From 6789dac20bff8030617091af90729bac6eace203 Mon Sep 17 00:00:00 2001
From: Jon Evans <jon@craftyjon.com>
Date: Sun, 6 Jun 2021 18:56:33 -0400
Subject: [PATCH] PNS: Keep most recent good placement around for a rainy day

Fixes https://gitlab.com/kicad/code/kicad/-/issues/8562
---
 pcbnew/router/pns_line_placer.cpp | 15 +++++++++++++--
 pcbnew/router/pns_line_placer.h   |  2 ++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/pcbnew/router/pns_line_placer.cpp b/pcbnew/router/pns_line_placer.cpp
index 9d02e5131a..2ec2f57da5 100644
--- a/pcbnew/router/pns_line_placer.cpp
+++ b/pcbnew/router/pns_line_placer.cpp
@@ -960,8 +960,19 @@ void LINE_PLACER::routeStep( const VECTOR2I& aP )
 
     if( fail )
     {
-        m_head.RemoveVia();
-        m_head.Clear();
+        if( m_last_head.PointCount() > 0 )
+        {
+            m_head = m_last_head;
+        }
+        else
+        {
+            m_head.RemoveVia();
+            m_head.Clear();
+        }
+    }
+    else
+    {
+        m_last_head = m_head;
     }
 
     if( !fail && Settings().FollowMouse() )
diff --git a/pcbnew/router/pns_line_placer.h b/pcbnew/router/pns_line_placer.h
index c48aadad31..5b4ad15b02 100644
--- a/pcbnew/router/pns_line_placer.h
+++ b/pcbnew/router/pns_line_placer.h
@@ -328,6 +328,8 @@ private:
     LINE           m_head;          ///< the volatile part of the track from the previously
                                     ///< analyzed point to the current routing destination
 
+    LINE           m_last_head;     ///< Most recent successful (non-colliding) head
+
     LINE           m_tail;          ///< routing "tail": part of the track that has been already
                                     ///< fixed due to collisions with obstacles