Fix accuracy of tuning path calculation when measuring arcs

Fixes https://gitlab.com/kicad/code/kicad/-/issues/8401

libs/kimath/include/geometry/shape_arc.h

@@ -148,6 +148,11 @@ public:
     double  GetStartAngle() const;
     double  GetEndAngle() const;
 
+    /**
+     * @return the length of the arc shape
+     */
+    double GetLength() const;
+
     /**
      * Constructs a SHAPE_LINE_CHAIN of segments from a given arc
      * @param aAccuracy maximum divergence from true arc given in internal units

libs/kimath/src/geometry/shape_arc.cpp

@@ -386,6 +386,15 @@ VECTOR2I SHAPE_ARC::GetCenter() const
 }
 
 
+double SHAPE_ARC::GetLength() const
+{
+    double radius = GetRadius();
+    double includedAngle  = std::abs( GetCentralAngle() );
+
+    return radius * M_PI * includedAngle / 180.0;
+}
+
+
 double SHAPE_ARC::GetCentralAngle() const
 {
     VECTOR2I center = GetCenter();

libs/kimath/src/geometry/shape_line_chain.cpp

@@ -234,7 +234,17 @@ long long int SHAPE_LINE_CHAIN::Length() const
     long long int l = 0;
 
     for( int i = 0; i < SegmentCount(); i++ )
-        l += CSegment( i ).Length();
+    {
+        // Only include segments that aren't part of arc shapes
+        if( m_shapes[i] == SHAPE_IS_PT || m_shapes[i + 1] == SHAPE_IS_PT ||
+            ( m_shapes[i] != m_shapes[i + 1] ) )
+        {
+            l += CSegment( i ).Length();
+        }
+    }
+
+    for( int i = 0; i < ArcCount(); i++ )
+        l += CArcs()[i].GetLength();
 
     return l;
 }