Reporting of segment set intersections is one of the fundamental problems of computational geometry. This library implements Ivan J. Balaban's Intermediate Algorithm for finding intersecting segment pairs from a given set of N segments in the plane.

This algorithm has complexity O( n*log²(n) + k ) (where k is the number of intersecting pairs), which is much faster than the more commonly implemented O( (n+k)*log(n) ) Bentley–Ottmann algorithm for the same problem. In practice therefore, this library processes sizeable segment sets orders of magnitude more quickly than existing (Java) Bentley–Ottmann implementations.

This repository improves the abandoned original project from Google Code Archive whose author was Taras, and is provided as an artifact via Jitpack for easy use in Maven/Gradle projects.

The algorithm processes a single collection of line segments.

The algorithm itself does not check for or handle degenerate inputs and will error (generally a stack overflow) on such inputs. The library offers the findDegenerateSegments() method to identify degenerate segments before processing.

Segment sets are degenerate when:

• Contains a vertical segment (whose vertices have the same X coordinate)
• Contains segments where a vertex from each share an x coordinate.

The algorithm does not natively compute points of intersection. Rather, it returns intersecting segment pairs.

You must provide a callback function to the solver: it provides intersecting pairs — you decide what to do with them.

Random r = new Random(0);

Collection<Segment> segments = new ArrayList<>();
for (int i = 0; i < 2000; i++) {
	segments.add(new Segment(r.nextDouble(), r.nextDouble(), r.nextDouble(), r.nextDouble()));
}

Collection<Point> points = new ArrayList<>();

BalabanSolver balabanSolver = new BalabanSolver((a, b) -> {
    	// will be called by the solver for each intersecting pair
	points.add(a.getIntersection(b));
});

balabanSolver.computeIntersections(segments);