defmodule Maxwell.Middleware.Headers do
  @moduledoc """
  Add fixed headers to request's headers

  ## Examples

        # Client.ex
        use Maxwell.Builder ~(get)a
        middleware Maxwell.Middleware.Headers, %{'User-Agent' => "zhongwencool"}

        def request do
          # headers is merge to %{'User-Agent' => "zhongwencool", 'username' => "zhongwencool"}
          new()
          |> put_req_header(%{'username' => "zhongwencool"})
          |> get!
        end

  """
  use Maxwell.Middleware
  alias Maxwell.Conn

  def init(headers) do
    check_headers!(headers)
    conn = Conn.put_req_headers(%Conn{}, headers)
    conn.req_headers
  end

  def request(%Conn{} = conn, req_headers) do
    %{conn | req_headers: Map.merge(req_headers, conn.req_headers)}
  end

  defp check_headers!(headers) do
    unless Enum.all?(headers, fn {key, _value} -> is_binary(key) end) do
      raise(ArgumentError, "Header keys must be strings, but got: #{inspect(headers)}")
    end
  end
end