From eaf8eb284a9499103527c8ca3d9f16ac23e57e51 Mon Sep 17 00:00:00 2001
From: Roberto Fernandez Bautista <roberto.fer.bau@gmail.com>
Date: Mon, 26 Jul 2021 17:14:05 +0100
Subject: [PATCH] Add true arc-to-polyline collisions

---
 libs/kimath/src/geometry/shape_collisions.cpp | 49 +++++++++++++++++--
 qa/libs/kimath/geometry/test_shape_arc.cpp    |  5 +-
 2 files changed, 47 insertions(+), 7 deletions(-)

diff --git a/libs/kimath/src/geometry/shape_collisions.cpp b/libs/kimath/src/geometry/shape_collisions.cpp
index eca244e584..7153cb8ee3 100644
--- a/libs/kimath/src/geometry/shape_collisions.cpp
+++ b/libs/kimath/src/geometry/shape_collisions.cpp
@@ -534,14 +534,53 @@ static inline bool Collide( const SHAPE_ARC& aA, const SHAPE_LINE_CHAIN_BASE& aB
                                            aA.Type(),
                                            aB.Type() ) );
 
-    const SHAPE_LINE_CHAIN lc = aA.ConvertToPolyline();
+    int      closest_dist = INT_MAX;
+    VECTOR2I nearest;
 
-    bool rv = Collide( lc, aB, aClearance + aA.GetWidth() / 2, aActual, aLocation, aMTV );
+    if( aB.IsClosed() && aB.PointInside( aA.GetP0() ) )
+    {
+        closest_dist = 0;
+        nearest = aA.GetP0();
+    }
+    else
+    {
+        for( size_t i = 0; i < aB.GetSegmentCount(); i++ )
+        {
+            int      collision_dist = 0;
+            VECTOR2I pn;
 
-    if( rv && aActual )
-        *aActual = std::max( 0, *aActual - aA.GetWidth() / 2 );
+            if( aA.Collide( aB.GetSegment( i ), aClearance,
+                            aActual || aLocation ? &collision_dist : nullptr,
+                            aLocation ? &pn : nullptr ) )
+            {
+                if( collision_dist < closest_dist )
+                {
+                    nearest = pn;
+                    closest_dist = collision_dist;
+                }
 
-    return rv;
+                if( closest_dist == 0 )
+                    break;
+
+                // If we're not looking for aActual then any collision will do
+                if( !aActual )
+                    break;
+            }
+        }
+    }
+
+    if( closest_dist == 0 || closest_dist < aClearance )
+    {
+        if( aLocation )
+            *aLocation = nearest;
+
+        if( aActual )
+            *aActual = closest_dist;
+
+        return true;
+    }
+
+    return false;
 }
 
 
diff --git a/qa/libs/kimath/geometry/test_shape_arc.cpp b/qa/libs/kimath/geometry/test_shape_arc.cpp
index 3005b30d97..7e7d63d92b 100644
--- a/qa/libs/kimath/geometry/test_shape_arc.cpp
+++ b/qa/libs/kimath/geometry/test_shape_arc.cpp
@@ -761,10 +761,11 @@ BOOST_AUTO_TEST_CASE( CollideArcToPolygonApproximation )
     // Create a polyset approximation from the arc - error outside (simulating the zone filler)
     SHAPE_POLY_SET arcBuffer;
     int            clearance = ( arc.GetWidth() * 3 ) / 2;
+    int            polygonApproximationError = SHAPE_ARC::DefaultAccuracyForPCB();
 
     TransformArcToPolygon( arcBuffer, wxPoint( arc.GetP0() ), wxPoint( arc.GetArcMid() ),
                            wxPoint( arc.GetP1() ), arc.GetWidth() + 2 * clearance,
-                           SHAPE_ARC::DefaultAccuracyForPCB(), ERROR_OUTSIDE );
+                           polygonApproximationError, ERROR_OUTSIDE );
 
     BOOST_REQUIRE_EQUAL( arcBuffer.OutlineCount(), 1 );
     BOOST_CHECK_EQUAL( arcBuffer.HoleCount( 0 ), 0 );
@@ -793,7 +794,7 @@ BOOST_AUTO_TEST_CASE( CollideArcToPolygonApproximation )
 
     BOOST_CHECK_EQUAL( zoneFill.Collide( &arc, clearance * 2, &actual, &location ), true );
 
-    BOOST_CHECK( KI_TEST::IsWithin( actual, clearance, tol ) );
+    BOOST_CHECK( KI_TEST::IsWithin( actual, clearance, polygonApproximationError ) );
 }