year2024/
day07.rs

1use utils::prelude::*;
2
3/// Finding operator combinations to make valid equations.
4#[derive(Clone, Debug)]
5pub struct Day07 {
6    part1: u64,
7    part2: u64,
8}
9
10impl Day07 {
11    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
12        let parser = parser::u64()
13            .with_suffix(": ")
14            .then(parser::number_range(0u64..=999).repeat_arrayvec::<12, _>(b' ', 2))
15            .with_suffix(parser::eol());
16
17        let (mut part1, mut part2) = (0, 0);
18        for item in parser.parse_iterator(input) {
19            let (target, numbers) = item?;
20            if Self::possible(target, &numbers, false) {
21                part1 += target;
22                part2 += target;
23            } else if Self::possible(target, &numbers, true) {
24                part2 += target;
25            }
26        }
27
28        Ok(Self { part1, part2 })
29    }
30
31    #[must_use]
32    pub fn part1(&self) -> u64 {
33        self.part1
34    }
35
36    #[must_use]
37    pub fn part2(&self) -> u64 {
38        self.part2
39    }
40
41    #[inline]
42    fn possible(target: u64, numbers: &[u64], concat: bool) -> bool {
43        let (&next, numbers) = numbers.split_last().unwrap();
44        if numbers.is_empty() {
45            return target == next;
46        }
47
48        (target % next == 0 && Self::possible(target / next, numbers, concat))
49            || (concat && {
50                // All the numbers are 1-3 digits, which makes this faster than
51                //  let pow = 10u64.pow(next.ilog10() + 1);
52                let pow = if next < 10 {
53                    10
54                } else if next < 100 {
55                    100
56                } else {
57                    1000
58                };
59                target % pow == next && Self::possible(target / pow, numbers, concat)
60            })
61            || (target >= next && Self::possible(target - next, numbers, concat))
62    }
63}
64
65examples!(Day07 -> (u64, u64) [
66    {file: "day07_example0.txt", part1: 3749, part2: 11387},
67]);
year2024/day07.rs

year2024/
day07.rs
