advent-of-code/2020/src/bin/day19.rs

#![feature(test, str_split_once, bool_to_option)]
extern crate test;
use aoc2020::common::*;
use itertools::Itertools;

#[derive(Debug, PartialEq, Clone, Copy)]
enum Rule {
    Char(u8),
    And((usize, usize)),
    AndOr((usize, usize), (usize, usize)),
    Or((usize, usize)),
    Useless(usize), // just delegates to another rule and makes parsing annoying
    // This is why you don’t change the requirements once the implementation is done. REEEEEE
    Special8(usize, (usize, usize)),
    Special11((usize, usize), (usize, usize, usize)),
    // If Rule was a recursive struct, this could have been defined as And(first, And(second, third)), but alas
    Triple((usize, usize, usize)),
}

type Parsed<'a> = (RuleSet, &'a str);

#[derive(Debug, Clone)]
struct RuleSet(Vec<Rule>);

impl RuleSet {
    fn matches(&self, s: &str, rule: Rule) -> Option<usize> {
        let res = match rule {
            Rule::Char(c) => (s.as_bytes().first() == Some(&c)).then_some(1),
            Rule::And((r1, r2)) => self
                .matches(s, self.0[r1])
                .and_then(|i| self.matches(&s[i..], self.0[r2]).map(|j| i + j)),
            Rule::AndOr((r1, r2), (r3, r4)) => self
                .matches(s, Rule::And((r1, r2)))
                .or_else(|| self.matches(s, Rule::And((r3, r4)))),
            Rule::Or((r1, r2)) => self.matches(s, Rule::Useless(r1)).or_else(|| self.matches(s, Rule::Useless(r2))),
            Rule::Useless(r) => self.matches(s, self.0[r]),
            // part 2 shit below:
            Rule::Special8(first, second) => self.matches(s, Rule::Useless(first)).or_else(|| self.matches(s, Rule::And(second))),
            Rule::Special11(first, second) => self.matches(s, Rule::And(first)).or_else(|| self.matches(s, Rule::Triple(second))),
            Rule::Triple((r1, r2, r3)) => self
                .matches(s, self.0[r1])
                .and_then(|i| self.matches(&s[i..], self.0[r2]).map(|j| i + j))
                .and_then(|i| self.matches(&s[i..], self.0[r3]).map(|j| i + j)),
        };
        res
    }
}

fn read_input() -> String {
    read_file(19)
}

fn parse_input<'a>(raw: &'a str) -> Parsed<'a> {
    let (rules, inputs) = raw.split_once("\n\n").unwrap();
    let rules = RuleSet(
        rules
            .lines()
            .filter_map(|l| l.split_once(": "))
            // why isn’t this already sorted like it was in the example?
            .sorted_by_key(|t| t.0.parse::<i32>().unwrap())
            .map(|(_, r)| match r {
                or if or.contains('|') => {
                    let mut split = or.split(' ').filter(|s| s != &"|").map(|s| s.parse().unwrap());
                    split
                        .clone()
                        .next_tuple()
                        .map(|(a, b, c, d)| Rule::AndOr((a, b), (c, d)))
                        .or_else(|| split.next_tuple().map(|(a, b)| Rule::Or((a, b))))
                        .unwrap()
                }
                chr if chr.contains('"') => Rule::Char(chr.bytes().skip_while(|&b| b != b'"').skip(1).next().unwrap()),
                and if and.contains(' ') => Rule::And(and.split(' ').map(|s| s.parse().unwrap()).next_tuple().unwrap()),
                useless => Rule::Useless(useless.parse().unwrap()),
            })
            .collect(),
    );
    (rules, inputs)
}

fn part1((rules, input): &Parsed) -> usize {
    input.lines().filter(|l| rules.matches(l, rules.0[0]) == Some(l.len())).count()
}

fn part2(parsed: &Parsed) -> usize {
    let mut rules = parsed.0.clone();
    rules.0[8] = Rule::Special8(42, (42, 8));
    rules.0[11] = Rule::Special11((42, 31), (42, 11, 31));
    dbg!(parsed.1.lines().map(|l| (l, rules.matches(l, rules.0[0]))).collect_vec());
    parsed.1.lines().filter(|l| rules.matches(l, rules.0[0]) == Some(l.len())).count()
}

fn main() {
    let raw = read_input();
    let input = parse_input(&raw);
    println!("Part 1: {}", part1(&input));
    // 265 too low, 395 too high. apparently 379? no idea how that’s supposed to work
    println!("Part 2: {}", part2(&input));
}

#[cfg(test)]
mod tests {
    use super::*;
    use aoc2020::*;
    use paste::paste;
    use test::black_box;

    const TEST_INPUT: &str = r#"0: 8 11
1: "a"
2: 1 24 | 14 4
3: 5 14 | 16 1
4: 1 1
5: 1 14 | 15 1
6: 14 14 | 1 14
7: 14 5 | 1 21
8: 42
9: 14 27 | 1 26
10: 23 14 | 28 1
11: 42 31
12: 24 14 | 19 1
13: 14 3 | 1 12
14: "b"
15: 1 | 14
16: 15 1 | 14 14
17: 14 2 | 1 7
18: 15 15
19: 14 1 | 14 14
20: 14 14 | 1 15
21: 14 1 | 1 14
22: 14 14
23: 25 1 | 22 14
24: 14 1
25: 1 1 | 1 14
26: 14 22 | 1 20
27: 1 6 | 14 18
28: 16 1
29: 29
30: 30
31: 14 17 | 1 13
32: 32
33: 33
34: 34
35: 35
36: 36
37: 37
38: 38
39: 39
40: 40
41: 41
42: 9 14 | 10 1

abbbbbabbbaaaababbaabbbbabababbbabbbbbbabaaaa
bbabbbbaabaabba
babbbbaabbbbbabbbbbbaabaaabaaa
aaabbbbbbaaaabaababaabababbabaaabbababababaaa
bbbbbbbaaaabbbbaaabbabaaa
bbbababbbbaaaaaaaabbababaaababaabab
ababaaaaaabaaab
ababaaaaabbbaba
baabbaaaabbaaaababbaababb
abbbbabbbbaaaababbbbbbaaaababb
aaaaabbaabaaaaababaa
aaaabbaaaabbaaa
aaaabbaabbaaaaaaabbbabbbaaabbaabaaa
babaaabbbaaabaababbaabababaaab
aabbbbbaabbbaaaaaabbbbbababaaaaabbaaabba"#;
    
    test!(part1() == 3);
    test!(part2() == 12);
    bench!(part1() == 235);
    // bench!(part2() == 0);
    // bench_input!(0.len == 0);
}