module Agents
  class DropboxWatchAgent < Agent
    include DropboxConcern

    cannot_receive_events!
    default_schedule "every_1m"

    description <<~MD
      The Dropbox Watch Agent watches the given `dir_to_watch` and emits events with the detected changes.

      #{'## Include the `dropbox-api` and `omniauth-dropbox` gems in your `Gemfile` and set `DROPBOX_OAUTH_KEY` and `DROPBOX_OAUTH_SECRET` in your environment to use Dropbox Agents.' if dependencies_missing?}
    MD

    event_description <<~MD
      The event payload will contain the following fields:

          {
            "added": [ {
              "path": "/path/to/added/file",
              "rev": "1526952fd5",
              "modified": "2017-10-14T18:39:41Z"
            } ],
            "removed": [ ... ],
            "updated": [ ... ]
          }
    MD

    def default_options
      {
        'dir_to_watch' => '/',
        'expected_update_period_in_days' => 1
      }
    end

    def validate_options
      errors.add(:base, 'The `dir_to_watch` property is required.') unless options['dir_to_watch'].present?
      errors.add(:base,
                 'Invalid `expected_update_period_in_days` format.') unless options['expected_update_period_in_days'].present? && is_positive_integer?(options['expected_update_period_in_days'])
    end

    def working?
      event_created_within?(interpolated['expected_update_period_in_days']) && !received_event_without_error?
    end

    def check
      current_contents = ls(interpolated['dir_to_watch'])
      diff = DropboxDirDiff.new(previous_contents, current_contents)
      create_event(payload: diff.to_hash) unless previous_contents.nil? || diff.empty?

      remember(current_contents)
    end

    private

    def ls(dir_to_watch)
      dropbox.ls(dir_to_watch)
        .select { |entry| entry.respond_to?(:rev) }
        .map { |file| { 'path' => file.path, 'rev' => file.rev, 'modified' => file.server_modified } }
    end

    def previous_contents
      self.memory['contents']
    end

    def remember(contents)
      self.memory['contents'] = contents
    end

    # == Auxiliary classes ==

    class DropboxDirDiff
      def initialize(previous, current)
        @previous = previous || []
        @current = current || []
      end

      def empty?
        (@previous == @current)
      end

      def to_hash
        calculate_diff
        { added: @added, removed: @removed, updated: @updated }
      end

      private

      def calculate_diff
        @updated = @current.select do |current_entry|
          previous_entry = find_by_path(@previous, current_entry['path'])
          (current_entry != previous_entry) && !previous_entry.nil?
        end

        updated_entries = @updated + @previous.select do |previous_entry|
          find_by_path(@updated, previous_entry['path'])
        end

        @added = @current - @previous - updated_entries
        @removed = @previous - @current - updated_entries
      end

      def find_by_path(array, path)
        array.find { |entry| entry['path'] == path }
      end
    end
  end
end