<BufReader<File>>;

fn get_input() -> InputIter {
    let f = File::open("input").unwrap();
    let br = BufReader::new(f);
    br.lines()
}

fn main() {
    println!("Problem 1 solution: {}", problem1(get_input()));
    println!("Problem 2 solution: {}", problem2(get_input()));
}

// PARSE

const LINE_REGEX: Lazy<Regex> = lazy_regex!(r"([A-Z0-9]{3}) = \(([A-Z0-9]{3}), ([A-Z0-9]{3})\)");

enum Instruction {
    LEFT,
    RIGHT,
}

impl From<char> for Instruction {
    fn from(c: char) -> Self {
        match c {
            'L' => Self::LEFT,
            'R' => Self::RIGHT,
            _ => panic!("Invalid instruction"),
        }
    }
}

#[derive(Debug)]
struct Element {
    left: String,
    right: String,
}

type Nodes = HashMap<String, Element>;

#[derive(Debug)]
struct Map {
    instructions: Vec<char>,
    nodes: Nodes,
}

fn parse_node(nodes: &mut Nodes, node: &str) {
    let matches = LINE_REGEX.captures(node).unwrap();

    let node_id = matches.get(1).unwrap().as_str();
    let left = matches.get(2).unwrap().as_str();
    let right = matches.get(3).unwrap().as_str();

    nodes.insert(
        node_id.to_string(),
        Element {
            left: left.to_string(),
            right: right.to_string(),
        },
    );
}

impl<T: BufRead> From<Lines<T>> for Map {
    fn from(mut lines: Lines<T>) -> Self {
        let instructions = lines.next().unwrap().unwrap().chars().collect();
        let mut nodes = HashMap::new();
        lines.next();
        for line in lines {
            parse_node(&mut nodes, line.unwrap().as_str());
        }

        Self {
            instructions,
            nodes,
        }
    }
}

// PROBLEM 1 solution

fn problem1<T: BufRead>(input: Lines<T>) -> u64 {
    let map = Map::from(input);

    let mut count = 0;
    let mut cur_node = "AAA";
    for side in map.instructions.iter().cycle() {
        count += 1;
        cur_node = match side {
            'L' => &map.nodes[cur_node].left,
            'R' => &map.nodes[cur_node].right,
            _ => panic!("invalid instruction"),
        };
        if cur_node == "ZZZ" {
            return count;
        }
    }

    0
}

fn node_cycle_length(map: &Map, node: &str) -> u64 {
    let mut count = 0;
    let mut cur_node = node;
    for side in map.instructions.iter().cycle() {
        count += 1;
        cur_node = match side {
            'L' => &map.nodes[cur_node].left,
            'R' => &map.nodes[cur_node].right,
            _ => panic!("invalid instruction"),
        };
        if cur_node.ends_with('Z') {
            return count;
        }
    }
    0
}

// PROBLEM 2 solution
fn problem2<T: BufRead>(input: Lines<T>) -> u64 {
    let map = Map::from(input);

    let starting_nodes: Vec<_> = map.nodes.keys().filter(|s| s.ends_with('A')).collect();

    starting_nodes.iter().map(|n| node_cycle_length(&map, *n)).fold(1, |accum, elem| num::integer::lcm(accum, elem))
}

#[cfg(test)]
mod tests {
    use crate::*;
    use std::io::Cursor;

    const EXAMPLE: &str = &"RL

AAA = (BBB, CCC)
BBB = (DDD, EEE)
CCC = (ZZZ, GGG)
DDD = (DDD, DDD)
EEE = (EEE, EEE)
GGG = (GGG, GGG)
ZZZ = (ZZZ, ZZZ)";

    const EXAMPLE2: &str = &"LR

11A = (11B, XXX)
11B = (XXX, 11Z)
11Z = (11B, XXX)
22A = (22B, XXX)
22B = (22C, 22C)
22C = (22Z, 22Z)
22Z = (22B, 22B)
XXX = (XXX, XXX)";

    #[test]
    fn problem1_example() {
        let c = Cursor::new(EXAMPLE);
        assert_eq!(problem1(c.lines()), 2);
    }

    #[test]
    fn problem2_example() {
        let c = Cursor::new(EXAMPLE2);
        assert_eq!(problem2(c.lines()), 6);
    }
}