class GenerateSendGridHeaders
  # (uses the attr_extras gem)
  method_object :call,
    [ :mailer!, :action!, :args!, :env ]

  def call
    {
      category: [ mailer_name, "#{mailer_name}##{action}" ],
      unique_args: {
        environment: env,
        arguments: args_hash.to_json,
      },
    }
  end

  private

  def args_hash
    method_params = mailer.method(action).parameters
    param_names = method_params.map(&:last)

    rest_param_count = method_params.count { |(type, _)| type == :rest }
    raise "Only handles a single splat!" if rest_param_count > 1

    non_rest_param_count = method_params.length - rest_param_count
    non_rest_param_names = param_names.first(non_rest_param_count)
    non_rest_values = args.first(non_rest_param_count)

    # NOTE: This assumes the splat is always the last argument.
    # That is likely but not guaranteed.
    rest_values = args.from(non_rest_param_count)

    hash = Hash[ non_rest_param_names.zip(non_rest_values) ]

    if rest_values.any?
      rest_param_name = param_names.last
      hash.merge!(rest_param_name => rest_values)
    end

    hash
  end

  def mailer_name
    mailer.class.name
  end
end