fixed triangulate_naive for inner valence 2

src/polymesh/algorithms/triangulate.cc

 #include "triangulate.hh"
 
+#include <polymesh/properties.hh>
+
 #include <vector>
 
 void polymesh::triangulate_naive(polymesh::Mesh& m)
 {
+    auto is_inner_valence2 = [](vertex_handle v) { return !v.is_boundary() && valence(v) == 2; };
+
     std::vector<vertex_handle> vs;
     for (auto f : m.faces())
     {
@@ -16,8 +20,23 @@ void polymesh::triangulate_naive(polymesh::Mesh& m)
         // remove
         m.faces().remove(f);
 
+        // find non-valence 2
+        auto si = 0;
+        for (auto i = 0u; i < vs.size(); ++i)
+        {
+            if (!is_inner_valence2(vs[i]))
+            {
+                si = i;
+                break;
+            }
+        }
+        POLYMESH_ASSERT(!is_inner_valence2(vs[si]) && "could not find start vertex (second vertex must not be inner valence 2)");
+        si--; // make sure v1 is never valence 2
+        if (si < 0)
+            si += int(vs.size());
+
         // triangulate
         for (auto i = 2u; i < vs.size(); ++i)
-            m.faces().add(vs[0], vs[i - 1], vs[i]);
+            m.faces().add(vs[si], vs[(si + i - 1) % vs.size()], vs[(si + i) % vs.size()]);
     }
 }