Year 2025 Day 2

Author: maneatingape

README.md

@@ -78,6 +78,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | Day | Problem | Solution | Benchmark (μs) |
 | --- | --- | --- | --: |
 | 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) | 1016 |
 
 ## 2024
 

benches/benchmark.rs

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

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
+    day01, day02
 );

src/main.rs

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

src/year2025/day02.rs

@@ -0,0 +1,56 @@
+use crate::util::iter::*;
+use crate::util::parse::*;
+
+type Input = (u64, u64);
+
+pub fn parse(input: &str) -> Input {
+    let ranges: Vec<_> = input.iter_unsigned::<u64>().chunk::<2>().collect();
+    let mut invalid = Vec::with_capacity(100_000);
+
+    for digits in (2..=10).step_by(2) {
+        let size = digits / 2;
+        let start = 10_u64.pow(size - 1);
+        let end = start * 10;
+
+        for n in start..end {
+            invalid.push(end * n + n);
+        }
+    }
+
+    let part_one = sum(&ranges, &invalid);
+
+    for digits in 2_u32..=10 {
+        for size in 1..=digits / 2 {
+            let repeat = digits / size;
+            if !digits.is_multiple_of(size) || repeat == 2 {
+                continue;
+            }
+
+            let start = 10_u64.pow(size - 1);
+            let end = start * 10;
+
+            for n in start..end {
+                invalid.push((0..repeat).fold(0, |acc, _| end * acc + n));
+            }
+        }
+    }
+
+    invalid.sort_unstable();
+    invalid.dedup();
+
+    let part_two = sum(&ranges, &invalid);
+    (part_one, part_two)
+}
+
+pub fn part1(input: &Input) -> u64 {
+    input.0
+}
+
+pub fn part2(input: &Input) -> u64 {
+    input.1
+}
+
+fn sum(ranges: &[[u64; 2]], invalid: &[u64]) -> u64 {
+    let index = |n: u64| invalid.binary_search(&n).unwrap_or_else(|i| i);
+    ranges.iter().flat_map(|&[from, to]| invalid[index(from)..index(to + 1)].iter()).sum()
+}

tests/test.rs

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

tests/year2025/day02.rs

@@ -0,0 +1,18 @@
+use aoc::year2025::day02::*;
+
+const EXAMPLE: &str = "
+11-22,95-115,998-1012,1188511880-1188511890,222220-222224,
+1698522-1698528,446443-446449,38593856-38593862,565653-565659,
+824824821-824824827,2121212118-2121212124";
+
+#[test]
+fn part1_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part1(&input), 1227775554);
+}
+
+#[test]
+fn part2_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part2(&input), 4174379265);
+}