Treat straight arcs as segments

When an arc has a very small curvature, we should not waste time with
the arc collision mode and instead just use the segment collision as it
is effectively the same result

Fixes https://gitlab.com/kicad/code/kicad/-/issues/19422

libs/kimath/include/geometry/shape_arc.h

@@ -175,6 +175,8 @@ public:
         return true;
     }
 
+    bool IsEffectiveLine() const;
+
     void Move( const VECTOR2I& aVector ) override;
 
     /**

libs/kimath/src/geometry/shape_arc.cpp

@@ -241,6 +241,15 @@ SHAPE_ARC& SHAPE_ARC::ConstructFromStartEndCenter( const VECTOR2I& aStart, const
 }
 
 
+bool SHAPE_ARC::IsEffectiveLine() const
+{
+    SEG v1 = SEG( m_start, m_mid );
+    SEG v2 = SEG( m_mid, m_end );
+
+    return v1.ApproxCollinear( v2 );
+}
+
+
 bool SHAPE_ARC::Collide( const SEG& aSeg, int aClearance, int* aActual, VECTOR2I* aLocation ) const
 {
     VECTOR2I center = GetCenter();

libs/kimath/src/geometry/shape_collisions.cpp

@@ -651,6 +651,13 @@ static inline bool Collide( const SHAPE_ARC& aA, const SHAPE_SEGMENT& aB, int aC
                                            aA.TypeName(),
                                            aB.TypeName() ) );
 
+    // If the arc radius is too large, it is effectively a line segment
+    if( aA.IsEffectiveLine() )
+    {
+        SHAPE_SEGMENT tmp( aA.GetP0(), aA.GetP1(), aA.GetWidth() );
+        return Collide( tmp, aB, aClearance, aActual, aLocation, aMTV );
+    }
+
     bool rv = aA.Collide( aB.GetSeg(), aClearance + aB.GetWidth() / 2, aActual, aLocation );
 
     if( rv && aActual )
@@ -663,6 +670,13 @@ static inline bool Collide( const SHAPE_ARC& aA, const SHAPE_SEGMENT& aB, int aC
 static inline bool Collide( const SHAPE_ARC& aA, const SHAPE_LINE_CHAIN_BASE& aB, int aClearance,
                             int* aActual, VECTOR2I* aLocation, VECTOR2I* aMTV )
 {
+    // If the arc radius is too large, it is effectively a line segment
+    if( aA.IsEffectiveLine() )
+    {
+        SHAPE_SEGMENT tmp( aA.GetP0(), aA.GetP1(), aA.GetWidth() );
+        return Collide( aB, tmp, aClearance, aActual, aLocation, aMTV );
+    }
+
     wxASSERT_MSG( !aMTV, wxString::Format( wxT( "MTV not implemented for %s : %s collisions" ),
                                            aA.TypeName(),
                                            aB.TypeName() ) );

pcbnew/pcb_track.cpp

@@ -1982,7 +1982,12 @@ std::shared_ptr<SHAPE> PCB_ARC::GetEffectiveShape( PCB_LAYER_ID aLayer, FLASHING
     if( IsSolderMaskLayer( aLayer ) )
         width += 2 * GetSolderMaskExpansion();
 
-    return std::make_shared<SHAPE_ARC>( GetStart(), GetMid(), GetEnd(), width );
+    SHAPE_ARC arc( GetStart(), GetMid(), GetEnd(), width );
+
+    if( arc.IsEffectiveLine() )
+        return std::make_shared<SHAPE_SEGMENT>( GetStart(), GetEnd(), width );
+
+    return std::make_shared<SHAPE_ARC>( arc );
 }