code: function(_number) {
      if ('undefined' == typeof _number) {
        return null;
      }
      else if ('number' != typeof(_number)) {
        throw new Error('Wrong parameter type');
      }
  
      var ret = '';
  
      for (var i = Math.floor(Math.log(parseInt(_number)) / Math.log(this.index.length)); i >= 0; i--) {
        ret = ret + this.index.substr((Math.floor(parseInt(_number) / this.bcpow(this.index.length, i)) % this.index.length), 1);
      }
  
      return ret.reverse();
    }
docs: [@function] AlphabeticID.encode
  [@description] Encode a number into short string
  [@param] integer
  [@return] string