indexCurve function
Implementation
indexCurve(Node start, minX, minY, invSize) {
var p = start;
do {
p.z ??= zOrder(p.x, p.y, minX, minY, invSize);
p.prevZ = p.prev;
p.nextZ = p.next;
p = p.next!;
} while (p != start);
p.prevZ!.nextZ = null;
p.prevZ = null;
sortLinked(p);
}