firestore.rules

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    function documentFieldsCheckOut(requiredFields, optionalFields) {
      let allFields = requiredFields.concat(optionalFields);
      return request.resource.data.keys().hasAll(requiredFields) 
        && request.resource.data.keys().hasOnly(allFields);
    }

    function isString(field) {
      return field is string;
    }

    function isTimestamp(field) {
      return field is timestamp;
    }

    function validateStringSize(field, min, max) {
      let stringLength = field.size();

      return stringLength >= min && stringLength <= max;
    }

    function validateTimeIsNow(field) {
      let millisNowWindow = 30 * 1000; // 30 seconds

      let serverMillis = request.time.toMillis();
      let fieldMillis = field.toMillis();

      return fieldMillis <= serverMillis + millisNowWindow 
        && fieldMillis >= serverMillis - millisNowWindow;
    }

    match /words/{wordId} {
      function validateCreateWordDocument() {
        return documentFieldsCheckOut(['word', 'createdAt'], [])
          && isString(request.resource.data.word)
          && validateStringSize(request.resource.data.word, 4, 16)
          && isTimestamp(request.resource.data.createdAt)
          && validateTimeIsNow(request.resource.data.createdAt);
      }

      allow create: if validateCreateArticleDocument();
    }
  }
}