use nom::{
    bytes::complete::tag,
    character::complete::{alpha1, anychar, line_ending},
    combinator::map,
    multi::{many0, separated_list1},
    sequence::tuple,
    IResult,
};
use std::{collections::BTreeMap, fs};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let input = fs::read_to_string("inputs/day_14.txt")?;
    let mut polymer = parse_polymer_expansion(&input).unwrap().1;

    let small_expansion_counts = polymer.count_after_expansions(10);
    dbg!(small_expansion_counts.most_common() - small_expansion_counts.least_common());

    let big_expansion_counts = polymer.count_after_expansions(40);
    dbg!(big_expansion_counts.most_common() - big_expansion_counts.least_common());

    Ok(())
}

#[derive(Debug)]
struct PolymerExpansion {
    polymer: Vec<char>,
    rules: Rules,
    cache: BTreeMap<(char, char, usize), ElementCount>,
}

impl PolymerExpansion {
    fn new(polymer: Vec<char>, rules: Rules) -> PolymerExpansion {
        PolymerExpansion {
            polymer,
            rules,
            cache: BTreeMap::new(),
        }
    }

    fn count_after_expansions(&mut self, expansions: usize) -> ElementCount {
        let mut counts = ElementCount::default();
        for i in 0..self.polymer.len() - 1 {
            counts.add(self.polymer[i]);
            counts.append(self.count_between(self.polymer[i], self.polymer[i + 1], expansions));
        }
        counts.add(self.polymer[self.polymer.len() - 1]);
        counts
    }

    fn count_between(
        &mut self,
        left: char,
        right: char,
        remaining_expansions: usize,
    ) -> ElementCount {
        if remaining_expansions == 0 {
            ElementCount::default()
        } else if let Some(cached) = self.cache.get(&(left, right, remaining_expansions)) {
            cached.clone()
        } else {
            let mut counts = ElementCount::default();
            let middle = self.rules.get(&[left, right]).expect("No rule!");
            counts.add(middle);
            counts.append(self.count_between(left, middle, remaining_expansions - 1));
            counts.append(self.count_between(middle, right, remaining_expansions - 1));
            self.cache
                .insert((left, right, remaining_expansions), counts.clone());
            counts
        }
    }
}

#[derive(Debug, Default, Clone)]
struct ElementCount(BTreeMap<char, u64>);

impl ElementCount {
    fn add(&mut self, c: char) {
        *self.0.entry(c).or_insert(0) += 1;
    }

    fn append(&mut self, other: ElementCount) {
        for (key, val) in other.0 {
            *self.0.entry(key).or_insert(0) += val;
        }
    }

    fn most_common(&self) -> u64 {
        self.0.values().max().cloned().unwrap_or(0)
    }

    fn least_common(&self) -> u64 {
        self.0.values().min().cloned().unwrap_or(0)
    }
}

#[derive(Debug)]
struct Rules {
    rules: BTreeMap<[char; 2], char>,
}

impl Rules {
    fn get(&self, key: &[char; 2]) -> Option<char> {
        self.rules.get(key).cloned()
    }
}

fn parse_polymer_expansion(input: &str) -> IResult<&str, PolymerExpansion> {
    let (input, polymer) = map(alpha1, |s: &str| s.chars().collect())(input)?;
    let (input, _) = many0(line_ending)(input)?;
    let (input, rules) = parse_rules(input)?;
    Ok((input, PolymerExpansion::new(polymer, rules)))
}

fn parse_rules(input: &str) -> IResult<&str, Rules> {
    map(separated_list1(line_ending, parse_rule), |rules| Rules {
        rules: rules.into_iter().collect(),
    })(input)
}

fn parse_rule(input: &str) -> IResult<&str, ([char; 2], char)> {
    map(
        tuple((anychar, anychar, tag(" -> "), anychar)),
        |(p1, p2, _, r)| ([p1, p2], r),
    )(input)
}