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) | 2 |
 
 ## 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 +1,50 @@
+//! # Gift Shop
+use crate::util::iter::*;
+use crate::util::parse::*;
+
+type Pair = [usize; 2];
+
+const FIRST: [Pair; 5] = [[2, 1], [4, 2], [6, 3], [8, 4], [10, 5]];
+const SECOND: [Pair; 6] = [[3, 1], [5, 1], [6, 2], [7, 1], [9, 3], [10, 2]];
+const THIRD: [Pair; 2] = [[6, 1], [10, 1]];
+
+pub fn parse(input: &str) -> Pair {
+    let ranges: Vec<_> = input.iter_unsigned::<usize>().chunk::<2>().collect();
+    let first = sum(&FIRST, &ranges);
+    let second = sum(&SECOND, &ranges);
+    let third = sum(&THIRD, &ranges);
+    [first, first + second - third]
+}
+
+pub fn part1(input: &Pair) -> usize {
+    input[0]
+}
+
+pub fn part2(input: &Pair) -> usize {
+    input[1]
+}
+
+fn sum(ranges: &[Pair], ids: &[Pair]) -> usize {
+    let mut result = 0;
+
+    for &[digits, size] in ranges {
+        let repeat = digits / size;
+        let power = 10_usize.pow(size as u32);
+        let factor = (0..repeat).fold(0, |acc, _| acc * power + 1);
+        let start = factor * (power / 10);
+        let end = factor * (power - 1);
+
+        for &[from, to] in ids {
+            let lower = from.next_multiple_of(factor).max(start);
+            let upper = to.min(end);
+
+            if lower <= upper {
+                let n = (upper - lower) / factor;
+                let triangular = n * (n + 1) / 2;
+                result += lower * (n + 1) + factor * triangular;
+            }
+        }
+    }
+
+    result
+}

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);
+}