From 26731083a355044d600f2935c5f8c81ff5607d6b Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Wed, 11 Dec 2024 22:29:10 +0900
Subject: [PATCH] 2024 Day 11 Part 2 + retroactively use for Part 1

---
 2024/day11.exs | 53 ++++++++++++++++++++++++++------------------------
 1 file changed, 28 insertions(+), 25 deletions(-)

diff --git a/2024/day11.exs b/2024/day11.exs
index 261069e..5deed00 100644
--- a/2024/day11.exs
+++ b/2024/day11.exs
@@ -1,34 +1,36 @@
 #!/usr/bin/env elixir
 defmodule Day11 do
   def part1(stones) do
-    1..25
-    |> Enum.reduce(stones, fn _, stones -> blink(stones) end)
-    |> Enum.count()
+    blink(stones, 25)
   end
 
-  def blink([]), do: []
-  def blink([0 | rest]), do: [1 | blink(rest)]
-
-  def blink([stone | rest]) do
-    string = Integer.to_string(stone)
-    size = byte_size(string)
-
-    if rem(size, 2) == 0 do
-      string
-      |> String.split_at(div(size, 2))
-      |> Tuple.to_list()
-      |> Enum.map(&String.to_integer/1)
-      |> Kernel.++(blink(rest))
-    else
-      [stone |> Kernel.*(2024) | blink(rest)]
-    end
-  end
-
-  # probably have to track the changes rather than keep the whole list?
   def part2(stones) do
-    1..75
-    |> Enum.reduce(stones, fn _, stones -> blink(stones) end)
-    |> Enum.count()
+    blink(stones, 75)
+  end
+
+  def blink(stones, 0), do: stones |> Map.values() |> Enum.sum()
+
+  def blink(stones, times) do
+    stones
+    |> Enum.reduce(%{}, fn
+      {0, count}, next ->
+        Map.update(next, 1, count, &(&1 + count))
+
+      {stone, count}, next ->
+        string = Integer.to_string(stone)
+        size = byte_size(string)
+
+        if rem(size, 2) == 0 do
+          {a, b} = String.split_at(string, div(size, 2))
+
+          next
+          |> Map.update(String.to_integer(a), count, &(&1 + count))
+          |> Map.update(String.to_integer(b), count, &(&1 + count))
+        else
+          Map.update(next, stone * 2024, count, &(&1 + count))
+        end
+    end)
+    |> blink(times - 1)
   end
 
   def input do
@@ -37,6 +39,7 @@ defmodule Day11 do
       input
       |> String.split([" ", "\n"], trim: true)
       |> Enum.map(&String.to_integer/1)
+      |> Enum.frequencies()
     else
       _ -> :error
     end