optimized connected component computation (#569)

* use union find for graph connected components

* update readme

* remove graphlite dep

README.md

@@ -38,8 +38,7 @@ System Dependencies (note that we will automatically download the dependency if
 - [`gtest`](https://github.com/google/googletest/): Google test library (only when test is enabled, i.e. `MANIFOLD_TEST=ON`)
 
 Other dependencies:
-- [`graphlite`](https://github.com/haasdo95/graphlite): connected components algorithm.
-- [`Clipper2`](https://github.com/AngusJohnson/Clipper2j): provides our 2D subsystem
+- [`Clipper2`](https://github.com/AngusJohnson/Clipper2): provides our 2D subsystem
 - [`quickhull`](https://github.com/akuukka/quickhull): 3D convex hull algorithm.
 
 ## What's here

bindings/c/CMakeLists.txt

@@ -14,7 +14,7 @@ endif()
 
 target_link_libraries(
   ${PROJECT_NAME}
-  PRIVATE manifold sdf graphlite cross_section
+  PRIVATE manifold sdf cross_section
 )
 
 target_include_directories(${PROJECT_NAME} PUBLIC ${PROJECT_SOURCE_DIR}/include)

src/manifold/CMakeLists.txt

@@ -20,7 +20,7 @@ add_library(${PROJECT_NAME} ${SOURCE_FILES})
 target_include_directories(${PROJECT_NAME} PUBLIC ${PROJECT_SOURCE_DIR}/include)
 target_link_libraries(${PROJECT_NAME}
     PUBLIC utilities cross_section
-    PRIVATE collider polygon ${MANIFOLD_INCLUDE} graphlite Clipper2 quickhull
+    PRIVATE collider polygon ${MANIFOLD_INCLUDE} Clipper2 quickhull
 )
 
 target_compile_options(${PROJECT_NAME} PRIVATE ${MANIFOLD_FLAGS})

src/manifold/src/constructors.cpp

@@ -16,7 +16,6 @@
 
 #include "cross_section.h"
 #include "csg_tree.h"
-#include "graph.h"
 #include "impl.h"
 #include "par.h"
 #include "polygon.h"
@@ -465,17 +464,14 @@ Manifold Manifold::Compose(const std::vector<Manifold>& manifolds) {
  * containing a copy of the original. It is the inverse operation of Compose().
  */
 std::vector<Manifold> Manifold::Decompose() const {
-  Graph graph;
+  UnionFind<> uf(NumVert());
+  // Graph graph;
   auto pImpl_ = GetCsgLeafNode().GetImpl();
-  for (int i = 0; i < NumVert(); ++i) {
-    graph.add_nodes(i);
-  }
   for (const Halfedge& halfedge : pImpl_->halfedge_) {
-    if (halfedge.IsForward())
-      graph.add_edge(halfedge.startVert, halfedge.endVert);
+    if (halfedge.IsForward()) uf.unionXY(halfedge.startVert, halfedge.endVert);
   }
   std::vector<int> componentIndices;
-  const int numComponents = ConnectedComponents(componentIndices, graph);
+  const int numComponents = uf.connectedComponents(componentIndices);
 
   if (numComponents == 1) {
     std::vector<Manifold> meshes(1);

src/manifold/src/impl.cpp

@@ -19,7 +19,6 @@
 #include <map>
 #include <numeric>
 
-#include "graph.h"
 #include "hashtable.h"
 #include "mesh_fixes.h"
 #include "par.h"
@@ -272,11 +271,11 @@ struct CheckCoplanarity {
   VecView<int> comp2tri;
   VecView<const Halfedge> halfedge;
   VecView<const glm::vec3> vertPos;
-  VecView<const int> components;
+  std::vector<int>* components;
   const float precision;
 
   void operator()(int tri) {
-    const int component = components[tri];
+    const int component = (*components)[tri];
     const int referenceTri = comp2tri[component];
     if (referenceTri < 0 || referenceTri == tri) return;
 
@@ -310,17 +309,13 @@ struct EdgeBox {
 
 int GetLabels(std::vector<int>& components,
               const Vec<thrust::pair<int, int>>& edges, int numNodes) {
-  Graph graph;
-  for (int i = 0; i < numNodes; ++i) {
-    graph.add_nodes(i);
-  }
-  for (int i = 0; i < edges.size(); ++i) {
-    const thrust::pair<int, int> edge = edges[i];
-    if (edge.first < 0) continue;
-    graph.add_edge(edge.first, edge.second);
+  UnionFind<> uf(numNodes);
+  for (auto edge : edges) {
+    if (edge.first == -1 || edge.second == -1) continue;
+    uf.unionXY(edge.first, edge.second);
   }
 
-  return ConnectedComponents(components, graph);
+  return uf.connectedComponents(components);
 }
 
 void DedupePropVerts(manifold::Vec<glm::ivec3>& triProp,
@@ -639,7 +634,7 @@ void Manifold::Impl::CreateFaces(const std::vector<float>& propertyTolerance) {
   std::vector<int> components;
   const int numComponent = GetLabels(components, face2face, NumTri());
 
-  std::vector<int> comp2tri(numComponent, -1);
+  Vec<int> comp2tri(numComponent, -1);
   for (int tri = 0; tri < NumTri(); ++tri) {
     const int comp = components[tri];
     const int current = comp2tri[comp];
@@ -649,15 +644,13 @@ void Manifold::Impl::CreateFaces(const std::vector<float>& propertyTolerance) {
     }
   }
 
-  Vec<int> componentsD(components);
-  Vec<int> comp2triD(comp2tri);
   for_each_n(autoPolicy(halfedge_.size()), countAt(0), NumTri(),
              CheckCoplanarity(
-                 {comp2triD, halfedge_, vertPos_, componentsD, precision_}));
+                 {comp2tri, halfedge_, vertPos_, &components, precision_}));
 
   Vec<TriRef>& triRef = meshRelation_.triRef;
   for (int tri = 0; tri < NumTri(); ++tri) {
-    const int referenceTri = comp2triD[components[tri]];
+    const int referenceTri = comp2tri[components[tri]];
     if (referenceTri >= 0) {
       triRef[tri].tri = referenceTri;
     }

src/manifold/src/sort.cpp

@@ -16,7 +16,6 @@
 #include <numeric>
 #include <set>
 
-#include "graph.h"
 #include "impl.h"
 #include "par.h"
 
@@ -583,30 +582,20 @@ bool MeshGL::Merge() {
   Collider collider(vertBox, vertMorton);
   SparseIndices toMerge = collider.Collisions<true>(vertBox.cview());
 
-  Graph graph;
-  for (int i = 0; i < numVert; ++i) {
-    graph.add_nodes(i);
-  }
+  UnionFind<> uf(numVert);
   for (int i = 0; i < mergeFromVert.size(); ++i) {
-    graph.add_edge(static_cast<int>(mergeFromVert[i]),
-                   static_cast<int>(mergeToVert[i]));
+    uf.unionXY(static_cast<int>(mergeFromVert[i]),
+               static_cast<int>(mergeToVert[i]));
   }
   for (int i = 0; i < toMerge.size(); ++i) {
-    graph.add_edge(openVerts[toMerge.Get(i, false)],
-                   openVerts[toMerge.Get(i, true)]);
-  }
-
-  std::vector<int> vertLabels;
-  const int numLabels = ConnectedComponents(vertLabels, graph);
-  std::vector<int> label2vert(numLabels);
-  for (int v = 0; v < numVert; ++v) {
-    label2vert[vertLabels[v]] = v;
+    uf.unionXY(openVerts[toMerge.Get(i, false)],
+               openVerts[toMerge.Get(i, true)]);
   }
 
   mergeToVert.clear();
   mergeFromVert.clear();
   for (int v = 0; v < numVert; ++v) {
-    const int mergeTo = label2vert[vertLabels[v]];
+    const int mergeTo = uf.find(v);
     if (mergeTo != v) {
       mergeFromVert.push_back(v);
       mergeToVert.push_back(mergeTo);

src/third_party/CMakeLists.txt

@@ -1,5 +1,3 @@
-add_subdirectory(graphlite)
-
 set(CLIPPER2_UTILS OFF)
 set(CLIPPER2_EXAMPLES OFF)
 set(CLIPPER2_TESTS OFF)