From 3ab886b747c4742d1662a415d683857003c0388b Mon Sep 17 00:00:00 2001
From: bubnikv <bubnikv@gmail.com>
Date: Tue, 4 Jun 2019 18:25:53 +0200
Subject: [PATCH] Fix of mesh decimation (the admesh library). Fixes "Unable to
 save project (#2445)"

---
 src/admesh/connect.cpp         | 474 ++++++++++++++++-----------------
 src/admesh/shared.cpp          | 417 ++++++++++++++---------------
 src/admesh/stl.h               |   2 +
 src/admesh/util.cpp            |  47 ++++
 src/libslic3r/TriangleMesh.cpp |  36 ++-
 5 files changed, 495 insertions(+), 481 deletions(-)

diff --git a/src/admesh/connect.cpp b/src/admesh/connect.cpp
index fb3213219..3069251d3 100644
--- a/src/admesh/connect.cpp
+++ b/src/admesh/connect.cpp
@@ -37,7 +37,6 @@ static void stl_match_neighbors_nearby(stl_file *stl,
                                        stl_hash_edge *edge_a, stl_hash_edge *edge_b);
 static void stl_record_neighbors(stl_file *stl,
                                  stl_hash_edge *edge_a, stl_hash_edge *edge_b);
-static void stl_initialize_facet_check_exact(stl_file *stl);
 static void stl_initialize_facet_check_nearby(stl_file *stl);
 static void stl_load_edge_exact(stl_file *stl, stl_hash_edge *edge, const stl_vertex *a, const stl_vertex *b);
 static int stl_load_edge_nearby(stl_file *stl, stl_hash_edge *edge,
@@ -47,63 +46,90 @@ static void insert_hash_edge(stl_file *stl, stl_hash_edge edge,
                                  stl_hash_edge *edge_a, stl_hash_edge *edge_b));
 static int stl_compare_function(stl_hash_edge *edge_a, stl_hash_edge *edge_b);
 static void stl_free_edges(stl_file *stl);
-static void stl_remove_facet(stl_file *stl, int facet_number);
 static void stl_change_vertices(stl_file *stl, int facet_num, int vnot,
                                 stl_vertex new_vertex);
 static void stl_which_vertices_to_change(stl_file *stl, stl_hash_edge *edge_a,
     stl_hash_edge *edge_b, int *facet1, int *vertex1,
     int *facet2, int *vertex2,
     stl_vertex *new_vertex1, stl_vertex *new_vertex2);
-static void stl_remove_degenerate(stl_file *stl, int facet);
 extern int stl_check_normal_vector(stl_file *stl,
                                    int facet_num, int normal_fix_flag);
-static void stl_update_connects_remove_1(stl_file *stl, int facet_num);
+
+static inline size_t hash_size_from_nr_faces(const size_t nr_faces)
+{
+	// Good primes for addressing a cca. 30 bit space.
+	// https://planetmath.org/goodhashtableprimes
+	static std::vector<uint32_t> primes{ 98317, 196613, 393241, 786433, 1572869, 3145739, 6291469, 12582917, 25165843, 50331653, 100663319, 201326611, 402653189, 805306457, 1610612741 };
+	// Find a prime number for 50% filling of the shared triangle edges in the mesh.
+	auto it = std::upper_bound(primes.begin(), primes.end(), nr_faces * 3 * 2 - 1);
+	return (it == primes.end()) ? primes.back() : *it;
+}
 
 // This function builds the neighbors list.  No modifications are made
 // to any of the facets.  The edges are said to match only if all six
 // floats of the first edge matches all six floats of the second edge.
 void stl_check_facets_exact(stl_file *stl)
 {
-  if (stl->error)
-	  return;
+	if (stl->error)
+		return;
 
-  stl->stats.connected_edges = 0;
-  stl->stats.connected_facets_1_edge = 0;
-  stl->stats.connected_facets_2_edge = 0;
-  stl->stats.connected_facets_3_edge = 0;
+  	stl->stats.connected_edges         = 0;
+  	stl->stats.connected_facets_1_edge = 0;
+  	stl->stats.connected_facets_2_edge = 0;
+  	stl->stats.connected_facets_3_edge = 0;
 
-  // If any two of the three vertices are found to be exactally the same, call them degenerate and remove the facet.
-  // Do it before the next step, as the next step stores references to the face indices in the hash tables and removing a facet
-  // will break the references.
-  for (int i = 0; i < stl->stats.number_of_facets;) {
-	  stl_facet &facet = stl->facet_start[i];
-	  if (facet.vertex[0] == facet.vertex[1] || facet.vertex[1] == facet.vertex[2] || facet.vertex[0] == facet.vertex[2]) {
-		  // Remove the degenerate facet.
-		  facet = stl->facet_start[--stl->stats.number_of_facets];
-		  stl->stats.facets_removed += 1;
-		  stl->stats.degenerate_facets += 1;
-	  } else
-		  ++ i;
-  }
+  	// If any two of the three vertices are found to be exactally the same, call them degenerate and remove the facet.
+  	// Do it before the next step, as the next step stores references to the face indices in the hash tables and removing a facet
+  	// will break the references.
+  	for (uint32_t i = 0; i < stl->stats.number_of_facets;) {
+		stl_facet &facet = stl->facet_start[i];
+	  	if (facet.vertex[0] == facet.vertex[1] || facet.vertex[1] == facet.vertex[2] || facet.vertex[0] == facet.vertex[2]) {
+		  	// Remove the degenerate facet.
+		  	facet = stl->facet_start[--stl->stats.number_of_facets];
+		  	stl->stats.facets_removed += 1;
+		  	stl->stats.degenerate_facets += 1;
+	  	} else
+		  	++ i;
+  	}
 
-  // Connect neighbor edges.
-  stl_initialize_facet_check_exact(stl);
-  for (int i = 0; i < stl->stats.number_of_facets; i++) {
-	const stl_facet &facet = stl->facet_start[i];
-    for (int j = 0; j < 3; j++) {
-	  stl_hash_edge  edge;
-	  edge.facet_number = i;
-      edge.which_edge = j;
-      stl_load_edge_exact(stl, &edge, &facet.vertex[j], &facet.vertex[(j + 1) % 3]);
-      insert_hash_edge(stl, edge, stl_record_neighbors);
-    }
-  }
-  stl_free_edges(stl);
+  	// Initialize hash table.
+	stl->stats.malloced   = 0;
+	stl->stats.freed      = 0;
+	stl->stats.collisions = 0;
+	stl->M = (int)hash_size_from_nr_faces(stl->stats.number_of_facets);
+	for (uint32_t i = 0; i < stl->stats.number_of_facets; ++ i) {
+		// initialize neighbors list to -1 to mark unconnected edges
+		stl->neighbors_start[i].neighbor[0] = -1;
+		stl->neighbors_start[i].neighbor[1] = -1;
+		stl->neighbors_start[i].neighbor[2] = -1;
+	}
+	stl->heads = (stl_hash_edge**)calloc(stl->M, sizeof(*stl->heads));
+	if (stl->heads == NULL)
+		perror("stl_initialize_facet_check_exact");
+	stl->tail = (stl_hash_edge*)malloc(sizeof(stl_hash_edge));
+	if (stl->tail == NULL) 
+		perror("stl_initialize_facet_check_exact");
+	stl->tail->next = stl->tail;
+	for (int i = 0; i < stl->M; ++ i)
+		stl->heads[i] = stl->tail;
+
+  	// Connect neighbor edges.
+	for (uint32_t i = 0; i < stl->stats.number_of_facets; i++) {
+		const stl_facet &facet = stl->facet_start[i];
+		for (int j = 0; j < 3; ++ j) {
+			stl_hash_edge  edge;
+			edge.facet_number = i;
+			edge.which_edge = j;
+			stl_load_edge_exact(stl, &edge, &facet.vertex[j], &facet.vertex[(j + 1) % 3]);
+			insert_hash_edge(stl, edge, stl_record_neighbors);
+		}
+	}
+	stl_free_edges(stl);
 
 #if 0
-  printf("Number of faces: %d, number of manifold edges: %d, number of connected edges: %d, number of unconnected edges: %d\r\n", 
-    stl->stats.number_of_facets, stl->stats.number_of_facets * 3, 
-    stl->stats.connected_edges, stl->stats.number_of_facets * 3 - stl->stats.connected_edges);
+	printf("Number of faces: %d, number of manifold edges: %d, number of connected edges: %d, number of unconnected edges: %d\r\n", 
+    	stl->stats.number_of_facets, stl->stats.number_of_facets * 3, 
+    	stl->stats.connected_edges, stl->stats.number_of_facets * 3 - stl->stats.connected_edges);
 #endif
 }
 
@@ -141,48 +167,6 @@ static void stl_load_edge_exact(stl_file *stl, stl_hash_edge *edge, const stl_ve
   }
 }
 
-static inline size_t hash_size_from_nr_faces(const size_t nr_faces)
-{
-	// Good primes for addressing a cca. 30 bit space.
-	// https://planetmath.org/goodhashtableprimes
-	static std::vector<uint32_t> primes{ 98317, 196613, 393241, 786433, 1572869, 3145739, 6291469, 12582917, 25165843, 50331653, 100663319, 201326611, 402653189, 805306457, 1610612741 };
-	// Find a prime number for 50% filling of the shared triangle edges in the mesh.
-	auto it = std::upper_bound(primes.begin(), primes.end(), nr_faces * 3 * 2 - 1);
-	return (it == primes.end()) ? primes.back() : *it;
-}
-
-static void
-stl_initialize_facet_check_exact(stl_file *stl) {
-  int i;
-
-  if (stl->error) return;
-
-  stl->stats.malloced = 0;
-  stl->stats.freed = 0;
-  stl->stats.collisions = 0;
-
-  stl->M = hash_size_from_nr_faces(stl->stats.number_of_facets);
-
-  for (i = 0; i < stl->stats.number_of_facets ; i++) {
-    /* initialize neighbors list to -1 to mark unconnected edges */
-    stl->neighbors_start[i].neighbor[0] = -1;
-    stl->neighbors_start[i].neighbor[1] = -1;
-    stl->neighbors_start[i].neighbor[2] = -1;
-  }
-
-  stl->heads = (stl_hash_edge**)calloc(stl->M, sizeof(*stl->heads));
-  if(stl->heads == NULL) perror("stl_initialize_facet_check_exact");
-
-  stl->tail = (stl_hash_edge*)malloc(sizeof(stl_hash_edge));
-  if(stl->tail == NULL) perror("stl_initialize_facet_check_exact");
-
-  stl->tail->next = stl->tail;
-
-  for(i = 0; i < stl->M; i++) {
-    stl->heads[i] = stl->tail;
-  }
-}
-
 static void insert_hash_edge(stl_file *stl, stl_hash_edge edge,
                  void (*match_neighbors)(stl_file *stl,
                      stl_hash_edge *edge_a, stl_hash_edge *edge_b))
@@ -264,7 +248,7 @@ void stl_check_facets_nearby(stl_file *stl, float tolerance)
 
   stl_initialize_facet_check_nearby(stl);
 
-  for (int i = 0; i < stl->stats.number_of_facets; ++ i) {
+  for (uint32_t i = 0; i < stl->stats.number_of_facets; ++ i) {
     //FIXME is the copy necessary?
     stl_facet facet = stl->facet_start[i];
     for (int j = 0; j < 3; j++) {
@@ -348,7 +332,7 @@ static void stl_initialize_facet_check_nearby(stl_file *stl)
   /*  tolerance = STL_MAX((stl->stats.bounding_diameter / 500000.0), tolerance);*/
   /*  tolerance *= 0.5;*/
 
-  stl->M = hash_size_from_nr_faces(stl->stats.number_of_facets);
+  stl->M = (int)hash_size_from_nr_faces(stl->stats.number_of_facets);
 
   stl->heads = (stl_hash_edge**)calloc(stl->M, sizeof(*stl->heads));
   if(stl->heads == NULL) perror("stl_initialize_facet_check_nearby");
@@ -611,181 +595,170 @@ stl_which_vertices_to_change(stl_file *stl, stl_hash_edge *edge_a,
   }
 }
 
-static void
-stl_remove_facet(stl_file *stl, int facet_number) {
-  int neighbor[3];
-  int vnot[3];
-  int i;
-  int j;
+static void remove_facet(stl_file *stl, int facet_number)
+{
+	assert(! stl->error);
+	++ stl->stats.facets_removed;
+	/* Update list of connected edges */
+	stl_neighbors &neighbors = stl->neighbors_start[facet_number];
+	// Update statistics on unconnected triangle edges.
+	switch ((neighbors.neighbor[0] == -1) + (neighbors.neighbor[1] == -1) + (neighbors.neighbor[2] == -1)) {
+	case 0: // Facet has 3 neighbors
+		-- stl->stats.connected_facets_3_edge;
+		-- stl->stats.connected_facets_2_edge;
+		-- stl->stats.connected_facets_1_edge;
+		break;
+	case 1: // Facet has 2 neighbors
+		-- stl->stats.connected_facets_2_edge;
+		-- stl->stats.connected_facets_1_edge;
+		break;
+	case 2: // Facet has 1 neighbor
+		-- stl->stats.connected_facets_1_edge;
+	case 3: // Facet has 0 neighbors
+		break;
+	default:
+		assert(false);
+	}
 
-  if (stl->error) return;
+  	if (facet_number == -- stl->stats.number_of_facets)
+  		// Removing the last face is easy, just forget the last face.
+  		return;
 
-  stl->stats.facets_removed += 1;
-  /* Update list of connected edges */
-  j = ((stl->neighbors_start[facet_number].neighbor[0] == -1) +
-       (stl->neighbors_start[facet_number].neighbor[1] == -1) +
-       (stl->neighbors_start[facet_number].neighbor[2] == -1));
-  if(j == 2) {
-    stl->stats.connected_facets_1_edge -= 1;
-  } else if(j == 1) {
-    stl->stats.connected_facets_2_edge -= 1;
-    stl->stats.connected_facets_1_edge -= 1;
-  } else if(j == 0) {
-    stl->stats.connected_facets_3_edge -= 1;
-    stl->stats.connected_facets_2_edge -= 1;
-    stl->stats.connected_facets_1_edge -= 1;
-  }
+  	// Copy the face and neighborship from the last face to facet_number.
+  	stl->facet_start[facet_number] = stl->facet_start[stl->stats.number_of_facets];
+  	neighbors = stl->neighbors_start[stl->stats.number_of_facets];
+  	// Update neighborship of faces, which used to point to the last face, now moved to facet_number.
+  	for (int i = 0; i < 3; ++ i)
+    	if (neighbors.neighbor[i] != -1) {
+	    	int &other_face_idx = stl->neighbors_start[neighbors.neighbor[i]].neighbor[(neighbors.which_vertex_not[i] + 1) % 3];
+	  		if (other_face_idx != stl->stats.number_of_facets) {
+	    		printf("in remove_facet: neighbor = %d numfacets = %d this is wrong\n", other_face_idx, stl->stats.number_of_facets);
+	    		return;
+	  		}
+	  		other_face_idx = facet_number;
+  		}
+}
 
-  stl->facet_start[facet_number] =
-    stl->facet_start[stl->stats.number_of_facets - 1];
-  /* I could reallocate at this point, but it is not really necessary. */
-  stl->neighbors_start[facet_number] =
-    stl->neighbors_start[stl->stats.number_of_facets - 1];
-  stl->stats.number_of_facets -= 1;
+static void remove_degenerate(stl_file *stl, int facet) 
+{
+	assert(! stl->error);
 
-  for(i = 0; i < 3; i++) {
-    neighbor[i] = stl->neighbors_start[facet_number].neighbor[i];
-    vnot[i] = stl->neighbors_start[facet_number].which_vertex_not[i];
-  }
+	// Update statistics on face connectivity.
+	auto stl_update_connects_remove_1 = [stl](int facet_num) {
+		assert(! stl->error);
+		//FIXME when decreasing 3_edge, should I increase 2_edge etc?
+		switch ((stl->neighbors_start[facet_num].neighbor[0] == -1) + (stl->neighbors_start[facet_num].neighbor[1] == -1) + (stl->neighbors_start[facet_num].neighbor[2] == -1)) {
+		case 0: // Facet has 3 neighbors
+			-- stl->stats.connected_facets_3_edge; break;
+		case 1: // Facet has 2 neighbors
+			-- stl->stats.connected_facets_2_edge; break;
+		case 2: // Facet has 1 neighbor
+			-- stl->stats.connected_facets_1_edge; break;
+		case 3: // Facet has 0 neighbors
+			break;
+		default:
+			assert(false);
+	  	}
+	};
 
-  for(i = 0; i < 3; i++) {
-    if(neighbor[i] != -1) {
-      if(stl->neighbors_start[neighbor[i]].neighbor[(vnot[i] + 1)% 3] !=
-          stl->stats.number_of_facets) {
-        printf("\
-in stl_remove_facet: neighbor = %d numfacets = %d this is wrong\n",
-               stl->neighbors_start[neighbor[i]].neighbor[(vnot[i] + 1)% 3],
-               stl->stats.number_of_facets);
-        return;
-      }
-      stl->neighbors_start[neighbor[i]].neighbor[(vnot[i] + 1)% 3]
-        = facet_number;
-    }
-  }
+	int edge_to_collapse = 0;
+   	if (stl->facet_start[facet].vertex[0] == stl->facet_start[facet].vertex[1]) {
+		if (stl->facet_start[facet].vertex[1] == stl->facet_start[facet].vertex[2]) {
+			// All 3 vertices are equal. Collapse the edge with no neighbor if it exists.
+			const int *nbr = stl->neighbors_start[facet].neighbor;
+			edge_to_collapse = (nbr[0] == -1) ? 0 : (nbr[1] == -1) ? 1 : 2;
+		} else {
+			edge_to_collapse = 0;
+		}
+  	} else if (stl->facet_start[facet].vertex[1] == stl->facet_start[facet].vertex[2]) {
+		edge_to_collapse = 1;
+  	} else if (stl->facet_start[facet].vertex[2] == stl->facet_start[facet].vertex[0]) {
+		edge_to_collapse = 2;
+  	} else {
+    	// No degenerate. Function shouldn't have been called.
+    	return;
+  	}
+
+	int edge[3] = { (edge_to_collapse + 1) % 3, (edge_to_collapse + 2) % 3, edge_to_collapse };
+	int neighbor[] = {
+		stl->neighbors_start[facet].neighbor[edge[0]],
+		stl->neighbors_start[facet].neighbor[edge[1]],
+		stl->neighbors_start[facet].neighbor[edge[2]]
+	};
+	int vnot[] = {
+		stl->neighbors_start[facet].which_vertex_not[edge[0]],
+		stl->neighbors_start[facet].which_vertex_not[edge[1]],
+		stl->neighbors_start[facet].which_vertex_not[edge[2]]
+	};
+	// Update statistics on edge connectivity.
+  	if (neighbor[0] == -1)
+    	stl_update_connects_remove_1(neighbor[1]);
+  	if (neighbor[1] == -1)
+    	stl_update_connects_remove_1(neighbor[0]);
+
+  	if (neighbor[0] >= 0) {
+		if (neighbor[1] >= 0) {
+			// Adjust the "flip" flag for the which_vertex_not values.
+			if (vnot[0] > 2) {
+				if (vnot[1] > 2) {
+					// The face to be removed has its normal flipped compared to the left & right neighbors, therefore after removing this face
+					// the two remaining neighbors will be oriented correctly.
+					vnot[0] -= 3;
+					vnot[1] -= 3;
+				} else
+					// One neighbor has its normal inverted compared to the face to be removed, the other is oriented equally.
+					// After removal, the two neighbors will have their normals flipped.
+					vnot[1] += 3;
+			} else if (vnot[1] > 2)
+				// One neighbor has its normal inverted compared to the face to be removed, the other is oriented equally.
+				// After removal, the two neighbors will have their normals flipped.
+				vnot[0] += 3;
+		}
+		stl->neighbors_start[neighbor[0]].neighbor[(vnot[0] + 1) % 3] = (neighbor[0] == neighbor[1]) ? -1 : neighbor[1];
+    	stl->neighbors_start[neighbor[0]].which_vertex_not[(vnot[0] + 1) % 3] = vnot[1];
+  	}
+  	if (neighbor[1] >= 0) {
+		stl->neighbors_start[neighbor[1]].neighbor[(vnot[1] + 1) % 3] = (neighbor[0] == neighbor[1]) ? -1 : neighbor[0];
+    	stl->neighbors_start[neighbor[1]].which_vertex_not[(vnot[1] + 1) % 3] = vnot[0];
+  	}
+	if (neighbor[2] >= 0) {
+		stl_update_connects_remove_1(neighbor[2]);
+		stl->neighbors_start[neighbor[2]].neighbor[(vnot[2] + 1) % 3] = -1;
+	}
+
+  	remove_facet(stl, facet);
 }
 
 void stl_remove_unconnected_facets(stl_file *stl)
 {
-  /* A couple of things need to be done here.  One is to remove any */
-  /* completely unconnected facets (0 edges connected) since these are */
-  /* useless and could be completely wrong.   The second thing that needs to */
-  /* be done is to remove any degenerate facets that were created during */
-  /* stl_check_facets_nearby(). */
-  if (stl->error)
-    return;
+	// A couple of things need to be done here.  One is to remove any completely unconnected facets (0 edges connected) since these are
+	// useless and could be completely wrong.   The second thing that needs to be done is to remove any degenerate facets that were created during
+	// stl_check_facets_nearby().
+	if (stl->error)
+		return;
 
-  // remove degenerate facets
-  for (int i = 0; i < stl->stats.number_of_facets; ++ i) {
-    if(stl->facet_start[i].vertex[0] == stl->facet_start[i].vertex[1] ||
-       stl->facet_start[i].vertex[0] == stl->facet_start[i].vertex[2] ||
-       stl->facet_start[i].vertex[1] == stl->facet_start[i].vertex[2]) {
-      stl_remove_degenerate(stl, i);
-      i--;
-    }
-  }
+	// remove degenerate facets
+	for (uint32_t i = 0; i < stl->stats.number_of_facets;)
+		if (stl->facet_start[i].vertex[0] == stl->facet_start[i].vertex[1] ||
+			stl->facet_start[i].vertex[0] == stl->facet_start[i].vertex[2] ||
+			stl->facet_start[i].vertex[1] == stl->facet_start[i].vertex[2]) {
+			remove_degenerate(stl, i);
+//			assert(stl_validate(stl));
+		} else
+			++ i;
 
-  if(stl->stats.connected_facets_1_edge < stl->stats.number_of_facets) {
-    // remove completely unconnected facets
-    for (int i = 0; i < stl->stats.number_of_facets; i++) {
-      if (stl->neighbors_start[i].neighbor[0] == -1 &&
-          stl->neighbors_start[i].neighbor[1] == -1 &&
-          stl->neighbors_start[i].neighbor[2] == -1) {
-        // This facet is completely unconnected.  Remove it.
-        stl_remove_facet(stl, i);
-        -- i;
-      }
-    }
-  }
-}
-
-static void
-stl_remove_degenerate(stl_file *stl, int facet) {
-  int edge1;
-  int edge2;
-  int edge3;
-  int neighbor1;
-  int neighbor2;
-  int neighbor3;
-  int vnot1;
-  int vnot2;
-  int vnot3;
-
-  if (stl->error) return;
-
-  if (stl->facet_start[facet].vertex[0] == stl->facet_start[facet].vertex[1] &&
-      stl->facet_start[facet].vertex[1] == stl->facet_start[facet].vertex[2]) {
-    /* all 3 vertices are equal.  Just remove the facet.  I don't think*/
-    /* this is really possible, but just in case... */
-    printf("removing a facet in stl_remove_degenerate\n");
-    stl_remove_facet(stl, facet);
-    return;
-  }
-
-  if (stl->facet_start[facet].vertex[0] == stl->facet_start[facet].vertex[1]) {
-    edge1 = 1;
-    edge2 = 2;
-    edge3 = 0;
-  } else if (stl->facet_start[facet].vertex[1] == stl->facet_start[facet].vertex[2]) {
-    edge1 = 0;
-    edge2 = 2;
-    edge3 = 1;
-  } else if (stl->facet_start[facet].vertex[2] == stl->facet_start[facet].vertex[0]) {
-    edge1 = 0;
-    edge2 = 1;
-    edge3 = 2;
-  } else {
-    /* No degenerate. Function shouldn't have been called. */
-    return;
-  }
-  neighbor1 = stl->neighbors_start[facet].neighbor[edge1];
-  neighbor2 = stl->neighbors_start[facet].neighbor[edge2];
-
-  if(neighbor1 == -1) {
-    stl_update_connects_remove_1(stl, neighbor2);
-  }
-  if(neighbor2 == -1) {
-    stl_update_connects_remove_1(stl, neighbor1);
-  }
-
-
-  neighbor3 = stl->neighbors_start[facet].neighbor[edge3];
-  vnot1 = stl->neighbors_start[facet].which_vertex_not[edge1];
-  vnot2 = stl->neighbors_start[facet].which_vertex_not[edge2];
-  vnot3 = stl->neighbors_start[facet].which_vertex_not[edge3];
-
-  if(neighbor1 >= 0){
-    stl->neighbors_start[neighbor1].neighbor[(vnot1 + 1) % 3] = neighbor2;
-    stl->neighbors_start[neighbor1].which_vertex_not[(vnot1 + 1) % 3] = vnot2;
-  }
-  if(neighbor2 >= 0){
-    stl->neighbors_start[neighbor2].neighbor[(vnot2 + 1) % 3] = neighbor1;
-    stl->neighbors_start[neighbor2].which_vertex_not[(vnot2 + 1) % 3] = vnot1;
-  }
-
-  stl_remove_facet(stl, facet);
-
-  if(neighbor3 >= 0) {
-    stl_update_connects_remove_1(stl, neighbor3);
-    stl->neighbors_start[neighbor3].neighbor[(vnot3 + 1) % 3] = -1;
-  }
-}
-
-void
-stl_update_connects_remove_1(stl_file *stl, int facet_num) {
-  int j;
-
-  if (stl->error) return;
-  /* Update list of connected edges */
-  j = ((stl->neighbors_start[facet_num].neighbor[0] == -1) +
-       (stl->neighbors_start[facet_num].neighbor[1] == -1) +
-       (stl->neighbors_start[facet_num].neighbor[2] == -1));
-  if(j == 0) {		       /* Facet has 3 neighbors */
-    stl->stats.connected_facets_3_edge -= 1;
-  } else if(j == 1) {	     /* Facet has 2 neighbors */
-    stl->stats.connected_facets_2_edge -= 1;
-  } else if(j == 2) {	     /* Facet has 1 neighbor  */
-    stl->stats.connected_facets_1_edge -= 1;
-  }
+	if (stl->stats.connected_facets_1_edge < (int)stl->stats.number_of_facets) {
+		// remove completely unconnected facets
+		for (uint32_t i = 0; i < stl->stats.number_of_facets;)
+			if (stl->neighbors_start[i].neighbor[0] == -1 &&
+				stl->neighbors_start[i].neighbor[1] == -1 &&
+				stl->neighbors_start[i].neighbor[2] == -1) {
+				// This facet is completely unconnected.  Remove it.
+				remove_facet(stl, i);
+				assert(stl_validate(stl));
+			} else
+				++ i;
+	}
 }
 
 void
@@ -801,7 +774,6 @@ stl_fill_holes(stl_file *stl) {
   int next_edge;
   int pivot_vertex;
   int next_facet;
-  int i;
   int j;
   int k;
 
@@ -809,7 +781,7 @@ stl_fill_holes(stl_file *stl) {
 
   /* Insert all unconnected edges into hash list */
   stl_initialize_facet_check_nearby(stl);
-  for(i = 0; i < stl->stats.number_of_facets; i++) {
+  for (uint32_t i = 0; i < stl->stats.number_of_facets; i++) {
     facet = stl->facet_start[i];
     for(j = 0; j < 3; j++) {
       if(stl->neighbors_start[i].neighbor[j] != -1) continue;
@@ -822,7 +794,7 @@ stl_fill_holes(stl_file *stl) {
     }
   }
 
-  for(i = 0; i < stl->stats.number_of_facets; i++) {
+  for (uint32_t i = 0; i < stl->stats.number_of_facets; i++) {
     facet = stl->facet_start[i];
     neighbors_initial[0] = stl->neighbors_start[i].neighbor[0];
     neighbors_initial[1] = stl->neighbors_start[i].neighbor[1];
@@ -900,7 +872,7 @@ stl_add_facet(stl_file *stl, stl_facet *new_facet) {
   if (stl->error) return;
 
   stl->stats.facets_added += 1;
-  if(stl->stats.facets_malloced < stl->stats.number_of_facets + 1) {
+  if(stl->stats.facets_malloced < (int)stl->stats.number_of_facets + 1) {
     stl->facet_start = (stl_facet*)realloc(stl->facet_start,
                                            (sizeof(stl_facet) * (stl->stats.facets_malloced + 256)));
     if(stl->facet_start == NULL) perror("stl_add_facet");
diff --git a/src/admesh/shared.cpp b/src/admesh/shared.cpp
index c8c17ccd5..2ad270903 100644
--- a/src/admesh/shared.cpp
+++ b/src/admesh/shared.cpp
@@ -23,242 +23,239 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include <vector>
+
 #include <boost/nowide/cstdio.hpp>
 
 #include "stl.h"
 
-void
-stl_invalidate_shared_vertices(stl_file *stl) {
-  if (stl->error) return;
+void stl_invalidate_shared_vertices(stl_file *stl)
+{
+ 	if (stl->error) 
+    	return;
 
-  if (stl->v_indices != NULL) {
-    free(stl->v_indices);
-    stl->v_indices = NULL;
-  }
-  if (stl->v_shared != NULL) {
-    free(stl->v_shared);
-    stl->v_shared = NULL;
-  }
+  	if (stl->v_indices != nullptr) {
+    	free(stl->v_indices);
+    	stl->v_indices = nullptr;
+  	}
+  	if (stl->v_shared != nullptr) {
+    	free(stl->v_shared);
+    	stl->v_shared = nullptr;
+	}
 }
 
-void
-stl_generate_shared_vertices(stl_file *stl) {
-  int i;
-  int j;
-  int first_facet;
-  int direction;
-  int facet_num;
-  int vnot;
-  int next_edge;
-  int pivot_vertex;
-  int next_facet;
-  int reversed;
+void stl_generate_shared_vertices(stl_file *stl)
+{
+	if (stl->error)
+		return;
 
-  if (stl->error) return;
+	/* make sure this function is idempotent and does not leak memory */
+	stl_invalidate_shared_vertices(stl);
 
-  /* make sure this function is idempotent and does not leak memory */
-  stl_invalidate_shared_vertices(stl);
+	// 3 indices to vertex per face
+	stl->v_indices = (v_indices_struct*)calloc(stl->stats.number_of_facets, sizeof(v_indices_struct));
+	if (stl->v_indices == nullptr) 
+		perror("stl_generate_shared_vertices");
+	// Shared vertices (3D coordinates)
+	stl->v_shared = (stl_vertex*)calloc((stl->stats.number_of_facets / 2), sizeof(stl_vertex));
+	if (stl->v_shared == nullptr) 
+		perror("stl_generate_shared_vertices");
+	stl->stats.shared_malloced = stl->stats.number_of_facets / 2;
+	stl->stats.shared_vertices = 0;
 
-  stl->v_indices = (v_indices_struct*)
-                   calloc(stl->stats.number_of_facets, sizeof(v_indices_struct));
-  if(stl->v_indices == NULL) perror("stl_generate_shared_vertices");
-  stl->v_shared = (stl_vertex*)
-                  calloc((stl->stats.number_of_facets / 2), sizeof(stl_vertex));
-  if(stl->v_shared == NULL) perror("stl_generate_shared_vertices");
-  stl->stats.shared_malloced = stl->stats.number_of_facets / 2;
-  stl->stats.shared_vertices = 0;
+	for (uint32_t i = 0; i < stl->stats.number_of_facets; ++ i) {
+		// vertex index -1 means no shared vertex was assigned yet.
+		stl->v_indices[i].vertex[0] = -1;
+		stl->v_indices[i].vertex[1] = -1;
+		stl->v_indices[i].vertex[2] = -1;
+	}
 
-  for(i = 0; i < stl->stats.number_of_facets; i++) {
-    stl->v_indices[i].vertex[0] = -1;
-    stl->v_indices[i].vertex[1] = -1;
-    stl->v_indices[i].vertex[2] = -1;
-  }
+	// A degenerate mesh may contain loops: Traversing a fan will end up in an endless loop
+	// while never reaching the starting face. To avoid these endless loops, traversed faces at each fan traversal
+	// are marked with a unique fan_traversal_stamp.
+	unsigned int			  fan_traversal_stamp = 0;
+	std::vector<unsigned int> fan_traversal_facet_visited(stl->stats.number_of_facets, 0);
 
+	for (uint32_t facet_idx = 0; facet_idx < stl->stats.number_of_facets; ++ facet_idx) {
+		for (int j = 0; j < 3; ++ j) {
+			if (stl->v_indices[facet_idx].vertex[j] != -1)
+				// Shared vertex was already assigned.
+				continue;
+			// Create a new shared vertex.
+			if (stl->stats.shared_vertices == stl->stats.shared_malloced) {
+				stl->stats.shared_malloced += 1024;
+				stl->v_shared = (stl_vertex*)realloc(stl->v_shared, stl->stats.shared_malloced * sizeof(stl_vertex));
+				if(stl->v_shared == nullptr) 
+					perror("stl_generate_shared_vertices");
+			}
+			stl->v_shared[stl->stats.shared_vertices] = stl->facet_start[facet_idx].vertex[j];
+			// Traverse the fan around the j-th vertex of the i-th face, assign the newly created shared vertex index to all the neighboring triangles in the triangle fan.
+			int  facet_in_fan_idx 	= facet_idx;
+			bool edge_direction 	= false;
+			bool traversal_reversed = false;
+			int  vnot      			= (j + 2) % 3;
+			// Increase the 
+			++ fan_traversal_stamp;
+			for (;;) {
+				// Next edge on facet_in_fan_idx to be traversed. The edge is indexed by its starting vertex index.
+				int next_edge    = 0;
+				// Vertex index in facet_in_fan_idx, which is being pivoted around, and which is being assigned a new shared vertex.
+				int pivot_vertex = 0;
+				if (vnot > 2) {
+					// The edge of facet_in_fan_idx opposite to vnot is equally oriented, therefore
+					// the neighboring facet is flipped.
+			  		if (! edge_direction) {
+			    		pivot_vertex = (vnot + 2) % 3;
+			    		next_edge    = pivot_vertex;			    		
+			  		} else {
+			    		pivot_vertex = (vnot + 1) % 3;
+			    		next_edge    = vnot % 3;
+			  		}
+			  		edge_direction = ! edge_direction;
+				} else {
+					// The neighboring facet is correctly oriented.
+			  		if (! edge_direction) {
+			    		pivot_vertex = (vnot + 1) % 3;
+			    		next_edge    = vnot;
+			  		} else {
+			    		pivot_vertex = (vnot + 2) % 3;
+			    		next_edge    = pivot_vertex;
+			  		}
+				}
+				stl->v_indices[facet_in_fan_idx].vertex[pivot_vertex] = stl->stats.shared_vertices;
+				fan_traversal_facet_visited[facet_in_fan_idx] = fan_traversal_stamp;
 
-  for(i = 0; i < stl->stats.number_of_facets; i++) {
-    first_facet = i;
-    for(j = 0; j < 3; j++) {
-      if(stl->v_indices[i].vertex[j] != -1) {
-        continue;
-      }
-      if(stl->stats.shared_vertices == stl->stats.shared_malloced) {
-        stl->stats.shared_malloced += 1024;
-        stl->v_shared = (stl_vertex*)realloc(stl->v_shared,
-                                             stl->stats.shared_malloced * sizeof(stl_vertex));
-        if(stl->v_shared == NULL) perror("stl_generate_shared_vertices");
-      }
+				// next_edge is an index of the starting vertex of the edge, not an index of the opposite vertex to the edge!
+				int next_facet = stl->neighbors_start[facet_in_fan_idx].neighbor[next_edge];
+				if (next_facet == -1) {
+					// No neighbor going in the current direction.
+					if (traversal_reversed) {
+						// Went to one limit, then turned back and reached the other limit. Quit the fan traversal.
+					    break;
+					} else {
+						// Reached the first limit. Now try to reverse and traverse up to the other limit.
+					    edge_direction        = true;
+					    vnot 	         	  = (j + 1) % 3;
+					    traversal_reversed    = true;
+				    	facet_in_fan_idx      = facet_idx;
+					}
+				} else if (next_facet == facet_idx) {
+					// Traversed a closed fan all around.
+//					assert(! traversal_reversed);
+					break;
+				} else if (next_facet >= (int)stl->stats.number_of_facets) {
+					// The mesh is not valid!
+					// assert(false);
+					break;
+				} else if (fan_traversal_facet_visited[next_facet] == fan_traversal_stamp) {
+					// Traversed a closed fan all around, but did not reach the starting face.
+					// This indicates an invalid geometry (non-manifold).
+					//assert(false);
+					break;
+				} else {
+					// Continue traversal.
+					// next_edge is an index of the starting vertex of the edge, not an index of the opposite vertex to the edge!
+					vnot = stl->neighbors_start[facet_in_fan_idx].which_vertex_not[next_edge];
+					facet_in_fan_idx = next_facet;
+				}
+			}
 
-      stl->v_shared[stl->stats.shared_vertices] =
-        stl->facet_start[i].vertex[j];
-
-      direction = 0;
-      reversed = 0;
-      facet_num = i;
-      vnot = (j + 2) % 3;
-
-      for(;;) {
-        if(vnot > 2) {
-          if(direction == 0) {
-            pivot_vertex = (vnot + 2) % 3;
-            next_edge = pivot_vertex;
-            direction = 1;
-          } else {
-            pivot_vertex = (vnot + 1) % 3;
-            next_edge = vnot % 3;
-            direction = 0;
-          }
-        } else {
-          if(direction == 0) {
-            pivot_vertex = (vnot + 1) % 3;
-            next_edge = vnot;
-          } else {
-            pivot_vertex = (vnot + 2) % 3;
-            next_edge = pivot_vertex;
-          }
-        }
-        stl->v_indices[facet_num].vertex[pivot_vertex] =
-          stl->stats.shared_vertices;
-
-        next_facet = stl->neighbors_start[facet_num].neighbor[next_edge];
-        if(next_facet == -1) {
-          if(reversed) {
-            break;
-          } else {
-            direction = 1;
-            vnot = (j + 1) % 3;
-            reversed = 1;
-            facet_num = first_facet;
-          }
-        } else if(next_facet != first_facet) {
-          vnot = stl->neighbors_start[facet_num].
-                 which_vertex_not[next_edge];
-          facet_num = next_facet;
-        } else {
-          break;
-        }
-      }
-      stl->stats.shared_vertices += 1;
-    }
-  }
+			++ stl->stats.shared_vertices;
+		}
+	}
 }
 
-void
-stl_write_off(stl_file *stl, const char *file) {
-  int i;
-  FILE      *fp;
-  char      *error_msg;
+void stl_write_off(stl_file *stl, const char *file)
+{
+	if (stl->error)
+		return;
 
-  if (stl->error) return;
+	/* Open the file */
+	FILE *fp = boost::nowide::fopen(file, "w");
+	if (fp == nullptr) {
+		char *error_msg = (char*)malloc(81 + strlen(file)); /* Allow 80 chars+file size for message */
+		sprintf(error_msg, "stl_write_ascii: Couldn't open %s for writing", file);
+		perror(error_msg);
+		free(error_msg);
+		stl->error = 1;
+		return;
+	}
 
-  /* Open the file */
-  fp = boost::nowide::fopen(file, "w");
-  if(fp == NULL) {
-    error_msg = (char*)
-                malloc(81 + strlen(file)); /* Allow 80 chars+file size for message */
-    sprintf(error_msg, "stl_write_ascii: Couldn't open %s for writing",
-            file);
-    perror(error_msg);
-    free(error_msg);
-    stl->error = 1;
-    return;
-  }
-
-  fprintf(fp, "OFF\n");
-  fprintf(fp, "%d %d 0\n",
-          stl->stats.shared_vertices, stl->stats.number_of_facets);
-
-  for(i = 0; i < stl->stats.shared_vertices; i++) {
-    fprintf(fp, "\t%f %f %f\n",
-            stl->v_shared[i](0), stl->v_shared[i](1), stl->v_shared[i](2));
-  }
-  for(i = 0; i < stl->stats.number_of_facets; i++) {
-    fprintf(fp, "\t3 %d %d %d\n", stl->v_indices[i].vertex[0],
-            stl->v_indices[i].vertex[1], stl->v_indices[i].vertex[2]);
-  }
-  fclose(fp);
+	fprintf(fp, "OFF\n");
+	fprintf(fp, "%d %d 0\n", stl->stats.shared_vertices, stl->stats.number_of_facets);
+	for (int i = 0; i < stl->stats.shared_vertices; ++ i)
+		fprintf(fp, "\t%f %f %f\n", stl->v_shared[i](0), stl->v_shared[i](1), stl->v_shared[i](2));
+	for (uint32_t i = 0; i < stl->stats.number_of_facets; ++ i)
+		fprintf(fp, "\t3 %d %d %d\n", stl->v_indices[i].vertex[0], stl->v_indices[i].vertex[1], stl->v_indices[i].vertex[2]);
+	fclose(fp);
 }
 
-void
-stl_write_vrml(stl_file *stl, const char *file) {
-  int i;
-  FILE      *fp;
-  char      *error_msg;
+void stl_write_vrml(stl_file *stl, const char *file)
+{
+  	if (stl->error) 
+  		return;
 
-  if (stl->error) return;
+	/* Open the file */
+  	FILE *fp = boost::nowide::fopen(file, "w");
+	if (fp == nullptr) {
+  		char *error_msg = (char*)malloc(81 + strlen(file)); /* Allow 80 chars+file size for message */
+		sprintf(error_msg, "stl_write_ascii: Couldn't open %s for writing", file);
+		perror(error_msg);
+		free(error_msg);
+		stl->error = 1;
+		return;
+	}
 
-  /* Open the file */
-  fp = boost::nowide::fopen(file, "w");
-  if(fp == NULL) {
-    error_msg = (char*)
-                malloc(81 + strlen(file)); /* Allow 80 chars+file size for message */
-    sprintf(error_msg, "stl_write_ascii: Couldn't open %s for writing",
-            file);
-    perror(error_msg);
-    free(error_msg);
-    stl->error = 1;
-    return;
-  }
+	fprintf(fp, "#VRML V1.0 ascii\n\n");
+	fprintf(fp, "Separator {\n");
+	fprintf(fp, "\tDEF STLShape ShapeHints {\n");
+	fprintf(fp, "\t\tvertexOrdering COUNTERCLOCKWISE\n");
+	fprintf(fp, "\t\tfaceType CONVEX\n");
+	fprintf(fp, "\t\tshapeType SOLID\n");
+	fprintf(fp, "\t\tcreaseAngle 0.0\n");
+	fprintf(fp, "\t}\n");
+	fprintf(fp, "\tDEF STLModel Separator {\n");
+	fprintf(fp, "\t\tDEF STLColor Material {\n");
+	fprintf(fp, "\t\t\temissiveColor 0.700000 0.700000 0.000000\n");
+	fprintf(fp, "\t\t}\n");
+	fprintf(fp, "\t\tDEF STLVertices Coordinate3 {\n");
+	fprintf(fp, "\t\t\tpoint [\n");
 
-  fprintf(fp, "#VRML V1.0 ascii\n\n");
-  fprintf(fp, "Separator {\n");
-  fprintf(fp, "\tDEF STLShape ShapeHints {\n");
-  fprintf(fp, "\t\tvertexOrdering COUNTERCLOCKWISE\n");
-  fprintf(fp, "\t\tfaceType CONVEX\n");
-  fprintf(fp, "\t\tshapeType SOLID\n");
-  fprintf(fp, "\t\tcreaseAngle 0.0\n");
-  fprintf(fp, "\t}\n");
-  fprintf(fp, "\tDEF STLModel Separator {\n");
-  fprintf(fp, "\t\tDEF STLColor Material {\n");
-  fprintf(fp, "\t\t\temissiveColor 0.700000 0.700000 0.000000\n");
-  fprintf(fp, "\t\t}\n");
-  fprintf(fp, "\t\tDEF STLVertices Coordinate3 {\n");
-  fprintf(fp, "\t\t\tpoint [\n");
+	int i = 0;
+	for (; i < (stl->stats.shared_vertices - 1); i++)
+		fprintf(fp, "\t\t\t\t%f %f %f,\n", stl->v_shared[i](0), stl->v_shared[i](1), stl->v_shared[i](2));
+	fprintf(fp, "\t\t\t\t%f %f %f]\n", stl->v_shared[i](0), stl->v_shared[i](1), stl->v_shared[i](2));
+	fprintf(fp, "\t\t}\n");
+	fprintf(fp, "\t\tDEF STLTriangles IndexedFaceSet {\n");
+	fprintf(fp, "\t\t\tcoordIndex [\n");
 
-  for(i = 0; i < (stl->stats.shared_vertices - 1); i++) {
-    fprintf(fp, "\t\t\t\t%f %f %f,\n",
-            stl->v_shared[i](0), stl->v_shared[i](1), stl->v_shared[i](2));
-  }
-  fprintf(fp, "\t\t\t\t%f %f %f]\n",
-          stl->v_shared[i](0), stl->v_shared[i](1), stl->v_shared[i](2));
-  fprintf(fp, "\t\t}\n");
-  fprintf(fp, "\t\tDEF STLTriangles IndexedFaceSet {\n");
-  fprintf(fp, "\t\t\tcoordIndex [\n");
-
-  for(i = 0; i < (stl->stats.number_of_facets - 1); i++) {
-    fprintf(fp, "\t\t\t\t%d, %d, %d, -1,\n", stl->v_indices[i].vertex[0],
-            stl->v_indices[i].vertex[1], stl->v_indices[i].vertex[2]);
-  }
-  fprintf(fp, "\t\t\t\t%d, %d, %d, -1]\n", stl->v_indices[i].vertex[0],
-          stl->v_indices[i].vertex[1], stl->v_indices[i].vertex[2]);
-  fprintf(fp, "\t\t}\n");
-  fprintf(fp, "\t}\n");
-  fprintf(fp, "}\n");
-  fclose(fp);
+	for (int i = 0; i + 1 < (int)stl->stats.number_of_facets; ++ i)
+		fprintf(fp, "\t\t\t\t%d, %d, %d, -1,\n", stl->v_indices[i].vertex[0], stl->v_indices[i].vertex[1], stl->v_indices[i].vertex[2]);
+	fprintf(fp, "\t\t\t\t%d, %d, %d, -1]\n", stl->v_indices[i].vertex[0], stl->v_indices[i].vertex[1], stl->v_indices[i].vertex[2]);
+	fprintf(fp, "\t\t}\n");
+	fprintf(fp, "\t}\n");
+	fprintf(fp, "}\n");
+	fclose(fp);
 }
 
-void stl_write_obj (stl_file *stl, const char *file) {
-  int i;
-  FILE* fp;
+void stl_write_obj (stl_file *stl, const char *file)
+{
+	if (stl->error)
+		return;
 
-  if (stl->error) return;
+  	FILE *fp = boost::nowide::fopen(file, "w");
+  	if (fp == nullptr) {
+    	char* error_msg = (char*)malloc(81 + strlen(file)); /* Allow 80 chars+file size for message */
+    	sprintf(error_msg, "stl_write_ascii: Couldn't open %s for writing", file);
+    	perror(error_msg);
+    	free(error_msg);
+    	stl->error = 1;
+    	return;
+  	}
 
-  /* Open the file */
-  fp = boost::nowide::fopen(file, "w");
-  if (fp == NULL) {
-    char* error_msg = (char*)malloc(81 + strlen(file)); /* Allow 80 chars+file size for message */
-    sprintf(error_msg, "stl_write_ascii: Couldn't open %s for writing", file);
-    perror(error_msg);
-    free(error_msg);
-    stl->error = 1;
-    return;
-  }
-
-  for (i = 0; i < stl->stats.shared_vertices; i++) {
-    fprintf(fp, "v %f %f %f\n", stl->v_shared[i](0), stl->v_shared[i](1), stl->v_shared[i](2));
-  }
-  for (i = 0; i < stl->stats.number_of_facets; i++) {
-    fprintf(fp, "f %d %d %d\n", stl->v_indices[i].vertex[0]+1, stl->v_indices[i].vertex[1]+1, stl->v_indices[i].vertex[2]+1);
-  }
-
-  fclose(fp);
+	for (int i = 0; i < stl->stats.shared_vertices; ++ i)
+    	fprintf(fp, "v %f %f %f\n", stl->v_shared[i](0), stl->v_shared[i](1), stl->v_shared[i](2));
+  	for (uint32_t i = 0; i < stl->stats.number_of_facets; ++ i)
+    	fprintf(fp, "f %d %d %d\n", stl->v_indices[i].vertex[0]+1, stl->v_indices[i].vertex[1]+1, stl->v_indices[i].vertex[2]+1);
+  	fclose(fp);
 }
diff --git a/src/admesh/stl.h b/src/admesh/stl.h
index f867e197b..5ecd94bd1 100644
--- a/src/admesh/stl.h
+++ b/src/admesh/stl.h
@@ -277,5 +277,7 @@ extern void stl_add_facet(stl_file *stl, stl_facet *new_facet);
 extern void stl_clear_error(stl_file *stl);
 extern int stl_get_error(stl_file *stl);
 extern void stl_exit_on_error(stl_file *stl);
+// Validate the mesh, assert on error.
+extern bool stl_validate(stl_file *stl);
 
 #endif
diff --git a/src/admesh/util.cpp b/src/admesh/util.cpp
index 305a58e22..c2d2c2726 100644
--- a/src/admesh/util.cpp
+++ b/src/admesh/util.cpp
@@ -457,3 +457,50 @@ All facets connected.  No further nearby check necessary.\n");
     stl_verify_neighbors(stl);
   }
 }
+
+// Check validity of the mesh, assert on error.
+bool stl_validate(stl_file *stl)
+{
+	assert(! stl->error);
+	assert(stl->fp == nullptr);
+	assert(stl->facet_start != nullptr);
+	assert(stl->heads == nullptr);
+	assert(stl->tail  == nullptr);
+	assert(stl->neighbors_start != nullptr);
+	assert((stl->v_indices == nullptr) == (stl->v_shared == nullptr));
+	assert(stl->stats.number_of_facets > 0);
+
+#ifdef _DEBUG
+    // Verify validity of neighborship data.
+    for (int facet_idx = 0; facet_idx < (int)stl->stats.number_of_facets; ++ facet_idx) {
+        const stl_neighbors &nbr 		= stl->neighbors_start[facet_idx];
+        const int 			*vertices 	= (stl->v_indices == nullptr) ? nullptr : stl->v_indices[facet_idx].vertex;
+        for (int nbr_idx = 0; nbr_idx < 3; ++ nbr_idx) {
+            int nbr_face = stl->neighbors_start[facet_idx].neighbor[nbr_idx];
+            assert(nbr_face < (int)stl->stats.number_of_facets);
+            if (nbr_face != -1) {
+            	int nbr_vnot = nbr.which_vertex_not[nbr_idx];
+				assert(nbr_vnot >= 0 && nbr_vnot < 6);
+				// Neighbor of the neighbor is the original face.
+				assert(stl->neighbors_start[nbr_face].neighbor[(nbr_vnot + 1) % 3] == facet_idx);
+				int vnot_back = stl->neighbors_start[nbr_face].which_vertex_not[(nbr_vnot + 1) % 3];
+				assert(vnot_back >= 0 && vnot_back < 6);
+				assert((nbr_vnot < 3) == (vnot_back < 3));
+				assert(vnot_back % 3 == (nbr_idx + 2) % 3);
+				if (vertices != nullptr) {
+					// Has shared vertices.
+	            	if (nbr_vnot < 3) {
+	            		// Faces facet_idx and nbr_face share two vertices accross the common edge. Faces are correctly oriented.
+						assert((stl->v_indices[nbr_face].vertex[(nbr_vnot + 1) % 3] == vertices[(nbr_idx + 1) % 3] && stl->v_indices[nbr_face].vertex[(nbr_vnot + 2) % 3] == vertices[nbr_idx]));
+					} else {
+	            		// Faces facet_idx and nbr_face share two vertices accross the common edge. Faces are incorrectly oriented, one of them is flipped.
+						assert((stl->v_indices[nbr_face].vertex[(nbr_vnot + 2) % 3] == vertices[(nbr_idx + 1) % 3] && stl->v_indices[nbr_face].vertex[(nbr_vnot + 1) % 3] == vertices[nbr_idx]));
+					}
+				}
+            }
+        }
+    }
+#endif /* _DEBUG */
+
+	return true;
+}
diff --git a/src/libslic3r/TriangleMesh.cpp b/src/libslic3r/TriangleMesh.cpp
index 4d35cabca..b7c6c07a7 100644
--- a/src/libslic3r/TriangleMesh.cpp
+++ b/src/libslic3r/TriangleMesh.cpp
@@ -104,18 +104,21 @@ TriangleMesh& TriangleMesh::operator=(const TriangleMesh &other)
 
 void TriangleMesh::repair()
 {
-    if (this->repaired) return;
+    if (this->repaired)
+    	return;
     
     // admesh fails when repairing empty meshes
-    if (this->stl.stats.number_of_facets == 0) return;
+    if (this->stl.stats.number_of_facets == 0)
+    	return;
 
     BOOST_LOG_TRIVIAL(debug) << "TriangleMesh::repair() started";
-    
+
     // checking exact
 #ifdef SLIC3R_TRACE_REPAIR
 	BOOST_LOG_TRIVIAL(trace) << "\tstl_check_faces_exact";
 #endif /* SLIC3R_TRACE_REPAIR */
 	stl_check_facets_exact(&stl);
+    assert(stl_validate(&this->stl));
     stl.stats.facets_w_1_bad_edge = (stl.stats.connected_facets_2_edge - stl.stats.connected_facets_3_edge);
     stl.stats.facets_w_2_bad_edge = (stl.stats.connected_facets_1_edge - stl.stats.connected_facets_2_edge);
     stl.stats.facets_w_3_bad_edge = (stl.stats.number_of_facets - stl.stats.connected_facets_1_edge);
@@ -141,6 +144,7 @@ void TriangleMesh::repair()
             }
         }
     }
+    assert(stl_validate(&this->stl));
     
     // remove_unconnected
     if (stl.stats.connected_facets_3_edge < (int)stl.stats.number_of_facets) {
@@ -148,6 +152,7 @@ void TriangleMesh::repair()
         BOOST_LOG_TRIVIAL(trace) << "\tstl_remove_unconnected_facets";
 #endif /* SLIC3R_TRACE_REPAIR */
         stl_remove_unconnected_facets(&stl);
+	    assert(stl_validate(&this->stl));
     }
     
     // fill_holes
@@ -168,24 +173,28 @@ void TriangleMesh::repair()
     BOOST_LOG_TRIVIAL(trace) << "\tstl_fix_normal_directions";
 #endif /* SLIC3R_TRACE_REPAIR */
     stl_fix_normal_directions(&stl);
+    assert(stl_validate(&this->stl));
 
     // normal_values
 #ifdef SLIC3R_TRACE_REPAIR
     BOOST_LOG_TRIVIAL(trace) << "\tstl_fix_normal_values";
 #endif /* SLIC3R_TRACE_REPAIR */
     stl_fix_normal_values(&stl);
+    assert(stl_validate(&this->stl));
     
     // always calculate the volume and reverse all normals if volume is negative
 #ifdef SLIC3R_TRACE_REPAIR
     BOOST_LOG_TRIVIAL(trace) << "\tstl_calculate_volume";
 #endif /* SLIC3R_TRACE_REPAIR */
     stl_calculate_volume(&stl);
+    assert(stl_validate(&this->stl));
     
     // neighbors
 #ifdef SLIC3R_TRACE_REPAIR
     BOOST_LOG_TRIVIAL(trace) << "\tstl_verify_neighbors";
 #endif /* SLIC3R_TRACE_REPAIR */
     stl_verify_neighbors(&stl);
+    assert(stl_validate(&this->stl));
 
     this->repaired = true;
 
@@ -594,27 +603,14 @@ TriangleMesh TriangleMesh::convex_hull_3d() const
 void TriangleMesh::require_shared_vertices()
 {
     BOOST_LOG_TRIVIAL(trace) << "TriangleMeshSlicer::require_shared_vertices - start";
-    if (!this->repaired) 
+    assert(stl_validate(&this->stl));
+    if (! this->repaired) 
         this->repair();
-    if (this->stl.v_shared == NULL) {
+    if (this->stl.v_shared == nullptr) {
         BOOST_LOG_TRIVIAL(trace) << "TriangleMeshSlicer::require_shared_vertices - stl_generate_shared_vertices";
         stl_generate_shared_vertices(&(this->stl));
     }
-#ifdef _DEBUG
-    // Verify validity of neighborship data.
-    for (int facet_idx = 0; facet_idx < stl.stats.number_of_facets; ++facet_idx) {
-        const stl_neighbors &nbr = stl.neighbors_start[facet_idx];
-        const int *vertices = stl.v_indices[facet_idx].vertex;
-        for (int nbr_idx = 0; nbr_idx < 3; ++nbr_idx) {
-            int nbr_face = this->stl.neighbors_start[facet_idx].neighbor[nbr_idx];
-            if (nbr_face != -1) {
-				assert(
-					(stl.v_indices[nbr_face].vertex[(nbr.which_vertex_not[nbr_idx] + 1) % 3] == vertices[(nbr_idx + 1) % 3] && stl.v_indices[nbr_face].vertex[(nbr.which_vertex_not[nbr_idx] + 2) % 3] == vertices[nbr_idx]) ||
-					(stl.v_indices[nbr_face].vertex[(nbr.which_vertex_not[nbr_idx] + 2) % 3] == vertices[(nbr_idx + 1) % 3] && stl.v_indices[nbr_face].vertex[(nbr.which_vertex_not[nbr_idx] + 1) % 3] == vertices[nbr_idx]));
-            }
-        }
-    }
-#endif /* _DEBUG */
+    assert(stl_validate(&this->stl));
     BOOST_LOG_TRIVIAL(trace) << "TriangleMeshSlicer::require_shared_vertices - end";
 }