From 0bfc53f5a53ca3794e184358c6cc110d0691b663 Mon Sep 17 00:00:00 2001
From: Vojtech Bubnik <bubnikv@gmail.com>
Date: Tue, 1 Jun 2021 19:30:26 +0200
Subject: [PATCH] VertexFaceIndex: vertex index to incident face indices built
 for indexed_triangle_set.

---
 src/libslic3r/TriangleMesh.cpp | 25 +++++++++++++++++++++++++
 src/libslic3r/TriangleMesh.hpp | 23 +++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/src/libslic3r/TriangleMesh.cpp b/src/libslic3r/TriangleMesh.cpp
index 4954ff46c..a8e952a2b 100644
--- a/src/libslic3r/TriangleMesh.cpp
+++ b/src/libslic3r/TriangleMesh.cpp
@@ -685,6 +685,31 @@ std::vector<std::vector<size_t>> create_vertex_faces_index(const indexed_triangl
     return index;
 }
 
+void VertexFaceIndex::create(const indexed_triangle_set &its)
+{
+    m_vertex_to_face_start.assign(its.vertices.size() + 1, 0);
+    // 1) Calculate vertex incidence by scatter.
+    for (auto &face : its.indices) {
+        ++ m_vertex_to_face_start[face(0) + 1];
+        ++ m_vertex_to_face_start[face(1) + 1];
+        ++ m_vertex_to_face_start[face(2) + 1];
+    }
+    // 2) Prefix sum to calculate offsets to m_vertex_faces_all.
+    for (size_t i = 2; i < m_vertex_to_face_start.size(); ++ i)
+        m_vertex_to_face_start[i] += m_vertex_to_face_start[i - 1];
+    // 3) Scatter indices of faces incident to a vertex into m_vertex_faces_all.
+    m_vertex_faces_all.assign(m_vertex_to_face_start.back(), 0);
+    for (size_t face_idx = 0; face_idx < its.indices.size(); ++ face_idx) {
+        auto &face = its.indices[face_idx];
+        for (int i = 0; i < 3; ++ i)
+            m_vertex_faces_all[m_vertex_to_face_start[face(i)] ++] = face_idx;
+    }
+    // 4) The previous loop modified m_vertex_to_face_start. Revert the change.
+    for (auto i = int(m_vertex_to_face_start.size()) - 1; i > 0; -- i)
+        m_vertex_to_face_start[i] = m_vertex_to_face_start[i - 1];
+    m_vertex_to_face_start.front() = 0;
+}
+
 // Map from a face edge to a unique edge identifier or -1 if no neighbor exists.
 // Two neighbor faces share a unique edge identifier even if they are flipped.
 template<typename ThrowOnCancelCallback>
diff --git a/src/libslic3r/TriangleMesh.hpp b/src/libslic3r/TriangleMesh.hpp
index 55fffa726..6d7fd48e4 100644
--- a/src/libslic3r/TriangleMesh.hpp
+++ b/src/libslic3r/TriangleMesh.hpp
@@ -93,6 +93,29 @@ private:
 // vertex.
 std::vector<std::vector<size_t>> create_vertex_faces_index(const indexed_triangle_set &its);
 
+// Index of face indices incident with a vertex index.
+struct VertexFaceIndex
+{
+public:
+    using iterator = std::vector<size_t>::const_iterator;
+
+    VertexFaceIndex(const indexed_triangle_set &its) { this->create(its); }
+    VertexFaceIndex() {}
+
+    void create(const indexed_triangle_set &its);
+    void clear() { m_vertex_to_face_start.clear(); m_vertex_faces_all.clear(); }
+
+    // Iterators of face indices incident with the input vertex_id.
+    iterator begin(size_t vertex_id) const throw() { return m_vertex_faces_all.begin() + m_vertex_to_face_start[vertex_id]; }
+    iterator end  (size_t vertex_id) const throw() { return m_vertex_faces_all.begin() + m_vertex_to_face_start[vertex_id + 1]; }
+    // Vertex incidence.
+    size_t   count(size_t vertex_id) const throw() { return m_vertex_to_face_start[vertex_id + 1] - m_vertex_to_face_start[vertex_id]; }
+
+private:
+    std::vector<size_t>     m_vertex_to_face_start;
+    std::vector<size_t>     m_vertex_faces_all;
+};
+
 // Map from a face edge to a unique edge identifier or -1 if no neighbor exists.
 // Two neighbor faces share a unique edge identifier even if they are flipped.
 // Used for chaining slice lines into polygons.