Throw in Triangle::NeighborAcross in case of null pointer

Add an example quad for which the exception is throwni in the tests

poly2tri/common/shapes.cc

@@ -352,12 +352,18 @@ void Triangle::SetDelunayEdgeCW(const Point& p, bool e)
 // The neighbor across to given point
 Triangle& Triangle::NeighborAcross(const Point& opoint)
 {
+  Triangle* neighbor = nullptr;
   if (&opoint == points_[0]) {
-    return *neighbors_[0];
+    neighbor = neighbors_[0];
   } else if (&opoint == points_[1]) {
-    return *neighbors_[1];
+    neighbor = neighbors_[1];
+  } else {
+    neighbor = neighbors_[2];
   }
-  return *neighbors_[2];
+  if (neighbor == nullptr) {
+      throw std::runtime_error("NeighborAcross - null neighbor");
+  }
+  return *neighbor;
 }
 
 void Triangle::DebugPrint()

testbed/data/deadly_quad.dat

@@ -0,0 +1,4 @@
+0.0     0.0
+1.0e-05 0.0
+1.1e-04 3.0e-07
+1.0e-04 3.0e-07

unittest/main.cpp

@@ -5,6 +5,7 @@
 #include <fstream>
 #include <iostream>
 #include <iterator>
+#include <stdexcept>
 
 BOOST_AUTO_TEST_CASE(BasicTest)
 {
@@ -40,6 +41,22 @@ BOOST_AUTO_TEST_CASE(QuadTest)
   }
 }
 
+BOOST_AUTO_TEST_CASE(QuadTestThrow)
+{
+  // Very narrow quad that demonstrates a failure case during triangulation
+  std::vector<p2t::Point*> polyline {
+    new p2t::Point(0.0,     0.0),
+    new p2t::Point(1.0e-05, 0.0),
+    new p2t::Point(1.1e-04, 3.0e-07),
+    new p2t::Point(1.0e-04, 3.0e-07)
+  };
+  p2t::CDT cdt{ polyline };
+  BOOST_CHECK_THROW(cdt.Triangulate(), std::runtime_error);
+  for (const auto p : polyline) {
+    delete p;
+  }
+}
+
 BOOST_AUTO_TEST_CASE(TestbedFilesTest)
 {
   for (const auto& filename : { "custom.dat", "diamond.dat", "star.dat", "test.dat" }) {