2024 day 7

Author: ictrobot

crates/year2024/examples/day07_example0.txt

@@ -0,0 +1,9 @@
+190: 10 19
+3267: 81 40 27
+83: 17 5
+156: 15 6
+7290: 6 8 6 15
+161011: 16 10 13
+192: 17 8 14
+21037: 9 7 18 13
+292: 11 6 16 20

crates/year2024/src/day07.rs

@@ -0,0 +1,67 @@
+use utils::prelude::*;
+
+/// Finding operator combinations to make valid equations.
+#[derive(Clone, Debug)]
+pub struct Day07 {
+    part1: u64,
+    part2: u64,
+}
+
+impl Day07 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let parser = parser::u64()
+            .with_suffix(": ")
+            .then(parser::number_range(0u64..=999).repeat_arrayvec::<12, _>(b' ', 2))
+            .with_suffix(parser::eol());
+
+        let (mut part1, mut part2) = (0, 0);
+        for item in parser.parse_iterator(input) {
+            let (target, numbers) = item?;
+            if Self::possible(target, &numbers, false) {
+                part1 += target;
+                part2 += target;
+            } else if Self::possible(target, &numbers, true) {
+                part2 += target;
+            }
+        }
+
+        Ok(Self { part1, part2 })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u64 {
+        self.part1
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> u64 {
+        self.part2
+    }
+
+    #[inline]
+    fn possible(target: u64, numbers: &[u64], concat: bool) -> bool {
+        let (&next, numbers) = numbers.split_last().unwrap();
+        if numbers.is_empty() {
+            return target == next;
+        }
+
+        (target % next == 0 && Self::possible(target / next, numbers, concat))
+            || (concat && {
+                // All the numbers are 1-3 digits, which makes this faster than
+                //  let pow = 10u64.pow(next.ilog10() + 1);
+                let pow = if next < 10 {
+                    10
+                } else if next < 100 {
+                    100
+                } else {
+                    1000
+                };
+                target % pow == next && Self::possible(target / pow, numbers, concat)
+            })
+            || (target >= next && Self::possible(target - next, numbers, concat))
+    }
+}
+
+examples!(Day07 -> (u64, u64) [
+    {file: "day07_example0.txt", part1: 3749, part2: 11387},
+]);

crates/year2024/src/lib.rs

@@ -8,4 +8,5 @@ utils::year!(2024 => year2024, ${
     4 => day04::Day04,
     5 => day05::Day05,
     6 => day06::Day06,
+    7 => day07::Day07,
 });