# A Mail delivery method that randomly balances the actual delivery between different
# methods.
#
# Usage:
#
# ```ruby
#   ActionMailer::Base.add_delivery_method :balancer, BalancerDeliveryMethod
#   config.action_mailer.balancer_settings = {
#     smtp: 25,
#     sendmail: 75
#   }
#   config.action_mailer.delivery_method = :balancer
# ```
#
# Be sure to restart your server when you modify this file.
class BalancerDeliveryMethod
  FORCE_DELIVERY_METHOD_HEADER = 'X-deliver-with'
  # Allows configuring the random number generator used for selecting a delivery method,
  # mostly for testing purposes.
  mattr_accessor :random, default: Random.new

  def initialize(settings)
    @delivery_methods = settings
  end

  def deliver!(mail)
    balanced_delivery_method = delivery_method(mail)
    ApplicationMailer.wrap_delivery_behavior(mail, balanced_delivery_method)

    # Because we don't want to invoke observers or interceptors twice,
    # we can't call again `mail.deliver` here to send the email with balanced method
    # (it was first called before by deliver_now in ActiveJob or application code, which leads us here).
    #
    # Instead, we directly deliver the email from the handler (set by the wrapper above)
    # like Mail::Message.deliver does.
    #
    # See https://github.com/mikel/mail/blob/199a76bed3fc518508b46135691914a1cfd8bff8/lib/mail/message.rb#L250
    mail.delivery_handler.deliver_mail(mail) { mail.send :do_delivery }
  end

  private

  def force_delivery_method?(mail)
    @delivery_methods.keys.map(&:to_s).include?(mail[FORCE_DELIVERY_METHOD_HEADER]&.value)
  end

  def delivery_method(mail)
    return mail[FORCE_DELIVERY_METHOD_HEADER].value.to_sym if force_delivery_method?(mail)

    compatible_delivery_methods_for(mail)
      .flat_map { |delivery_method, weight| [delivery_method] * weight }
      .sample(random: self.class.random)
  end

  def compatible_delivery_methods_for(mail)
    @delivery_methods.reject { |delivery_method, _weight| delivery_method.to_s == 'dolist_api' && !Dolist::API.sendable?(mail) }
  end
end