Faster approach skipping empty rows and columns

Author: maneatingape

README.md

@@ -83,7 +83,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | 4 | [Printing Department](https://adventofcode.com/2025/day/4) | [Source](src/year2025/day04.rs) | 177 |
 | 5 | [Cafeteria](https://adventofcode.com/2025/day/5) | [Source](src/year2025/day05.rs) | 20 |
 | 6 | [Trash Compactor](https://adventofcode.com/2025/day/6) | [Source](src/year2025/day06.rs) | 20 |
-| 7 | [Laboratories](https://adventofcode.com/2025/day/7) | [Source](src/year2025/day07.rs) | 15 |
+| 7 | [Laboratories](https://adventofcode.com/2025/day/7) | [Source](src/year2025/day07.rs) | 5 |
 
 ## 2024
 

src/year2025/day07.rs

@@ -4,37 +4,26 @@ type Input = (u64, u64);
 pub fn parse(input: &str) -> Input {
     let lines: Vec<_> = input.lines().map(str::as_bytes).collect();
     let width = lines[0].len();
-    let start = lines[0].iter().position(|&b| b == b'S').unwrap();
+    let center = width / 2;
 
     let mut splits = 0;
-    let mut current = vec![0; width];
-    let mut next = vec![0; width];
-
-    current[start] = 1;
-
-    for row in lines {
-        for (i, &count) in current.iter().enumerate() {
-            if count > 0 {
-                if row[i] == b'^' {
-                    splits += 1;
-
-                    if i > 0 {
-                        next[i - 1] += count;
-                    }
-                    if i < width - 1 {
-                        next[i + 1] += count;
-                    }
-                } else {
-                    next[i] += count;
-                }
+    let mut timelines = vec![0; width];
+    timelines[center] = 1;
+
+    for (y, row) in lines.iter().skip(2).step_by(2).enumerate() {
+        for x in ((center - y)..(center + y + 1)).step_by(2) {
+            let count = timelines[x];
+
+            if count > 0 && row[x] == b'^' {
+                splits += 1;
+                timelines[x] = 0;
+                timelines[x - 1] += count;
+                timelines[x + 1] += count;
             }
         }
-
-        (current, next) = (next, current);
-        next.fill(0);
     }
 
-    (splits, current.iter().sum())
+    (splits, timelines.iter().sum())
 }
 
 pub fn part1(input: &Input) -> u64 {