// Faster equivalent of approachSlow()
private Cell approach(ChildType direction, int levels)
{
if (levels == 0)
{
return(this);
}
if (index < 2)
{
throw new ArgumentException("Cannot find child of a polar cell");
}
var a = (uint)Misc.IntPow(3, (uint)levels);
return(new Cell(this.index * a - (uint)((5 * (4 << (2 * this.Level)) * (a - (1 << (2 * levels))) - ((byte)direction - 4) * (a - 1)) / 2)));
}