2024: Add more elegant concat implementation for Day 7

Author: dcoles

2024/day07/src/main.rs

@@ -16,26 +16,43 @@ fn main() {
     println!("Part 2: {}", part2(&input));
 }
 
+#[derive(Debug, Copy, Clone)]
+enum Op {
+    Add,
+    Mul,
+    Concat,
+}
+
+impl Op {
+    fn apply(&self, left: u64, right: u64) -> u64 {
+        match self {
+            Op::Add => left + right,
+            Op::Mul => left * right,
+            Op::Concat => left * right.next_multiple_of(10) + right,
+        }
+    }
+}
+
 fn part1(input: &Input) -> u64 {
     let mut sum = 0;
 
     for (result, values) in input.values.iter() {
+        let result = *result;
+
         let mut edge = vec![(1, values[0])];
         while let Some((n, x)) = edge.pop() {
             if n == values.len() {
-                if x == *result {
-                    sum += *result;
+                if x == result {
+                    // solved!
+                    sum += result;
                     break;
                 }
             } else {
-                let add = x + values[n];
-                if add <= *result {
-                    edge.push((n + 1, add));
-                }
-
-                let mul = x * values[n];
-                if mul <= *result {
-                    edge.push((n + 1, mul));
+                for op in [Op::Add, Op::Mul] {
+                    let val = op.apply(x, values[n]);
+                    if val <= result {
+                        edge.push((n + 1, val));
+                    }
                 }
             }
         }
@@ -48,27 +65,22 @@ fn part2(input: &Input) -> u64 {
     let mut sum = 0;
 
     for (result, values) in input.values.iter() {
+        let result = *result;
+
         let mut edge = vec![(1, values[0])];
         while let Some((n, x)) = edge.pop() {
             if n == values.len() {
-                if x == *result {
-                    sum += *result;
+                // solved!
+                if x == result {
+                    sum += result;
                     break;
                 }
             } else {
-                let add = x + values[n];
-                if add <= *result {
-                    edge.push((n + 1, add));
-                }
-
-                let mul = x * values[n];
-                if mul <= *result {
-                    edge.push((n + 1, mul));
-                }
-
-                let concat = format!("{}{}", x, values[n]).parse().unwrap();
-                if concat <= *result {
-                    edge.push((n + 1, concat));
+                for op in [Op::Add, Op::Mul, Op::Concat] {
+                    let val = op.apply(x, values[n]);
+                    if val <= result {
+                        edge.push((n + 1, val));
+                    }
                 }
             }
         }