An OAuth2 authentication token.

class Token {
 /// The token type, usually "Bearer"
 final String type;
 /// The raw token data used for authentication
 final String data;
 /// Time at which the token will be expired
 final DateTime expiry;
 /// The email address of the user, only set if the scopes include
 /// https://www.googleapis.com/auth/userinfo.email
 String get email => _email;
 /// A unique identifier of the user, only set if the scopes include
 /// https://www.googleapis.com/auth/userinfo.profile
 String get userId => _userId;
 String _email;
 String _userId;

 Token(String this.type, String this.data, DateTime this.expiry);

 factory Token.fromJson(String json) {
   final map = JSON.parse(json);
   final token = new Token(map['type'], map['data'],
       new DateTime.fromMillisecondsSinceEpoch(map['expiry']));
   token._email = map['email'];
   token._userId = map['userId'];
   return token;
 }

 bool get expired => new DateTime.now().compareTo(expiry) > 0;

 String toString() => "[Token type=$type, data=$data, expired=$expired, "
     "expiry=$expiry, email=$email, userId=$userId]";

 /// Query whether this token is still valid.
 Future<bool> validate(String clientId,
     {String service: "https://www.googleapis.com/oauth2/v1/tokeninfo"}) {
   String url = UrlPattern.generatePattern(service, {}, {"access_token": data});
   var completer = new Completer();
   var request = new HttpRequest();
   request.onLoadEnd.listen((Event e) {
     if (request.status == 200) {
       completer.complete(request.responseText);
     }
   });
   request.open("GET", url);
   request.send();

   return completer.future.then((json) {
     final data = JSON.parse(json);
     final valid = clientId == data['audience'];
     if (valid) {
       _email = data['email'];
       _userId = data['user_id'];
     }
     return valid;
   });
 }

 String toJson() {
   return JSON.stringify({
     "type": type,
     "data": data,
     "expiry": expiry.millisecondsSinceEpoch,
     "email": email,
     "userId": userId,
   });
 }

 static Token _parse(String data) {
   if (data == null) {
     throw new Exception("No auth token data");
   }

   Map<String, String> params = {};
   for (String kv in _tokenizeRelativeUrl(data)) {
     if (kv.isEmpty) {
       continue;
     }
     int eqIndex = kv.indexOf('=');
     if (eqIndex < 0) {
       params[kv] = "";
     } else {
       params[kv.substring(0, eqIndex)] = kv.substring(eqIndex + 1);
     }
   }

   if (params.containsKey('error')) {
     throw new AuthException(params['error'], params);
   }
   for (String param in ['access_token', 'token_type', 'expires_in']) {
     if (!params.containsKey(param)) {
       throw new Exception("Missing parameter $param");
     }
   }

   // Mark tokens as 'expired' 20 seconds early so it's still valid when used.
   Duration duration =
       new Duration(seconds: int.parse(params['expires_in']) - 20);
   return new Token(params['token_type'], params['access_token'],
       new DateTime.now().add(duration));
 }

 /// Extracts &-separated tokens from the path, query, and fragment of [uri].
 static List<String> _tokenizeRelativeUrl(String uri) {
   final u = Uri.parse(uri);
   final result = [];
   [u.path, u.query, u.fragment].forEach((x) {
     if (x != null) result.addAll(_tokenize(x));
   });
   return result;
 }

 static List<String> _tokenize(String data) {
   return data.isEmpty ? [] : data.split('&');
 }
}