From c30040b691ab788a68fa35fe542899861ecfa88e Mon Sep 17 00:00:00 2001
From: Petr Ledvina <ledvinap@gmail.com>
Date: Tue, 22 Apr 2014 15:19:35 +0200
Subject: [PATCH] Apply bug fix from upstream clipper Clipper fix in commit
 [r463] for bug repport #92
 (http://sourceforge.net/p/polyclipping/code/463/tree//trunk/cpp/clipper.cpp?diff=504b9404fd48f873331e913b:462)
 is applied here.

---
 xs/src/clipper.cpp | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/xs/src/clipper.cpp b/xs/src/clipper.cpp
index 76522ac08..4a720b71b 100755
--- a/xs/src/clipper.cpp
+++ b/xs/src/clipper.cpp
@@ -1030,10 +1030,20 @@ TEdge* ClipperBase::ProcessBound(TEdge* E, bool IsClockwise)
   cInt StartX;
   if (IsHorizontal(*E))
   {
-    //it's possible for adjacent overlapping horz edges to start heading left
-    //before finishing right, so ...
-    if (IsClockwise) StartX = E->Prev->Bot.X;
-    else StartX = E->Next->Bot.X;
+    //first we need to be careful here with open paths because this
+    //may not be a true local minima (ie may be following a skip edge).
+    //also, watch for adjacent horz edges to start heading left
+    //before finishing right ...
+    if (IsClockwise)
+      {
+        if (E->Prev->Bot.Y == E->Bot.Y) StartX = E->Prev->Bot.X;
+        else StartX = E->Prev->Top.X;
+      }
+    else
+      {
+        if (E->Next->Bot.Y == E->Bot.Y) StartX = E->Next->Bot.X;
+        else StartX = E->Next->Top.X;
+      }
     if (E->Bot.X != StartX) ReverseHorizontal(*E);
   }