function WhitespaceComputer() {
            this._heights = [];
            this._ids = [];
            this._afterLineNumbers = [];
            this._ordinals = [];
            this._prefixSum = [];
            this._prefixSumValidIndex = -1;
            this._whitespaceId2Index = {};
            this._lastWhitespaceId = 0;
        }
Represent whitespaces in between lines and provide fast CRUD management methods.
The whitespaces are sorted ascending by `afterLineNumber`.