MKPolyline detect self-intersecting line OBJECTIVE C - ios

Question

How can I detect if an MKPolyline intersects itself? I tried researching this but only found problems that has two or more lines. How can I detect if I only have one line/one stroke? I want to detect it after the user releases the touch.
I currently have this code in touchEnded function.
CGPoint location = [touch locationInView:self.mapView];
CLLocationCoordinate2D coordinate = [self.mapView convertPoint:location toCoordinateFromView:self.mapView];
[self.coordinates addObject:[NSValue valueWithMKCoordinate:coordinate]];
NSInteger numberOfPoints = [self.coordinates count];
if(numberOfPoints > 2)
{
[self setLineLength:[self getLengthArea]];
if([self lineLength] < 401)
{
if (numberOfPoints > 2)
{
CLLocationCoordinate2D points[numberOfPoints];
for (NSInteger i = 0; i < numberOfPoints; i++) {
points[i] = [self.coordinates[i] MKCoordinateValue];
}
[self.mapView addOverlay:[MKPolyline polylineWithCoordinates:points count:numberOfPoints]];
}
PCAnnotation *ann = [[PCAnnotation alloc] init];
[ann setCoordinate:coordinate];
ann.title = #"End";
[self.mapView addAnnotation:ann];
}
else
{
NSArray *overlayItems = [self.mapView overlays];
NSArray *annotations = [self.mapView annotations];
[self.mapView removeOverlays:overlayItems];
[self.mapView removeAnnotations:annotations];
}
}

Answer 1

MKPolyline inherits form MKMultiPoint
which has a - (MKMapPoint *)points; method,
You could try to check for intersections between all line segments.
"The points are connected end-to-end in the order they are provided."
So you can make your own line segments between each 2 points,
and after you have an Array of line segments you can check for their intersections.
Here is a C++ code snippet for checking the intersections:
It can be easily translated to Objective-C and whatever else.
public static bool LineSegmentsCross(Vector2 a, Vector2 b, Vector2 c, Vector2 d)
{
float denominator = ((b.X - a.X) * (d.Y - c.Y)) - ((b.Y - a.Y) * (d.X - c.X));
if (denominator == 0)
{
return false;
}
float numerator1 = ((a.Y - c.Y) * (d.X - c.X)) - ((a.X - c.X) * (d.Y - c.Y));
float numerator2 = ((a.Y - c.Y) * (b.X - a.X)) - ((a.X - c.X) * (b.Y - a.Y));
if (numerator1 == 0 || numerator2 == 0)
{
return false;
}
float r = numerator1 / denominator;
float s = numerator2 / denominator;
return (r > 0 && r < 1) && (s > 0 && s < 1);
}