src/vs/editor/contrib/folding/browser/foldingModel.ts/FoldingModel/_getLinesChecksum
class FoldingModel:	private _getLinesChecksum(lineNumber1: number, lineNumber2: number): number {
		const h = hash(this._textModel.getLineContent(lineNumber1)
			+ this._textModel.getLineContent(lineNumber2));
		return h % 1000000; // 6 digits is plenty
	}