From ae87dc686a8ecf78a3887c4c91b79bd2fe9a8a93 Mon Sep 17 00:00:00 2001
From: Roberto Fernandez Bautista <roberto.fer.bau@gmail.com>
Date: Sun, 8 Aug 2021 20:56:46 +0100
Subject: [PATCH] Fix SHAPE_LINE_CHAIN::NearestPoint when
 aAllowInternalShapePoints=false Fixes
 https://gitlab.com/kicad/code/kicad/-/issues/8770

---
 libs/kimath/src/geometry/shape_line_chain.cpp | 45 ++++++++++++-------
 .../kimath/geometry/test_shape_line_chain.cpp | 31 +++++++++++++
 2 files changed, 60 insertions(+), 16 deletions(-)

diff --git a/libs/kimath/src/geometry/shape_line_chain.cpp b/libs/kimath/src/geometry/shape_line_chain.cpp
index ca0bf81d43..c65e659198 100644
--- a/libs/kimath/src/geometry/shape_line_chain.cpp
+++ b/libs/kimath/src/geometry/shape_line_chain.cpp
@@ -1557,29 +1557,42 @@ const VECTOR2I SHAPE_LINE_CHAIN::NearestPoint( const VECTOR2I& aP,
     {
         int d = CSegment( i ).Distance( aP );
 
-        bool isInternalShapePoint = false;
-
-        // An internal shape point here is everything after the start of an arc and before the
-        // second-to-last vertex of the arc, because we are looking at segments here!
-        if( i > 0 && i < SegmentCount() - 1 && m_shapes[i] != SHAPES_ARE_PT
-            && ( ( m_shapes[i - 1] != SHAPES_ARE_PT && m_shapes[i - 1] == m_shapes[i] )
-                 && ( m_shapes[i + 2] != SHAPES_ARE_PT && m_shapes[i + 2] == m_shapes[i] ) ) )
-        {
-            isInternalShapePoint = true;
-        }
-
-        if( ( d < min_d ) && ( aAllowInternalShapePoints || !isInternalShapePoint ) )
+        if( d < min_d )
         {
             min_d = d;
             nearest = i;
         }
     }
 
-    // Is this the start or end of an arc?  If so, return it directly
-    if( !aAllowInternalShapePoints && ( IsArcStart( nearest ) || IsArcEnd( nearest ) ) )
+    if( !aAllowInternalShapePoints )
     {
-        //@todo should we calculate the nearest point to the "true" arc?
-        return m_points[nearest];
+        //Snap to arc end points if the closest found segment is part of an arc segment
+        if( nearest > 0 && nearest < PointCount() && IsArcSegment( nearest ) )
+        {
+            VECTOR2I ptToSegStart = CSegment( nearest ).A - aP;
+            VECTOR2I ptToSegEnd = CSegment( nearest ).B - aP;
+
+            if( ptToSegStart.EuclideanNorm() > ptToSegEnd.EuclideanNorm() )
+                nearest++;
+
+            // Is this the start or end of an arc?  If so, return it directly
+            if( IsArcStart( nearest ) || IsArcEnd( nearest ) )
+            {
+                return m_points[nearest];
+            }
+            else
+            {
+                const SHAPE_ARC& nearestArc = Arc( ArcIndex( nearest ) );
+                VECTOR2I         ptToArcStart = nearestArc.GetP0() - aP;
+                VECTOR2I         ptToArcEnd = nearestArc.GetP1() - aP;
+
+                if( ptToArcStart.EuclideanNorm() > ptToArcEnd.EuclideanNorm() )
+                    return nearestArc.GetP1();
+                else
+                    return nearestArc.GetP0();
+            }
+
+        }
     }
 
     return CSegment( nearest ).NearestPoint( aP );
diff --git a/qa/libs/kimath/geometry/test_shape_line_chain.cpp b/qa/libs/kimath/geometry/test_shape_line_chain.cpp
index dd380bd87a..7c3acdc873 100644
--- a/qa/libs/kimath/geometry/test_shape_line_chain.cpp
+++ b/qa/libs/kimath/geometry/test_shape_line_chain.cpp
@@ -260,4 +260,35 @@ BOOST_AUTO_TEST_CASE( Slice )
 }
 
 
+// Test SHAPE_LINE_CHAIN::NearestPoint( VECTOR2I )
+BOOST_AUTO_TEST_CASE( NearestPointPt )
+{
+    SEG       seg1( VECTOR2I( 0, 100000 ), VECTOR2I( 50000, 0 ) );
+    SEG       seg2( VECTOR2I( 200000, 0 ), VECTOR2I( 300000, 0 ) );
+    SHAPE_ARC arc( VECTOR2I( 200000, 0 ), VECTOR2I( 300000, 0 ), 180.0 );
+
+    // Start a chain with 2 points (seg1)
+    SHAPE_LINE_CHAIN chain( { seg1.A, seg1.B } );
+    BOOST_CHECK_EQUAL( chain.PointCount(), 2 );
+    // Add first arc
+    chain.Append( arc );
+    BOOST_CHECK_EQUAL( chain.PointCount(), 9 );
+    // Add two points (seg2)
+    chain.Append( seg2.A );
+    chain.Append( seg2.B );
+    BOOST_CHECK_EQUAL( chain.PointCount(), 11 );
+    BOOST_CHECK( GEOM_TEST::IsOutlineValid( chain ) );
+
+    VECTOR2I ptOnArcCloseToStart( 297553, 31697 ); //should be index 3 in chain
+    VECTOR2I ptOnArcCloseToEnd( 139709, 82983 ); //should be index 6 in chain
+
+    BOOST_CHECK_EQUAL( chain.NearestPoint( ptOnArcCloseToStart, true ), ptOnArcCloseToStart );
+    BOOST_CHECK_EQUAL( chain.NearestPoint( ptOnArcCloseToStart, false ), arc.GetP0() );
+
+    BOOST_CHECK_EQUAL( chain.NearestPoint( ptOnArcCloseToEnd, true ), ptOnArcCloseToEnd );
+    BOOST_CHECK_EQUAL( chain.NearestPoint( ptOnArcCloseToEnd, false ), arc.GetP1() );
+}
+
+
+
 BOOST_AUTO_TEST_SUITE_END()