2023 Day 8 (Part2 doesn't complete)

2023/day8.exs

@@ -0,0 +1,86 @@
+defmodule Day8 do
+  def part1({instructions, network}) do
+    instructions
+    |> Stream.cycle()
+    |> Enum.reduce_while({"AAA", 0}, fn
+      _side, {"ZZZ", count} -> {:halt, count}
+      side, {node, count} -> {:cont, {elem(network[node], side), count + 1}}
+    end)
+  end
+
+  # Runs forever.
+  # Need a better answer. Start at the end and validate can get there or something?
+  def part2({instructions, network}) do
+    starting_nodes =
+      network
+      |> Map.keys()
+      |> Enum.filter(&match?(<<_::binary-2, "A">>, &1))
+
+    instructions
+    |> Stream.cycle()
+    |> Enum.reduce_while({starting_nodes, 0}, fn side, {current_nodes, count} ->
+      if Enum.all?(current_nodes, &match?(<<_::binary-2, "Z">>, &1)) do
+        {:halt, count}
+      else
+        next_nodes = Enum.map(current_nodes, fn node -> elem(network[node], side) end)
+        {:cont, {next_nodes, count + 1}}
+      end
+    end)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      [instructions, nodes] = String.split(input, "\n\n")
+
+      instructions =
+        instructions
+        |> String.graphemes()
+        |> Enum.map(fn
+          "L" -> 0
+          "R" -> 1
+        end)
+
+      network =
+        for <<key::binary-3, " = (", left::binary-3, ", ", right::binary-3, ")\n" <- nodes>>,
+          into: %{},
+          do: {key, {left, right}}
+
+      {instructions, network}
+    else
+      _ -> :error
+    end
+  end
+
+  #######################
+  # HERE BE BOILERPLATE #
+  #######################
+
+  def run do
+    case input() do
+      :error -> print_usage()
+      input -> run_parts_with_timer(input)
+    end
+  end
+
+  defp run_parts_with_timer(input) do
+    run_with_timer(1, fn -> part1(input) end)
+    run_with_timer(2, fn -> part2(input) end)
+  end
+
+  defp run_with_timer(part, fun) do
+    {time, result} = :timer.tc(fun)
+    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
+    IO.puts("#{result}\n")
+  end
+
+  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
+  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
+  defp format_time(μsec), do: "#{μsec / 1_000_000}s"
+
+  defp print_usage do
+    IO.puts("Usage: elixir day8.exs input_filename")
+  end
+end
+
+Day8.run()