advent-of-code 2025.5.0

use crate::input::Input;

#[derive(Clone)]
enum Rule {
    /// A literal character represented by its ASCII value.
    Character(u8),

    /// A list of options, where each option is a sequence of rule id:s.
    ///
    /// Example: The rule
    ///     "119 92 | 87 13"
    /// is represented as:
    ///     [[119, 92], [87, 13]]
    Sequences(Vec<Vec<RuleId>>),
}

impl Rule {
    fn parse(rule_str: &str) -> Result<Self, ()> {
        if rule_str.is_empty() {
            return Err(());
        }

        let bytes = rule_str.as_bytes();
        Ok(if bytes[0] == b'"' {
            if bytes.len() != 3 || bytes[2] != b'"' {
                return Err(());
            }
            Self::Character(bytes[1])
        } else {
            let sequences = rule_str
                .split(" | ")
                .map(|s| s.split(' ').map(str::parse).collect())
                .collect::<Result<_, _>>()
                .map_err(|_| ())?;
            Self::Sequences(sequences)
        })
    }
}

type RuleId = u8;

struct Rules {
    /// The rules indexed by rule id:s.
    rules: Vec<Rule>,
}

impl Rules {
    fn parse(rules_str: &str) -> Result<Self, ()> {
        let mut rules = Self {
            rules: vec![Rule::Character(0); 255],
        };
        for rule_line in rules_str.lines() {
            rules.add_line(rule_line)?;
        }
        Ok(rules)
    }

    fn add_line(&mut self, rule_line: &str) -> Result<(), ()> {
        let (rule_idx_str, pattern_str) = rule_line.rsplit_once(": ").ok_or(())?;

        let rule_idx = rule_idx_str.parse::<RuleId>().map_err(|_| ())?;
        let pattern = Rule::parse(pattern_str)?;

        self.rules[rule_idx as usize] = pattern;
        Ok(())
    }

    fn matches(&self, line: &str) -> bool {
        struct PartialMatch<'a> {
            remaining_input: &'a [u8],
            remaining_sequence: Vec<RuleId>,
        }

        if line.is_empty() {
            return false;
        }

        let mut stack = vec![PartialMatch {
            remaining_input: line.as_bytes(),
            remaining_sequence: vec![0],
        }];

        while let Some(partial_match) = stack.pop() {
            match &self.rules[partial_match.remaining_sequence[0] as usize] {
                &Rule::Character(value) => {
                    if partial_match.remaining_input[0] == value {
                        let end_of_input = partial_match.remaining_input.len() == 1;
                        let end_of_rule_sequence = partial_match.remaining_sequence.len() == 1;

                        match (end_of_input, end_of_rule_sequence) {
                            (true, true) => {
                                return true;
                            }
                            (false, false) => {
                                stack.push(PartialMatch {
                                    remaining_input: &partial_match.remaining_input[1..],
                                    remaining_sequence: partial_match.remaining_sequence[1..]
                                        .to_vec(),
                                });
                            }
                            _ => {}
                        }
                    }
                }
                Rule::Sequences(sequences) => {
                    for chosen_sequence in sequences.iter() {
                        let mut remaining_sequence = chosen_sequence.clone();
                        remaining_sequence.extend(&partial_match.remaining_sequence[1..]);

                        stack.push(PartialMatch {
                            remaining_input: partial_match.remaining_input,
                            remaining_sequence,
                        });
                    }
                }
            }
        }
        false
    }
}

pub fn solve(input: &Input) -> Result<u64, String> {
    let on_error = || "Invalid input".to_string();
    let map_error = |_| on_error();

    let (rules_str, messages_str) = input.text.split_once("\n\n").ok_or_else(on_error)?;

    let mut rules = Rules::parse(rules_str).map_err(map_error)?;

    if input.is_part_two() {
        rules.add_line("8: 42 | 42 8").map_err(map_error)?;
        rules.add_line("11: 42 31 | 42 11 31").map_err(map_error)?;
    }

    Ok(messages_str
        .lines()
        .filter(|line| rules.matches(line))
        .count() as u64)
}

#[test]
pub fn tests() {
    let example_part_one = "0: 4 1 5
1: 2 3 | 3 2
2: 4 4 | 5 5
3: 4 5 | 5 4
4: \"a\"
5: \"b\"

ababbb
bababa
abbbab
aaabbb
aaaabbb";
    test_part_one!(example_part_one => 2);
    let example_part_two = "42: 9 14 | 10 1
9: 14 27 | 1 26
10: 23 14 | 28 1
1: \"a\"
11: 42 31
5: 1 14 | 15 1
19: 14 1 | 14 14
12: 24 14 | 19 1
16: 15 1 | 14 14
31: 14 17 | 1 13
6: 14 14 | 1 14
2: 1 24 | 14 4
0: 8 11
13: 14 3 | 1 12
15: 1 | 14
17: 14 2 | 1 7
23: 25 1 | 22 14
28: 16 1
4: 1 1
20: 14 14 | 1 15
3: 5 14 | 16 1
27: 1 6 | 14 18
14: \"b\"
21: 14 1 | 1 14
25: 1 1 | 1 14
22: 14 14
8: 42
26: 14 22 | 1 20
18: 15 15
7: 14 5 | 1 21
24: 14 1

abbbbbabbbaaaababbaabbbbabababbbabbbbbbabaaaa
bbabbbbaabaabba
babbbbaabbbbbabbbbbbaabaaabaaa
aaabbbbbbaaaabaababaabababbabaaabbababababaaa
bbbbbbbaaaabbbbaaabbabaaa
bbbababbbbaaaaaaaabbababaaababaabab
ababaaaaaabaaab
ababaaaaabbbaba
baabbaaaabbaaaababbaababb
abbbbabbbbaaaababbbbbbaaaababb
aaaaabbaabaaaaababaa
aaaabbaaaabbaaa
aaaabbaabbaaaaaaabbbabbbaaabbaabaaa
babaaabbbaaabaababbaabababaaab
aabbbbbaabbbaaaaaabbbbbababaaaaabbaaabba";
    test_part_two!(example_part_two => 12);

    let real_input = include_str!("day19_input.txt");
    test_part_one!(real_input => 126);
    test_part_two!(real_input => 282);
}