module Core
  module Utils
    # A client for accessing the Facebook Graph.
    # Designed for account integration.
    class FacebookGraph
      def initialize(token)
        @token = token

        validate_configuration
      end

      def user
        client.get('me', appsecret_proof: proof).parsed
      rescue OAuth2::Error
        raise Errors::TokenExchangeError, 'The token provided was invalid.'
      end

      private

      def validate_configuration
        unless Core.configuration.facebook_key
          fail Errors::ConfigurationError, 'facebook_key is missing'
        end

        unless Core.configuration.facebook_secret
          fail Errors::ConfigurationError, 'facebook_secret is missing'
        end

        fail Errors::ConfigurationError, 'token is missing' unless @token
      end

      def client
        client = OAuth2::Client.new(Core.configuration.facebook_key,
                                    Core.configuration.facebook_secret,
                                    site: 'https://graph.facebook.com')

        OAuth2::AccessToken.from_hash(client, access_token: @token)
      end

      def proof
        OpenSSL::HMAC.hexdigest(OpenSSL::Digest::SHA256.new,
                                Core.configuration.facebook_secret, @token)
      end
    end
  end
end