2024 day 11

Author: ictrobot

crates/year2024/src/day11.rs

@@ -0,0 +1,57 @@
+use std::collections::HashMap;
+use utils::prelude::*;
+
+/// Counting dividing stones.
+#[derive(Clone, Debug)]
+pub struct Day11 {
+    stones: HashMap<u64, u64>,
+}
+
+impl Day11 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let mut stones = HashMap::new();
+        for v in parser::u64().repeat(b' ', 1).parse_complete(input)? {
+            *stones.entry(v).or_insert(0) += 1;
+        }
+        Ok(Self { stones })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u64 {
+        self.blink(25)
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> u64 {
+        self.blink(75)
+    }
+
+    fn blink(&self, times: u32) -> u64 {
+        let mut stones = self.stones.clone();
+        let mut next = HashMap::new();
+        for _ in 0..times {
+            for (&x, &c) in stones.iter() {
+                if x == 0 {
+                    next.entry(1).and_modify(|v| *v += c).or_insert(c);
+                } else {
+                    let log = x.ilog10() + 1;
+                    if log % 2 == 0 {
+                        let pow = 10u64.pow(log / 2);
+                        next.entry(x / pow).and_modify(|v| *v += c).or_insert(c);
+                        next.entry(x % pow).and_modify(|v| *v += c).or_insert(c);
+                    } else {
+                        next.entry(x * 2024).and_modify(|v| *v += c).or_insert(c);
+                    }
+                }
+            }
+
+            (stones, next) = (next, stones);
+            next.clear();
+        }
+        stones.values().sum()
+    }
+}
+
+examples!(Day11 -> (u64, u64) [
+    {input: "125 17", part1: 55312},
+]);

crates/year2024/src/lib.rs

@@ -12,4 +12,5 @@ utils::year!(2024 => year2024, ${
     8 => day08::Day08,
     9 => day09::Day09<'_>,
     10 => day10::Day10,
+    11 => day11::Day11,
 });