From c3b4b495eb3a2230a133c932a11abc2928158f2f Mon Sep 17 00:00:00 2001
From: Rico Hageman <rico@packfleet.com>
Date: Wed, 11 Dec 2024 22:16:40 +0000
Subject: [PATCH] Solve day 11

---
 src/day11/mod.rs | 93 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs       |  1 +
 2 files changed, 94 insertions(+)
 create mode 100644 src/day11/mod.rs

diff --git a/src/day11/mod.rs b/src/day11/mod.rs
new file mode 100644
index 0000000..3907085
--- /dev/null
+++ b/src/day11/mod.rs
@@ -0,0 +1,93 @@
+use rustc_hash::FxHashMap;
+
+type Stone = usize;
+type Iteration = usize;
+
+fn blink(
+    stone: Stone,
+    iterations_left: Iteration,
+    cache: &mut FxHashMap<(Stone, Iteration), usize>,
+) -> usize {
+    // Base case: If there are no more iterations there is just one stone
+    if iterations_left == 0 {
+        return 1;
+    }
+
+    // Base case: If there is a cache hit, use that value
+    if let Some(output) = cache.get(&(stone, iterations_left)) {
+        return *output;
+    }
+
+    // Rule 1: If the stone is engraved with the number 0, it is replaced by a stone engraved with the number 1.
+    if stone == 0 {
+        // Optimisation: This iteration zero is converted into one and in the next iteration it is
+        //  converted into 2024. So let's skip ahead a little bit.
+        if iterations_left <= 2 {
+            return 1;
+        }
+
+        let value = blink(2024, iterations_left - 2, cache);
+        cache.insert((0, iterations_left), value);
+        cache.insert((1, iterations_left - 1), value);
+        return value;
+    }
+
+    // Rule 2: If the stone is engraved with a number that has an even number of digits, it is replaced
+    //          by two stones. The left half of the digits are engraved on the new left stone, and the
+    //          right half of the digits are engraved on the new right stone.
+    let number_of_digits = stone.ilog10() + 1;
+    if number_of_digits % 2 == 0 {
+        let number_of_digits = number_of_digits / 2;
+
+        let value = blink(
+            stone % 10_usize.pow(number_of_digits),
+            iterations_left - 1,
+            cache,
+        ) + blink(
+            stone / 10_usize.pow(number_of_digits),
+            iterations_left - 1,
+            cache,
+        );
+
+        cache.insert((stone, iterations_left), value);
+        return value;
+    }
+
+    // Rule 3: The stone is replaced by a new stone; the old stone's number multiplied by 2024 is
+    //          engraved on the new stone.
+    let value = blink(stone * 2024, iterations_left - 1, cache);
+    cache.insert((stone, iterations_left), value);
+    value
+}
+
+#[aoc(day11, part1)]
+pub fn part1(input: &str) -> usize {
+    inner(input, 25)
+}
+
+#[aoc(day11, part2)]
+pub fn part2(input: &str) -> usize {
+    inner(input, 75)
+}
+
+fn inner(input: &str, iterations: usize) -> usize {
+    let mut cache: FxHashMap<(Stone, Iteration), usize> = FxHashMap::default();
+
+    input
+        .lines()
+        .next()
+        .unwrap()
+        .split_whitespace()
+        .map(|stone| stone.parse::<Stone>().unwrap())
+        .map(|stone| blink(stone, iterations, &mut cache))
+        .sum()
+}
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part1() {
+        assert_eq!(55312, part1("125 17"));
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 893d10d..e34adbf 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -5,6 +5,7 @@ extern crate aoc_runner_derive;
 
 pub mod day1;
 pub mod day10;
+pub mod day11;
 pub mod day2;
 pub mod day3;
 pub mod day4;