From bea59957563d102853ec3f32c29971c088a7f8cd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mael=20Rouxel-Labb=C3=A9?=
 <mael.rouxel.labbe@geometryfactory.com>
Date: Mon, 29 Mar 2021 14:27:16 +0200
Subject: [PATCH] Handle "right" folds

The following configurations of collinear points:
  x-2 x x-1      AND        x+2 x x+1
cannot be handled with the typical left swap because the interval
has range n-1. Instead, just manually swap like it is done
when the fold is on the left (which can and is immediately detected
at the beginning of the insertion event)
---
 .../include/CGAL/Random_polygon_2_sweep.h     | 46 +++++++++++++++++++
 .../CGAL/Polygon_2/Polygon_2_simplicity.h     |  2 +
 2 files changed, 48 insertions(+)

diff --git a/Generator/include/CGAL/Random_polygon_2_sweep.h b/Generator/include/CGAL/Random_polygon_2_sweep.h
index 6a059662ba6..b60b8bfdf4e 100644
--- a/Generator/include/CGAL/Random_polygon_2_sweep.h
+++ b/Generator/include/CGAL/Random_polygon_2_sweep.h
@@ -179,6 +179,52 @@ less_than_in_tree(Vertex_index new_edge, Vertex_index tree_edge) const
                                 m_vertex_data->point(mid),
                                 m_vertex_data->point(right)));
     m_vertex_data->is_simple_result = false;
+
+    // need to handle the specific combinations:
+    // as the standard x;x+k swap (see below) will not work
+    if (m_vertex_data->edges[tree_edge.as_int()].is_left_to_right)
+    {
+      // x+1 x x+2
+      // This is actually already handled in the insertion event, when the orientation returns "COPLANAR"
+      // but leaving the code below for completion.
+      if (m_vertex_data->next(mid) == left || m_vertex_data->next(mid) == right)
+      {
+        // swap mid & left
+        m_vertex_data->conflict1 = m_vertex_data->prev(mid);
+        m_vertex_data->conflict2 = left;
+        return true;
+      }
+      // x-2 x x-1
+      else if (m_vertex_data->prev(mid) == left || m_vertex_data->prev(mid) == right)
+      {
+        // swap mid & right
+        m_vertex_data->conflict1 = left;
+        m_vertex_data->conflict2 = mid;
+        return true;
+      }
+    }
+    else // right to left
+    {
+      // x+2 x x+1
+      if (m_vertex_data->next(mid) == left || m_vertex_data->next(mid) == right)
+      {
+        // swap mid & right
+        m_vertex_data->conflict1 = m_vertex_data->prev(mid);
+        m_vertex_data->conflict2 = right;
+        return true;
+      }
+      // x-1 x x-2
+      // This is actually already handled in the insertion event, when the orientation returns "COPLANAR"
+      // but leaving the code below for completion.
+      else if (m_vertex_data->prev(mid) == left || m_vertex_data->prev(mid) == right)
+      {
+        // swap mid & left
+        m_vertex_data->conflict1 = right;
+        m_vertex_data->conflict2 = mid;
+        return true;
+      }
+    }
+
     Vertex_index mid_succ = m_vertex_data->next(mid);
     if (mid_succ.as_int() <= (std::min)(left.as_int(), right.as_int()))
     {
diff --git a/Polygon/include/CGAL/Polygon_2/Polygon_2_simplicity.h b/Polygon/include/CGAL/Polygon_2/Polygon_2_simplicity.h
index 03a6856f162..b222b02cd08 100644
--- a/Polygon/include/CGAL/Polygon_2/Polygon_2_simplicity.h
+++ b/Polygon/include/CGAL/Polygon_2/Polygon_2_simplicity.h
@@ -74,6 +74,8 @@ struct Vertex_index {
     explicit Vertex_index(Index_t i): m_i(i) {}
     Index_t as_int() const {return m_i;}
     Vertex_index operator++() {++m_i; return *this; }
+    bool operator==(const Vertex_index& other) const = default;
+
 private:
     Index_t m_i;
 };