2025: day02: part 2

aoc2025/benches/aoc2025_bench.rs

@@ -1,9 +1,11 @@
 use criterion::{criterion_group, criterion_main, Criterion};
 
 use aoc2025::day01;
+use aoc2025::day02;
 
 fn aoc2025_all(c: &mut Criterion) {
     c.bench_function("day01", |b| b.iter(|| day01::run().unwrap()));
+    c.bench_function("day02", |b| b.iter(|| day02::run().unwrap()));
 }
 
 criterion_group! {

aoc2025/input/day02.txt

@@ -0,0 +1 @@
+52-75,71615244-71792700,89451761-89562523,594077-672686,31503-39016,733-976,1-20,400309-479672,458-635,836793365-836858811,3395595155-3395672258,290-391,5168-7482,4545413413-4545538932,65590172-65702074,25-42,221412-256187,873499-1078482,118-154,68597355-68768392,102907-146478,4251706-4487069,64895-87330,8664371543-8664413195,4091-5065,537300-565631,77-115,83892238-83982935,6631446-6694349,1112-1649,7725-9776,1453397-1493799,10240-12328,15873-20410,1925-2744,4362535948-4362554186,3078725-3256936,710512-853550,279817-346202,45515-60928,3240-3952

aoc2025/input/day02_provided.txt

@@ -0,0 +1 @@
+11-22,95-115,998-1012,1188511880-1188511890,222220-222224,1698522-1698528,446443-446449,38593856-38593862,565653-565659,824824821-824824827,2121212118-2121212124

aoc2025/src/day02.rs

@@ -0,0 +1,186 @@
+use anyhow::{anyhow, Context, Result};
+use std::{fmt::Write, ops::RangeInclusive, str::FromStr};
+
+const INPUT: &str = include_str!("../input/day02.txt");
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+    writeln!(res, "part 2: {}", part2(INPUT)?)?;
+    Ok(res)
+}
+
+struct IdRange(RangeInclusive<u64>);
+
+impl FromStr for IdRange {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> Result<Self> {
+        let (left, right) = s
+            .split_once('-')
+            .ok_or_else(|| anyhow!("couldn't find dash in range: `{}'", s))?;
+        let (left, right) = (
+            left.parse()
+                .with_context(|| format!("couldn't parse left member of range: `{}'", left))?,
+            right
+                .trim()
+                .parse()
+                .with_context(|| format!("couldn't parse right member of range: `{}'", right))?,
+        );
+
+        Ok(IdRange(left..=right))
+    }
+}
+
+impl Iterator for IdRange {
+    type Item = <RangeInclusive<u64> as Iterator>::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next()
+    }
+}
+
+fn get_num_digits(num: &u64) -> usize {
+    let mut digits = 0;
+    let mut num = *num;
+    while num != 0 {
+        num /= 10;
+        digits += 1;
+    }
+    digits
+}
+
+fn is_repeated_twice(num: &u64) -> bool {
+    let num_digits = get_num_digits(num);
+    if !num_digits.is_multiple_of(2) {
+        return false;
+    }
+    let half = num_digits / 2;
+    let mask = 10_u64.pow(half as u32);
+
+    *num / mask == *num % mask
+}
+
+fn part1(input: &str) -> Result<u64> {
+    let ranges = input
+        .split(',')
+        .map(IdRange::from_str)
+        .collect::<Result<Vec<_>>>()?;
+
+    Ok(ranges
+        .into_iter()
+        .map(|range| range.filter(is_repeated_twice).sum::<u64>())
+        .sum())
+}
+
+fn has_identical_chunks(num: &u64, chunk_size: usize) -> bool {
+    // build a "mask" for our chunk size
+    let mask = 10_u64.pow(chunk_size as u32);
+
+    // this is what all chunks should look like to satisfy our condition
+    let target = *num % mask;
+
+    let mut num = *num;
+    while num > target {
+        // shift num right by 10^(sub_digit_num)
+        num /= mask;
+        if num % mask != target {
+            return false;
+        }
+    }
+
+    true
+}
+
+fn is_repeated_at_least_twice(num: &u64) -> bool {
+    let num_digits = get_num_digits(num);
+    if num_digits < 2 {
+        return false;
+    }
+
+    has_identical_chunks(num, 1)
+        || (2..=(num_digits / 2)).any(|n| {
+            // walk all possible divisors of our number of digits
+            if !num_digits.is_multiple_of(n) {
+                return false;
+            }
+            let chunk_size = num_digits / n;
+            has_identical_chunks(num, chunk_size)
+        })
+}
+
+fn part2(input: &str) -> Result<u64> {
+    let ranges = input
+        .split(',')
+        .map(IdRange::from_str)
+        .collect::<Result<Vec<_>>>()?;
+
+    Ok(ranges
+        .into_iter()
+        .map(|range| range.filter(is_repeated_at_least_twice).sum::<u64>())
+        .sum())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day02_provided.txt");
+
+    #[test]
+    fn repeated_twice() {
+        assert!(is_repeated_twice(&11));
+        assert!(is_repeated_twice(&22));
+        assert!(is_repeated_twice(&99));
+        assert!(is_repeated_twice(&1010));
+        assert!(is_repeated_twice(&1188511885));
+        assert!(is_repeated_twice(&222222));
+        assert!(is_repeated_twice(&446446));
+        assert!(is_repeated_twice(&38593859));
+
+        assert!(!is_repeated_twice(&111));
+        assert!(!is_repeated_twice(&121));
+        assert!(!is_repeated_twice(&1));
+    }
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1(PROVIDED).unwrap(), 1227775554);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 26255179562);
+    }
+
+    #[test]
+    fn repeated_at_least_twice() {
+        assert!(is_repeated_at_least_twice(&11));
+        assert!(is_repeated_at_least_twice(&22));
+        assert!(is_repeated_at_least_twice(&99));
+        assert!(is_repeated_at_least_twice(&111));
+        assert!(is_repeated_at_least_twice(&999));
+        assert!(is_repeated_at_least_twice(&1010));
+        assert!(is_repeated_at_least_twice(&1188511885));
+        assert!(is_repeated_at_least_twice(&222222));
+        assert!(is_repeated_at_least_twice(&446446));
+        assert!(is_repeated_at_least_twice(&38593859));
+        assert!(is_repeated_at_least_twice(&565656));
+        assert!(is_repeated_at_least_twice(&824824824));
+        assert!(is_repeated_at_least_twice(&2121212121));
+
+        assert!(!is_repeated_at_least_twice(&1121));
+        assert!(!is_repeated_at_least_twice(&121));
+        assert!(!is_repeated_at_least_twice(&1));
+    }
+
+    #[test]
+    fn part2_provided() {
+        assert_eq!(part2(PROVIDED).unwrap(), 4174379265);
+    }
+
+    #[test]
+    fn part2_real() {
+        assert_eq!(part2(INPUT).unwrap(), 31680313976);
+    }
+}

aoc2025/src/lib.rs

@@ -1 +1,2 @@
 pub mod day01;
+pub mod day02;

aoc2025/src/main.rs

@@ -3,9 +3,10 @@ use anyhow::Result;
 use aoc::DayFunc;
 
 use aoc2025::day01;
+use aoc2025::day02;
 
 fn main() -> Result<()> {
-    let days: &[DayFunc] = &[day01::run];
+    let days: &[DayFunc] = &[day01::run, day02::run];
 
     aoc::run(days)
 }