From bee665ddb44742ae127a2534675948d00935c840 Mon Sep 17 00:00:00 2001
From: uku <hi@uku.moe>
Date: Wed, 11 Dec 2024 22:14:22 +0100
Subject: [PATCH] feat(day11): init

---
 src/solutions/day_11.rs | 69 +++++++++++++++++++++++++++++++++++++++++
 src/solutions/mod.rs    |  2 ++
 2 files changed, 71 insertions(+)
 create mode 100644 src/solutions/day_11.rs

diff --git a/src/solutions/day_11.rs b/src/solutions/day_11.rs
new file mode 100644
index 0000000..a9b61b0
--- /dev/null
+++ b/src/solutions/day_11.rs
@@ -0,0 +1,69 @@
+use itertools::Itertools;
+
+use crate::common::{Answer, Solution};
+
+pub struct Day11;
+
+impl Solution for Day11 {
+    fn name(&self) -> &'static str {
+        "Plutonian Pebbles"
+    }
+
+    fn part_a(&self, input: &str) -> Answer {
+        compute_stones(input, 25).into()
+    }
+
+    fn part_b(&self, input: &str) -> Answer {
+        compute_stones(input, 75).into()
+    }
+}
+
+fn compute_stones(input: &str, iterations: u64) -> usize {
+    let mut stones = input
+        .split_whitespace()
+        .map(|n| n.parse::<u64>().unwrap())
+        .counts();
+
+    for _ in 0..iterations {
+        stones = stones
+            .into_iter()
+            .flat_map(|(n, count)| {
+                if n == 0 {
+                    vec![(1, count)]
+                } else {
+                    let s = n.to_string();
+                    if s.len() % 2 == 0 {
+                        let (one, two) = s.split_at(s.len() / 2);
+                        vec![
+                            (one.parse::<u64>().unwrap(), count),
+                            (two.parse::<u64>().unwrap(), count),
+                        ]
+                    } else {
+                        vec![(n * 2024, count)]
+                    }
+                }
+            })
+            .into_grouping_map()
+            .sum();
+    }
+
+    stones.values().sum::<usize>()
+}
+
+#[cfg(test)]
+mod test {
+    use super::Day11;
+    use crate::common::Solution;
+
+    const INPUT: &str = "125 17";
+
+    #[test]
+    fn part_a() {
+        assert_eq!(Day11.part_a(INPUT), 55312.into());
+    }
+
+    #[test]
+    fn part_b() {
+        assert_eq!(Day11.part_b(INPUT), 65601038650482u64.into());
+    }
+}
diff --git a/src/solutions/mod.rs b/src/solutions/mod.rs
index c4d30ca..e5c7a19 100644
--- a/src/solutions/mod.rs
+++ b/src/solutions/mod.rs
@@ -10,6 +10,7 @@ mod day_07;
 mod day_08;
 mod day_09;
 mod day_10;
+mod day_11;
 
 pub const SOLUTIONS: &[&dyn Solution] = &[
     &day_01::Day01,
@@ -22,4 +23,5 @@ pub const SOLUTIONS: &[&dyn Solution] = &[
     &day_08::Day08,
     &day_09::Day09,
     &day_10::Day10,
+    &day_11::Day11,
 ];