defmodule Shift73kWeb.Redirector do
  import Phoenix.Controller, only: [redirect: 2]

  def init([to: _] = opts), do: opts
  def init([external: _] = opts), do: opts
  def init(_default), do: raise("Missing required to: / external: option in redirect")

  def call(conn, [to: to]) do
    redirect(conn, to: append_query_string(conn, to))
  end
  def call(conn, [external: url]) do
    external = url
    |> URI.parse
    |> merge_query_string(conn)
    |> URI.to_string

    redirect(conn, external: external)
  end


  defp append_query_string(%Plug.Conn{query_string: ""}, path), do: path
  defp append_query_string(%Plug.Conn{query_string: query}, path), do: "#{path}?#{query}"

  defp merge_query_string(%URI{query: nil} = destination_uri, %Plug.Conn{query_string: source}) do
    %{destination_uri | query: source}
  end

  defp merge_query_string(%URI{query: destination} = destination_uri, %Plug.Conn{query_string: source}) do
    merged_query = Map.merge(
      URI.decode_query(destination),
      URI.decode_query(source)
    )

    %{destination_uri | query: URI.encode_query(merged_query)}
  end
end