require 'google/api_client'
require 'oauth2'
require 'legato'

module Sufia
  module Analytics
    # Loads configuration options from config/analytics.yml. Expected structure:
    # `analytics:`
    # `  app_name: GOOGLE_OAUTH_APP_NAME`
    # `  app_version: GOOGLE_OAUTH_APP_VERSION`
    # `  privkey_path: GOOGLE_OAUTH_PRIVATE_KEY_PATH`
    # `  privkey_secret: GOOGLE_OAUTH_PRIVATE_KEY_SECRET`
    # `  client_email: GOOGLE_OAUTH_CLIENT_EMAIL`
    # @return [Hash] A hash containing five keys: 'app_name', 'app_version', 'client_email', 'privkey_path', 'privkey_secret'
    def self.config
      @config ||= load_config
    end
    private_class_method :config

    def self.load_config
      filename = File.join(Rails.root, 'config', 'analytics.yml')
      yaml = YAML.load(File.read(filename))
      unless yaml
        Rails.logger.error("Unable to fetch any keys from #{filename}.")
        return
      end
      yaml.fetch('analytics')
    end
    private_class_method :load_config

    # Generate an OAuth2 token for Google Analytics
    # @return [OAuth2::AccessToken] An OAuth2 access token for GA
    def self.token
      scope = 'https://www.googleapis.com/auth/analytics.readonly'
      client = Google::APIClient.new(application_name: config.fetch('app_name'),
                                     application_version: config.fetch('app_version'))
      key = Google::APIClient::PKCS12.load_key(config.fetch('privkey_path'),
                                               config.fetch('privkey_secret'))
      service_account = Google::APIClient::JWTAsserter.new(config.fetch('client_email'),
                                                           scope,
                                                           key)
      client.authorization = service_account.authorize
      oauth_client = OAuth2::Client.new('',
                                        '',
                                        authorize_url: 'https://accounts.google.com/o/oauth2/auth',
                                        token_url: 'https://accounts.google.com/o/oauth2/token')
      OAuth2::AccessToken.new(oauth_client, client.authorization.access_token)
    end
    private_class_method :token

    # Return a user object linked to a Google Analytics account
    # @return [Legato::User] A user account wit GA access
    def self.user
      Legato::User.new(token)
    end
    private_class_method :user

    # Return a Google Analytics profile matching specified ID
    # @ return [Legato::Management::Profile] A user profile associated with GA
    def self.profile
      user.profiles.detect do |profile|
        profile.web_property_id == Sufia.config.google_analytics_id
      end
    end
  end
end