2025: day02: part 2

aoc2025/src/day02.rs

@@ -6,6 +6,7 @@ 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)
 }
 
@@ -72,6 +73,54 @@ fn part1(input: &str) -> Result<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::*;
@@ -104,13 +153,34 @@ mod tests {
         assert_eq!(part1(INPUT).unwrap(), 26255179562);
     }
 
-    //#[test]
-    //fn part2_provided() {
-    //    assert_eq!(part2(PROVIDED).unwrap(), 6);
-    //}
+    #[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));
 
-    //#[test]
-    //fn part2_real() {
-    //    assert_eq!(part2(INPUT).unwrap(), 6616);
-    //}
+        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);
+    }
 }