From f51dc9c14514d7c04ce8196e6b80fbd8855e7fe6 Mon Sep 17 00:00:00 2001
From: Keenan Tims <ktims@gotroot.ca>
Date: Fri, 5 Dec 2025 04:29:55 -0800
Subject: [PATCH] day2: clever and fast solution

---
 src/day2.rs | 53 ++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 50 insertions(+), 3 deletions(-)

diff --git a/src/day2.rs b/src/day2.rs
index 79dd47b..a9c2ae3 100644
--- a/src/day2.rs
+++ b/src/day2.rs
@@ -1,6 +1,7 @@
 use aoc_runner_derive::{aoc, aoc_generator};
 use itertools::Itertools;
 use misc::POW10;
+use std::cmp::{max, min};
 use std::ops::RangeInclusive;
 
 #[aoc_generator(day2)]
@@ -88,15 +89,15 @@ fn part2_arithmetic_brute(input: &[RangeInclusive<u64>]) -> u64 {
             let n_digits = (product.ilog10() + 1) as usize;
 
             for n in 1..=n_digits / 2 {
-                let repetitions = n_digits / n;
                 if !n_digits.is_multiple_of(n) {
                     continue;
                 }
-                let decade = POW10[n_digits - n];
+                let repetitions = n_digits / n;
+                let decade = POW10[n_digits - n]; // get the power of 10 to split the leftmost n digits
                 let lhs = product / decade;
                 let remainder = product % decade;
 
-                // for each repetition we multiply by 10^(rep * n)
+                // for each repetition we multiply by 10^(rep * n) to add the appropriate zeros
                 let expected_remainder = (0..repetitions - 1).map(|rep| lhs * POW10[rep * n]).sum();
 
                 if remainder == expected_remainder {
@@ -139,6 +140,48 @@ fn part1_clever(input: &[RangeInclusive<u64>]) -> u64 {
     invalid_sum
 }
 
+const fn n_digits(n: &u64) -> usize {
+    (n.ilog10() + 1) as usize
+}
+
+fn generate_repeaters(r: &RangeInclusive<u64>, n: usize) -> Vec<u64> {
+    let mut invalids = Vec::new();
+    for r in split_by_decade(r) {
+        let n_digits = n_digits(r.start());
+        if !n_digits.is_multiple_of(n) || n_digits < 2 {
+            continue;
+        }
+
+        let repetitions = n_digits / n;
+        let decade = POW10[n_digits - n];
+        for lhs in (r.start() / decade)..=(r.end() / decade) {
+            let repeater = (0..repetitions)
+                .map(|rep| lhs * POW10[rep * n])
+                .sum::<u64>();
+            if r.contains(&repeater) {
+                invalids.push(repeater)
+            }
+        }
+    }
+    invalids
+}
+
+fn split_by_decade(r: &RangeInclusive<u64>) -> impl Iterator<Item = RangeInclusive<u64>> {
+    let (start, end) = (*r.start(), *r.end());
+    (n_digits(r.start())..n_digits(r.end()) + 1)
+        .map(move |nd| RangeInclusive::new(max(start, POW10[nd - 1]), min(end, POW10[nd] - 1)))
+}
+
+#[aoc(day2, part2, Clever)]
+fn part2_clever(input: &[RangeInclusive<u64>]) -> u64 {
+    input
+        .iter()
+        .flat_map(|r| (1..=n_digits(r.end()) / 2).map(|nd| generate_repeaters(r, nd)))
+        .flatten()
+        .unique()
+        .sum()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -167,4 +210,8 @@ mod tests {
     fn part1_clever_example() {
         assert_eq!(part1_clever(&parse(EXAMPLE)), 1227775554);
     }
+    #[test]
+    fn part2_clever_example() {
+        assert_eq!(part2_clever(&parse(EXAMPLE)), 4174379265);
+    }
 }