From 688846610526465bb8ddd0d1fa6823988c9003a1 Mon Sep 17 00:00:00 2001
From: Keenan Tims <ktims@gotroot.ca>
Date: Wed, 3 Dec 2025 00:46:44 -0800
Subject: [PATCH] day3: add clever solution

---
 src/day3.rs           | 53 +++++++++++++++++++++++++++++++------------
 utils/misc/src/lib.rs | 20 +++++++++++++---
 2 files changed, 56 insertions(+), 17 deletions(-)

diff --git a/src/day3.rs b/src/day3.rs
index 2be5275..a8f7b4a 100644
--- a/src/day3.rs
+++ b/src/day3.rs
@@ -1,5 +1,6 @@
 use aoc_runner_derive::{aoc, aoc_generator};
 use memoize::memoize;
+use misc::POW10;
 
 #[aoc_generator(day3)]
 fn parse(input: &str) -> Vec<Vec<u8>> {
@@ -17,33 +18,47 @@ fn max_joltage(bank: Vec<u8>, n: usize) -> u64 {
 
     (0..bank.len() - n + 1)
         .map(|start| {
-            bank[start] as u64 * 10u64.pow(n as u32 - 1)
-                + max_joltage(bank[start + 1..].to_vec(), n - 1)
+            bank[start] as u64 * POW10[n - 1] + max_joltage(bank[start + 1..].to_vec(), n - 1)
         })
         .max()
         .unwrap()
-
-    // let mut max = 0;
-    // for start in 0..bank.len() - n + 1 {
-    //     let cur = bank[start] as u64 * 10u64.pow(n as u32 - 1)
-    //         + *bank[start + 1..].iter().max().unwrap() as u64;
-    //     if cur > max {
-    //         max = cur
-    //     }
-    // }
-    // max
 }
 
-#[aoc(day3, part1)]
+fn max_joltage_2(bank: &[u8], n: usize) -> u64 {
+    if n == 1 {
+        return *bank.iter().max().unwrap() as u64;
+    }
+    let choices = &bank[..bank.len() - (n - 1)];
+
+    // Note: can't use position_max() here since it returns the _last_ instance and we need the _first_.
+    // get the max, then find the first instance of that value. There is definitely a one-pass solution.
+    let max = choices.iter().max().unwrap();
+    let choice = choices.iter().position(|n| max == n).unwrap();
+    let rest = &bank[choice + 1..];
+
+    bank[choice] as u64 * POW10[n - 1] + max_joltage_2(rest, n - 1)
+}
+
+#[aoc(day3, part1, Brute)]
 fn part1(input: &[Vec<u8>]) -> u64 {
     input.iter().map(|bank| max_joltage(bank.clone(), 2)).sum()
 }
 
-#[aoc(day3, part2)]
+#[aoc(day3, part2, Brute)]
 fn part2(input: &[Vec<u8>]) -> u64 {
     input.iter().map(|bank| max_joltage(bank.clone(), 12)).sum()
 }
 
+#[aoc(day3, part1, Smart)]
+fn part1_smart(input: &[Vec<u8>]) -> u64 {
+    input.iter().map(|bank| max_joltage_2(bank, 2)).sum()
+}
+
+#[aoc(day3, part2, Smart)]
+fn part2_smart(input: &[Vec<u8>]) -> u64 {
+    input.iter().map(|bank| max_joltage_2(bank, 12)).sum()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -62,4 +77,14 @@ mod tests {
     fn part2_example() {
         assert_eq!(part2(&parse(EXAMPLE)), 3121910778619);
     }
+
+    #[test]
+    fn part1_smart_example() {
+        assert_eq!(part1_smart(&parse(EXAMPLE)), 357);
+    }
+
+    #[test]
+    fn part2_smart_example() {
+        assert_eq!(part2_smart(&parse(EXAMPLE)), 3121910778619);
+    }
 }
diff --git a/utils/misc/src/lib.rs b/utils/misc/src/lib.rs
index b7d4b5c..46d447f 100644
--- a/utils/misc/src/lib.rs
+++ b/utils/misc/src/lib.rs
@@ -2,6 +2,19 @@ use num_traits::Signed;
 use std::fmt::Display;
 use std::ops::{Add, AddAssign};
 
+const POW10MAX: usize = u64::MAX.ilog10() as usize;
+pub const POW10: [u64; POW10MAX] = pow10_lut();
+
+const fn pow10_lut<const N: usize>() -> [u64; N] {
+    let mut res = [0; N];
+    let mut i = 0;
+    while i < N {
+        res[i] = 10u64.pow(i as u32);
+        i += 1;
+    }
+    res
+}
+
 /// Wrapped signed integer with custom upper bound with wrapping of 0s to the upper bound
 #[derive(Eq, Clone, Copy)]
 pub struct CustomWrapped<T: Signed + Copy> {
@@ -91,8 +104,9 @@ mod tests {
     use super::*;
 
     #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
+    fn test_pow10() {
+        for i in 0..POW10MAX {
+            assert_eq!(POW10[i], 10u64.pow(i as u32))
+        }
     }
 }