monitorex

defmodule Monitorex.Assets do
  @moduledoc """
  Plug for serving Monitorex dashboard static assets (CSS and JS).

  Reads the asset files at **compile time** and serves them with
  far-future cache headers.

  ## Usage

  In your router:

      get "/dashboard-assets/*path", Monitorex.Assets, :call

  Or in a plug pipeline:

      plug Monitorex.Assets, at: "/dashboard-assets"
  """

  use Plug.Builder

  alias Plug.Conn

  @external_resource Path.join(__DIR__, "../../priv/static/app.css")
  @external_resource Path.join(__DIR__, "../../priv/static/app.js")

  @css_path Path.join(__DIR__, "../../priv/static/app.css")
  @js_path Path.join(__DIR__, "../../priv/static/app.js")

  @css_content File.read!(@css_path)
  @js_content File.read!(@js_path)

  @css_hash Base.encode16(:crypto.hash(:md5, @css_content), case: :lower)
  @js_hash Base.encode16(:crypto.hash(:md5, @js_content), case: :lower)

  @doc """
  Returns the MD5 hex digest of the CSS file contents.
  """
  def css_hash, do: @css_hash

  @doc """
  Returns the MD5 hex digest of the JS file contents.
  """
  def js_hash, do: @js_hash

  @doc """
  Plug init callback. Accepts options including `:at` for the mount path.

  ## Options

    * `:at` — the path prefix where assets are mounted (default: `"/dashboard-assets"`)
  """
  def init(_opts) do
    %{at: "/dashboard-assets"}
  end

  @doc """
  Plug call callback. Serves the requested asset file or returns 404.
  """
  def call(conn, %{at: at}) do
    path = conn.path_info
    base_path = at |> String.trim("/") |> String.split("/")

    case path -- base_path do
      ["app.css"] -> serve_css(conn)
      ["app.js"] -> serve_js(conn)
      _ -> serve_not_found(conn)
    end
  end

  defp serve_css(conn) do
    conn
    |> put_resp_content_type("text/css")
    |> put_cache_headers()
    |> send_resp(200, @css_content)
  end

  defp serve_js(conn) do
    conn
    |> put_resp_content_type("application/javascript")
    |> put_cache_headers()
    |> send_resp(200, @js_content)
  end

  defp serve_not_found(conn) do
    conn
    |> put_resp_content_type("text/plain")
    |> send_resp(404, "Not Found")
  end

  defp put_cache_headers(conn) do
    Conn.put_resp_header(conn, "cache-control", "public, max-age=31536000, immutable")
  end
end