version_router.ex

defmodule Event.Plug.API.VersionRouter do
  import Plug.Conn

  @default_version 1

  def init(options) do
    case Keyword.fetch!(options, :versions) do
      versions ->
        for {version, router} <- versions, into: %{} do
          {version, {router, router.init([])}}
        end
      _ ->
        options
    end
  end

  def call(conn, options) do
    case Map.fetch!(options, get_api_version_from_conn(conn)) do
      {router, router_options} ->
        router.call(conn, router_options)
      _ ->
        {router, router_options} = Map.fetch!(options, :default)
        router.call(conn, router_options)
    end
  end

  defp get_api_version_from_conn(conn) do
    case get_accept_header(conn) do
      [header | _] ->
        case get_version_from_accept_header(header) do
          [version | _] when byte_size(version) > 0 ->
            {version, _} = Integer.parse(version)
            version
          _ ->
            @default_version
        end
      _ ->
        @default_version
    end
  end

  defp get_accept_header(conn) do
    Plug.Conn.get_req_header(conn, "accept")
  end

  defp get_version_from_accept_header([header | _]) do
    Regex.run(~r/version=(\d)/, header, capture: :all_but_first)
  end
end