src/vs/editor/common/model/pieceTreeTextBuffer/pieceTreeBase.ts/PieceTreeBase/getAccumulatedValue
class PieceTreeBase:	private getAccumulatedValue(node: TreeNode, index: number) {
		if (index < 0) {
			return 0;
		}
		const piece = node.piece;
		const lineStarts = this._buffers[piece.bufferIndex].lineStarts;
		const expectedLineStartIndex = piece.start.line + index + 1;
		if (expectedLineStartIndex > piece.end.line) {
			return lineStarts[piece.end.line] + piece.end.column - lineStarts[piece.start.line] - piece.start.column;
		} else {
			return lineStarts[expectedLineStartIndex] - lineStarts[piece.start.line] - piece.start.column;
		}
	}