From 69a360c48fd70085337d155429e2bac7ede4dc1a Mon Sep 17 00:00:00 2001
From: Patrick Brosi <brosi@informatik.uni-freiburg.de>
Date: Fri, 17 Aug 2018 15:07:20 +0200
Subject: [PATCH] handle cases where multiple endpoints are colinear as
 non-intersecting for line segments (because otherwise it cannot be used to
 check if intersection() has undefined behaviour, as there will be an infinite
 number of intersections)

---
 src/util/geo/Geo.h          | 8 ++++++--
 src/util/tests/TestMain.cpp | 6 ++++--
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/util/geo/Geo.h b/src/util/geo/Geo.h
index 0a7c0e6..cd93423 100644
--- a/src/util/geo/Geo.h
+++ b/src/util/geo/Geo.h
@@ -398,9 +398,13 @@ inline bool contains(const std::vector<GeometryA<T>>& multigeo,
 // _____________________________________________________________________________
 template <typename T>
 inline bool intersects(const LineSegment<T>& ls1, const LineSegment<T>& ls2) {
+  // two line segments intersect of there is a single, well-defined intersection
+  // point between them. If more than 1 endpoint is colinear with any line,
+  // the segments have infinite intersections. We handle this case as non-
+  // intersecting
   return intersects(getBoundingBox(ls1), getBoundingBox(ls2)) &&
-         (contains(ls1.first, ls2) || contains(ls1.second, ls2) ||
-          contains(ls2.first, ls1) || contains(ls2.second, ls1) ||
+         (((contains(ls1.first, ls2) ^ contains(ls1.second, ls2)) ^
+          (contains(ls2.first, ls1) ^ contains(ls2.second, ls1))) ||
           (((crossProd(ls1.first, ls2) < 0) ^
             (crossProd(ls1.second, ls2) < 0)) &&
            ((crossProd(ls2.first, ls1) < 0) ^
diff --git a/src/util/tests/TestMain.cpp b/src/util/tests/TestMain.cpp
index 26b35a8..677d574 100644
--- a/src/util/tests/TestMain.cpp
+++ b/src/util/tests/TestMain.cpp
@@ -972,12 +972,14 @@ CASE("geometry") {
 
   EXPECT(geo::intersects(lsa, lsb));
 
-  EXPECT(geo::intersects(lsa, lsa));
-  EXPECT(geo::intersects(lsb, lsb));
+  EXPECT(!geo::intersects(lsa, lsa));
+  EXPECT(!geo::intersects(lsb, lsb));
   EXPECT(!geo::intersects(lsa, lsc));
 
   EXPECT(!geo::intersects(geo::Point<double>(871569.2, 6104550.4), geo::Point<double>(871581.2, 6104536), geo::Point<double>(871580.3, 6104541.3), geo::Point<double>(871625.7, 6104510.1)));
 
+  EXPECT(!geo::intersects(geo::Point<double>(0, 0), geo::Point<double>(1, 1), geo::Point<double>(0.5, 0.5), geo::Point<double>(1.5, 1.5)));
+
   geo::Line<double> l{geo::Point<double>(1, 1), geo::Point<double>(2, 2), geo::Point<double>(2, 4)};
   EXPECT(!geo::contains(geo::Point<double>(1, 2), l));
   EXPECT(geo::contains(geo::Point<double>(2, 2), l));