part 2

2024/day11/src/lib.rs

@@ -13,7 +13,7 @@ helper::define_variants! {
         basic => crate::part1;
     }
     part2 {
-        basic => crate::part2;
+        basic => crate::part2,sample_count=10;
     }
 }
 
@@ -72,8 +72,49 @@ fn part1(input: &str) -> u64 {
         .sum()
 }
 
-fn part2(_input: &str) -> u64 {
+fn part2(input: &str) -> u64 {
-    0
+    let mut cache = FxHashMap::default();
+
+    fn expanded_count(cache: &mut FxHashMap<(u64, u64), u64>, stone: u64, blinks: u64) -> u64 {
+        let result_count = if blinks == 0 {
+            1
+        } else {
+            if let Some(result) = cache.get(&(stone, blinks)) {
+                return *result;
+            }
+
+            if stone == 0 {
+                expanded_count(cache, 1, blinks - 1)
+            } else if (stone.ilog10() + 1) % 2 == 0 {
+                //   3456 -> ilog=3, split =  100 (10^2)
+                // 123456 -> ilog=5, split = 1000 (10^3)
+                let split = 10_u64.pow((stone.ilog10() / 2) + 1);
+                let lhs = stone / split;
+                let rhs = stone % split;
+
+                let lhs_count = expanded_count(cache, lhs, blinks - 1);
+                let rhs_count = expanded_count(cache, rhs, blinks - 1);
+                lhs_count + rhs_count
+            } else {
+                expanded_count(cache, stone * 2024, blinks - 1)
+            }
+        };
+
+        cache.insert((stone, blinks), result_count);
+
+        result_count
+    }
+
+    let is_example = input.len() < 10;
+
+    let total_blinks = if is_example { 6 } else { 75 };
+
+    input
+        .trim()
+        .split(" ")
+        .map(parse_unwrap)
+        .map(|x| expanded_count(&mut cache, x, total_blinks))
+        .sum()
 }
 
 helper::tests! {
@@ -83,7 +124,7 @@ helper::tests! {
         default => 204022;
     }
     part2 {
-        small => 0;
+        small => 22;
         default => 0;
     }
 }