Solved AOC 2024 day 7

2024/src/day07.rs

@@ -0,0 +1,119 @@
+const INPUT: &str = include_str!("../../2024/input/day07.txt");
+
+pub(crate) fn part1() -> usize {
+    solve_part1(INPUT)
+}
+
+pub(crate) fn part2() -> usize {
+    solve_part2(INPUT)
+}
+
+fn solve_part1(input: &str) -> usize {
+    parse(input)
+        .filter(Equation::has_solution_without_concatenation)
+        .map(Equation::left_side)
+        .sum()
+}
+
+fn solve_part2(input: &str) -> usize {
+    parse(input)
+        .filter(Equation::has_solution_with_concatenation)
+        .map(Equation::left_side)
+        .sum()
+}
+
+#[derive(Debug)]
+struct Equation {
+    left_side: usize,
+    right_side: Vec<usize>,
+}
+impl Equation {
+    fn left_side(self) -> usize {
+        self.left_side
+    }
+    fn has_solution_without_concatenation(&self) -> bool {
+        Equation::has_solution(self.left_side, &self.right_side, false)
+    }
+    fn has_solution_with_concatenation(&self) -> bool {
+        Equation::has_solution(self.left_side, &self.right_side, true)
+    }
+    fn has_solution(left_side: usize, right_side: &[usize], allow_concat: bool) -> bool {
+        let last_idx = right_side.len() - 1;
+        let last_num = right_side[last_idx];
+        if right_side.len() == 1 {
+            return left_side == last_num;
+        }
+        let front_nums = &right_side[..last_idx];
+
+        let last_operation_is_addition = || {
+            left_side > last_num
+                && Equation::has_solution(left_side - last_num, front_nums, allow_concat)
+        };
+        let last_op_is_multiplication = || {
+            left_side % last_num == 0
+                && Equation::has_solution(left_side / last_num, front_nums, allow_concat)
+        };
+        let last_op_is_concatenation = || {
+            let last_num_string = last_num.to_string();
+            let concat_divisor = 10usize.pow(last_num_string.len() as u32);
+            left_side.to_string().ends_with(&last_num_string)
+                && Equation::has_solution(left_side / concat_divisor, front_nums, allow_concat)
+        };
+
+        last_operation_is_addition()
+            || last_op_is_multiplication()
+            || allow_concat && last_op_is_concatenation()
+    }
+}
+
+fn parse(input: &str) -> impl Iterator<Item = Equation> + use<'_> {
+    input.trim().lines().map(|line| {
+        let (left, right) = line.split_once(": ").unwrap();
+        let left = left.parse().unwrap();
+        let right = right
+            .split_whitespace()
+            .map(|n| n.parse().unwrap())
+            .collect();
+        Equation {
+            left_side: left,
+            right_side: right,
+        }
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const EXAMPLE: &str = "\
+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
+";
+
+    #[test]
+    fn test_part1_example() {
+        assert_eq!(190 + 3267 + 292, solve_part1(EXAMPLE));
+    }
+
+    #[test]
+    fn test_part1() {
+        assert_eq!(42_283_209_483_350, solve_part1(INPUT));
+    }
+
+    #[test]
+    fn test_part2_example() {
+        assert_eq!(190 + 3267 + 156 + 7290 + 192 + 292, solve_part2(EXAMPLE));
+    }
+
+    #[test]
+    fn test_part2() {
+        assert_eq!(1_026_766_857_276_279, solve_part2(INPUT));
+    }
+}

2024/src/main.rs

@@ -6,6 +6,7 @@ mod day03;
 mod day04;
 mod day05;
 mod day06;
+mod day07;
 
 fn main() {
     print_result(1, day01::part1(), day01::part2());
@@ -14,6 +15,7 @@ fn main() {
     print_result(1, day04::part1(), day04::part2());
     print_result(1, day05::part1(), day05::part2());
     print_result(1, day06::part1(), day06::part2());
+    print_result(1, day07::part1(), day07::part2());
 }
 
 fn print_result(day: i32, part1: impl Display, part2: impl Display) {