2024 Day 22 Part 2

2024/day22.exs

@@ -8,8 +8,7 @@ defmodule Day22 do
 
   def next(secret) do
     secret
-    |> Kernel.*(64)
+    |> then(fn secret -> (secret * 64) |> mix(secret) end)
-    |> mix(secret)
     |> prune()
     |> then(fn secret -> div(secret, 32) |> mix(secret) end)
     |> prune()
@@ -20,8 +19,25 @@ defmodule Day22 do
   def mix(num, secret), do: Bitwise.bxor(secret, num)
   def prune(secret), do: rem(secret, 16_777_216)
 
-  def part2(_input) do
+  def part2(input) do
-    :not_implemented
+    input
+    |> Enum.map(fn secret ->
+      secret
+      |> Stream.iterate(&next/1)
+      |> Stream.map(&bananas/1)
+      |> Stream.take(2000)
+      |> Enum.chunk_every(5, 1, :discard)
+      |> Enum.reduce(%{}, fn [a, b, c, d, bananas], changes ->
+        Map.put_new(changes, {b - a, c - b, d - c, bananas - d}, bananas)
+      end)
+    end)
+    |> Enum.reduce(&Map.merge(&1, &2, fn _k, v1, v2 -> v1 + v2 end))
+    |> Enum.max_by(fn {_sequence, bananas} -> bananas end)
+    |> elem(1)
+  end
+
+  def bananas(secret) do
+    secret |> Integer.to_string() |> String.last() |> String.to_integer()
   end
 
   def input do

dayN.exs

@@ -5,7 +5,7 @@ defmodule DayREPLACE_ME do
   end
 
   def part2(_input) do
-    :ok
+    :not_implemented
   end
 
   def input do
@@ -36,7 +36,7 @@ defmodule DayREPLACE_ME do
   defp run_with_timer(part, fun) do
     {time, result} = :timer.tc(fun)
     IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
-    IO.puts("#{inspect result}\n")
+    IO.puts("#{inspect(result)}\n")
   end
 
   defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"