Year 2025 Day 3

Author: maneatingape

README.md

@@ -79,6 +79,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | --- | --- | --- | --: |
 | 1 | [Secret Entrance](https://adventofcode.com/2025/day/1) | [Source](src/year2025/day01.rs) | 25 |
 | 2 | [Gift Shop](https://adventofcode.com/2025/day/2) | [Source](src/year2025/day02.rs) | 2 |
+| 3 | [Lobby](https://adventofcode.com/2025/day/3) | [Source](src/year2025/day03.rs) | 611 |
 
 ## 2024
 

benches/benchmark.rs

@@ -95,5 +95,5 @@ benchmark!(year2024
 );
 
 benchmark!(year2025
-    day01, day02
+    day01, day02, day03
 );

src/lib.rs

@@ -74,5 +74,5 @@ library!(year2024 "Locate the Chief Historian in time for the big Christmas slei
 );
 
 library!(year2025 "Finish the North Pole decorations in time for Christmas."
-    day01, day02
+    day01, day02, day03
 );

src/main.rs

@@ -141,5 +141,5 @@ run!(year2024
 );
 
 run!(year2025
-    day01, day02
+    day01, day02, day03
 );

src/year2025/day03.rs

@@ -0,0 +1,39 @@
+pub fn parse(input: &str) -> Vec<&str> {
+    input.lines().collect()
+}
+
+pub fn part1(input: &[&str]) -> u64 {
+    solve(input, 2)
+}
+
+pub fn part2(input: &[&str]) -> u64 {
+    solve(input, 12)
+}
+
+fn solve(lines: &[&str], limit: usize) -> u64 {
+    let mut bank = [0; 101];
+    let mut result = 0;
+
+    for line in lines {
+        for (i, b) in line.bytes().enumerate() {
+            bank[i] = (b - b'0') as u64;
+        }
+
+        let mut row = [0; 101];
+
+        for exp in 0..limit {
+            let tens = 10_u64.pow(exp as u32);
+            let mut next = [0; 101];
+
+            for i in ((limit - exp - 1)..(line.len() - exp)).rev() {
+                next[i] = (tens * bank[i] + row[i + 1]).max(next[i + 1]);
+            }
+
+            row = next;
+        }
+
+        result += row[0];
+    }
+
+    result
+}

tests/test.rs

@@ -87,5 +87,5 @@ test!(year2024
 );
 
 test!(year2025
-    day01, day02
+    day01, day02, day03
 );

tests/year2025/day03.rs

@@ -0,0 +1,19 @@
+use aoc::year2025::day03::*;
+
+const EXAMPLE: &str = "\
+987654321111111
+811111111111119
+234234234234278
+818181911112111";
+
+#[test]
+fn part1_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part1(&input), 357);
+}
+
+#[test]
+fn part2_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part2(&input), 3121910778619);
+}