Native watch

lib/mix_tasks.ex

@@ -100,6 +100,25 @@ defmodule Mix.Tasks.Aoc do
     create_file("#{tests_dir(day)}#{test}.#{part}", IO.read(:stdio, :eof))
   end
 
+  defp run(day, ["test", "watch"]) do
+    {:ok, pid} = Mix.Tasks.Aoc.Watcher.start_link({[dirs: ["lib/", "test/day#{day}/"]], self()})
+    Process.monitor(pid)
+    loop = fn (loop) ->
+      IO.puts(IO.ANSI.clear() <> IO.ANSI.cursor(0, 0))
+      IO.puts "At #{DateTime.utc_now(:second)}:"
+      System.shell("mix compile")
+      :code.purge(module(day))
+      :code.purge(Aoc2023.Common)
+      :code.load_file(module(day))
+      run(day, ["test"])
+      receive do
+        :update -> loop.(loop)
+        {:DOWN, _, :process, ^pid, _} -> nil
+      end
+    end
+    loop.(loop)
+  end
+
   defp run(day, ["test"]) do
     mod = module(day)
     tests = tests(day)
@@ -166,4 +185,28 @@ defmodule Mix.Tasks.Aoc do
   defp run_file(day, file \\ nil) do
     module(day).main(if file, do: read(file), else: IO.read(:stdio, :eof))
   end
+
+
+  defmodule Watcher do
+    use GenServer
+
+    def start_link(args) do
+      GenServer.start_link(__MODULE__, args)
+    end
+
+    def init({args, callback}) do
+      {:ok, watcher_pid} = FileSystem.start_link(args)
+      FileSystem.subscribe(watcher_pid)
+      {:ok, %{watcher_pid: watcher_pid, callback: callback}}
+    end
+
+    def handle_info({:file_event, watcher_pid, {_path, _events}}, %{watcher_pid: watcher_pid, callback: callback} = state) do
+      send(callback, :update)
+      {:noreply, state}
+    end
+
+    def handle_info({:file_event, _watcher_pid, :stop}, state) do
+      {:noreply, state}
+    end
+  end
 end