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) | 1 |
+| 3 | [Lobby](https://adventofcode.com/2025/day/3) | [Source](src/year2025/day03.rs) | 60 |
 
 ## 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 +1,32 @@
+//! # Lobby
+pub fn parse(input: &str) -> Vec<&[u8]> {
+    input.lines().map(str::as_bytes).collect()
+}
+
+pub fn part1(input: &[&[u8]]) -> u64 {
+    solve(input, 2)
+}
+
+pub fn part2(input: &[&[u8]]) -> u64 {
+    solve(input, 12)
+}
+
+fn solve(banks: &[&[u8]], limit: usize) -> u64 {
+    banks
+        .iter()
+        .map(|&bank| {
+            let mut max = 0;
+            let mut start = 0;
+
+            (0..limit).rev().fold(0, |joltage, digit| {
+                let end = bank.len() - digit;
+
+                (max, start) = (start..end).fold((0, 0), |(max, start), i| {
+                    if bank[i] > max { (bank[i], i + 1) } else { (max, start) }
+                });
+
+                10 * joltage + (max - b'0') as u64
+            })
+        })
+        .sum()
+}

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);
+}