From 3ba98e7f810f7ce36fbbae9f8d22e826c2d8ea6c Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Fri, 28 Jun 2024 18:51:49 +0900
Subject: [PATCH] Add initial PoC

---
 lib/token_server.ex             | 70 +++++++++++++++++++++++++++------
 lib/token_server/api_client.ex  | 10 +++++
 lib/token_server/application.ex |  3 +-
 3 files changed, 69 insertions(+), 14 deletions(-)
 create mode 100644 lib/token_server/api_client.ex

diff --git a/lib/token_server.ex b/lib/token_server.ex
index f7ef771..9eb3a81 100644
--- a/lib/token_server.ex
+++ b/lib/token_server.ex
@@ -1,18 +1,64 @@
 defmodule TokenServer do
-  @moduledoc """
-  Documentation for `TokenServer`.
-  """
+  use GenServer
+  require Logger
+  alias TokenServer.ApiClient
+
+  # Server
+
+  @impl GenServer
+  @spec init(any()) :: {:ok, DateTime.t()}
+  def init(_) do
+    :ets.new(:token_store, [:named_table, read_concurrency: true])
+    expires_at = get_and_store_token()
+    {:ok, expires_at}
+  end
+
+  @impl GenServer
+  @spec handle_info(any(), DateTime.t()) :: {:noreply, DateTime.t()}
+  def handle_info(:refresh, _expires_at) do
+    expires_at = get_and_store_token()
+    {:noreply, expires_at}
+  end
+
+  def handle_info(msg, expires_at) do
+    Logger.info("Received unknown message: #{inspect(msg)}")
+    {:noreply, expires_at}
+  end
+
+  @impl GenServer
+  @spec handle_call(:force_refresh, GenServer.from(), DateTime.t()) :: {:reply, :ok, DateTime.t()}
+  def handle_call(:force_refresh, _from, _expires_at) do
+    expires_at = get_and_store_token()
+    {:reply, :ok, expires_at}
+  end
+
+  defp get_and_store_token() do
+    {token, expires_at} = ApiClient.get_token()
+    :ets.insert(:token_store, {:token, token})
+
+    next_refresh = DateTime.add(expires_at, -5, :minute)
+    ms_to_refresh = DateTime.diff(next_refresh, DateTime.utc_now(), :millisecond)
+    Process.send_after(self(), :refresh, ms_to_refresh)
+
+    expires_at
+  end
+
+  # Client
 
   @doc """
-  Hello world.
-
-  ## Examples
-
-      iex> TokenServer.hello()
-      :world
-
+  A function to start this GenServer as part of the supervision tree, registered with the
+  name of the module.
   """
-  def hello do
-    :world
+  @spec start_link(any()) :: GenServer.on_start()
+  def start_link(_) do
+    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
+  end
+
+  @spec get() :: String.t()
+  def get(), do: :ets.lookup_element(:token_store, :token, 2)
+
+  @spec force_refresh() :: :ok
+  def force_refresh() do
+    GenServer.call(__MODULE__, :force_refresh)
   end
 end
diff --git a/lib/token_server/api_client.ex b/lib/token_server/api_client.ex
new file mode 100644
index 0000000..0b3fb2d
--- /dev/null
+++ b/lib/token_server/api_client.ex
@@ -0,0 +1,10 @@
+defmodule TokenServer.ApiClient do
+  def get_token do
+    Process.sleep(3000)
+
+    token = Base.encode32(:rand.bytes(10))
+    expires_at = DateTime.utc_now() |> DateTime.add(1, :hour)
+
+    {token, expires_at}
+  end
+end
diff --git a/lib/token_server/application.ex b/lib/token_server/application.ex
index c5a7995..ecd14fe 100644
--- a/lib/token_server/application.ex
+++ b/lib/token_server/application.ex
@@ -8,8 +8,7 @@ defmodule TokenServer.Application do
   @impl true
   def start(_type, _args) do
     children = [
-      # Starts a worker by calling: TokenServer.Worker.start_link(arg)
-      # {TokenServer.Worker, arg}
+      {TokenServer, nil}
     ]
 
     # See https://hexdocs.pm/elixir/Supervisor.html