class Mandrill::WebHook::Processor

  attr_accessor :params, :callback_host, :mandrill_events
  attr_accessor :on_unhandled_mandrill_events

  # Command initialise the processor with +params+ Hash.
  # +params+ is expected to contain an array of mandrill_events.
  # +callback_host+ is a handle to the controller making the request.
  def initialize(params={},callback_host=nil)
    self.params = params
    self.callback_host = callback_host
  end

  def mandrill_events
    @mandrill_events ||= JSON.parse(params['mandrill_events'] || '[]')
  rescue
    @mandrill_events = []
  end

  # Command: processes all +mandrill_events+
  def run!
    mandrill_events.each do |raw_payload|
      event_payload = wrap_payload(raw_payload)
      handler = "handle_#{event_payload.event_type}".to_sym
      if callback_host && callback_host.respond_to?(handler, true)
        callback_host.send(handler,event_payload)
      elsif self.respond_to?(handler)
        self.send(handler,event_payload)
      else
        error_message = "Expected handler method `#{handler}` for event type `#{event_payload.event_type}`"
        case on_unhandled_mandrill_events
        when :ignore
          # NOP
        when :raise_exception
          raise Mandrill::Rails::Errors::MissingEventHandler, error_message
        else
          Rails.logger.error error_message rescue nil
        end
      end
    end
  end

  # Returns a suitably ecapsulated +raw_event_payload+
  def wrap_payload(raw_event_payload)
    Mandrill::WebHook::EventDecorator[raw_event_payload]
  end

  class << self

    # Returns true if +params+ sent to +original_url+ are authentic given +expected_signature+ and +mandrill_webhook_keys+.
    def authentic?(expected_signature, mandrill_webhook_keys, original_url, params)
      result = true
      Array(mandrill_webhook_keys).each do |key|
        signature = generate_signature(key, original_url, params)
        result = (signature == expected_signature)
        break if result
      end
      result
    end

    # Method described in docs: http://help.mandrill.com/entries/23704122-Authenticating-webhook-requests
    def generate_signature(webhook_key, original_url, params)
      signed_data = original_url.dup
      params.keys.sort.each do |key|
        signed_data << key
        signed_data << params[key]
      end
      Base64.encode64("#{OpenSSL::HMAC.digest('sha1', webhook_key, signed_data)}").strip
    end

  end

end