src/vs/editor/common/model/pieceTreeTextBuffer/pieceTreeBase.ts/PieceTreeBase/getOffsetAt
class PieceTreeBase:	public getOffsetAt(lineNumber: number, column: number): number {
		let leftLen = 0; // inorder

		let x = this.root;

		while (x !== SENTINEL) {
			if (x.left !== SENTINEL && x.lf_left + 1 >= lineNumber) {
				x = x.left;
			} else if (x.lf_left + x.piece.lineFeedCnt + 1 >= lineNumber) {
				leftLen += x.size_left;
				// lineNumber >= 2
				const accumualtedValInCurrentIndex = this.getAccumulatedValue(x, lineNumber - x.lf_left - 2);
				return leftLen += accumualtedValInCurrentIndex + column - 1;
			} else {
				lineNumber -= x.lf_left + x.piece.lineFeedCnt;
				leftLen += x.size_left + x.piece.length;
				x = x.right;
			}
		}

		return leftLen;
	}