use anyhow::Result;
use std::cmp::Ordering;
use std::fs;

#[derive(Debug, Clone, Eq, PartialEq)]
enum Packet {
    Num(i32),
    List(Vec<Packet>),
}

fn parse(s: &str) -> Packet {
    if &s[0..1] == "[" {
        let mut stack: i32 = 0;
        Packet::List(
            s[1..s.len() - 1]
                .split(|c| {
                    if c == '[' {
                        stack += 1
                    } else if c == ']' {
                        stack -= 1
                    }
                    c == ',' && stack == 0
                })
                .filter_map(|s| (!s.is_empty()).then(|| parse(s)))
                .collect(),
        )
    } else {
        Packet::Num(s.parse().unwrap())
    }
}

impl Ord for Packet {
    fn cmp(&self, other: &Self) -> Ordering {
        match (self, other) {
            (Packet::Num(a), Packet::Num(b)) => a.cmp(b),
            (Packet::Num(_), _) => Packet::List(vec![self.clone()]).cmp(other),
            (_, Packet::Num(_)) => self.cmp(&Packet::List(vec![other.clone()])),
            (Packet::List(a), Packet::List(b)) => a.cmp(b),
        }
    }
}

impl PartialOrd for Packet {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

pub fn run() -> Result<()> {
    #[cfg(not(feature = "test_input"))]
    let file_contents = fs::read_to_string("day13.txt")?;

    #[cfg(feature = "test_input")]
    let file_contents = fs::read_to_string("tests/day13.txt")?;

    let mut packets: Vec<Packet> = file_contents
        .trim()
        .lines()
        .filter_map(|s| (!s.is_empty()).then(|| parse(s)))
        .collect();

    let mut part1 = 0;
    for (packet_index, packet_pair) in packets.chunks(2).enumerate() {
        let o = packet_pair[0].cmp(&packet_pair[1]);
        assert!(o != Ordering::Equal);
        if o == Ordering::Less {
            part1 += packet_index + 1;
        }
    }
    println!("part one: {}", part1);

    let div1 = parse("[[2]]");
    let div2 = parse("[[6]]");
    packets.push(div1.clone());
    packets.push(div2.clone());
    packets.sort();
    let i1 = packets.binary_search(&div1).unwrap() + 1;
    let i2 = packets.binary_search(&div2).unwrap() + 1;
    println!("part two: {} * {} = {}", i1, i2, i1 * i2);

    Ok(())
}