Cache read arc data for stability

Arcs can be altered by the process of changing from on-disk
representation to in-memory representation.  Saving back to disk without
modifying the arc should not modify the calculated values.

This stores a copy of the on-disk representation that is only used to
save back to disk in the event that the arc is not modified during
editing.

Fixes https://gitlab.com/kicad/code/kicad/issues/10442

common/eda_shape.cpp

@@ -437,6 +437,12 @@ void EDA_SHAPE::SetCenter( const VECTOR2I& aCenter )
 
 VECTOR2I EDA_SHAPE::GetArcMid() const
 {
+    // If none of the input data have changed since we loaded the arc,
+    // keep the original mid point data to minimize churn
+    if( m_arcMidData.start == m_start && m_arcMidData.end == m_end
+            && m_arcMidData.center == m_arcCenter )
+        return m_arcMidData.mid;
+
     VECTOR2I mid = m_start;
     RotatePoint( mid, m_arcCenter, -GetArcAngle() / 2.0 );
     return mid;
@@ -487,19 +493,34 @@ int EDA_SHAPE::GetRadius() const
 }
 
 
+void EDA_SHAPE::SetCachedArcData( const VECTOR2I& aStart, const VECTOR2I& aMid, const VECTOR2I& aEnd, const VECTOR2I& aCenter )
+{
+    m_arcMidData.start = aStart;
+    m_arcMidData.end = aEnd;
+    m_arcMidData.center = aCenter;
+    m_arcMidData.mid = aMid;
+}
+
+
 void EDA_SHAPE::SetArcGeometry( const VECTOR2I& aStart, const VECTOR2I& aMid, const VECTOR2I& aEnd )
 {
+    m_arcMidData = {};
     m_start = aStart;
     m_end = aEnd;
     m_arcCenter = CalcArcCenter( aStart, aMid, aEnd );
+    VECTOR2I new_mid = GetArcMid();
+
     m_endsSwapped = false;
 
+    // Watch the ordering here.  GetArcMid above needs to be called prior to initializing the
+    // m_arcMidData structure in order to ensure we get the calculated variant, not the cached
+    SetCachedArcData( aStart, aMid, aEnd, m_arcCenter );
+
     /*
      * If the input winding doesn't match our internal winding, the calculated midpoint will end
      * up on the other side of the arc.  In this case, we need to flip the start/end points and
      * flag this change for the system.
      */
-    VECTOR2I new_mid = GetArcMid();
     VECTOR2D dist( new_mid - aMid );
     VECTOR2D dist2( new_mid - m_arcCenter );
 

eeschema/sch_plugins/kicad/sch_sexpr_parser.cpp

@@ -1000,12 +1000,9 @@ LIB_SHAPE* SCH_SEXPR_PARSER::parseArc()
         }
     }
 
-    arc->SetStart( startPoint );
-    arc->SetEnd( endPoint );
-
     if( hasMidPoint )
     {
-        arc->SetCenter( CalcArcCenter( arc->GetStart(), midPoint, arc->GetEnd() ) );
+        arc->SetArcGeometry( startPoint, midPoint, endPoint);
 
 #if 1   // This code will be removed if the current code in Eeschema is modified to allow
         // full support of arcs >= 180 deg without issue.
@@ -1030,6 +1027,7 @@ LIB_SHAPE* SCH_SEXPR_PARSER::parseArc()
             VECTOR2I new_center = CalcArcCenter( arc->GetStart(), arc->GetEnd(),
                                   EDA_ANGLE( 179.5, DEGREES_T ) );
             arc->SetCenter( new_center );
+            arc->SetCachedArcData( startPoint, midPoint, endPoint, new_center );
         }
 #endif
     }
@@ -1041,9 +1039,8 @@ LIB_SHAPE* SCH_SEXPR_PARSER::parseArc()
          * between LibEdit and PCBNew.  Since we now use a common class (EDA_SHAPE) for both we
          * need to flip one of them.  LibEdit drew the short straw.
          */
-        VECTOR2I temp = arc->GetStart();
-        arc->SetStart( arc->GetEnd() );
-        arc->SetEnd( temp );
+        arc->SetStart( endPoint );
+        arc->SetEnd( startPoint );
     }
     else
     {

include/eda_shape.h

@@ -61,6 +61,15 @@ enum class FILL_T : int
 };
 
 
+// Holding struct to keep originating midpoint
+struct ARC_MID
+{
+    VECTOR2I mid;
+    VECTOR2I start;
+    VECTOR2I end;
+    VECTOR2I center;
+};
+
 class EDA_SHAPE
 {
 public:
@@ -193,6 +202,17 @@ public:
      */
     void SetArcGeometry( const VECTOR2I& aStart, const VECTOR2I& aMid, const VECTOR2I& aEnd );
 
+    /**
+     * Set the data used for mid point caching.  If the controlling points remain constant, then
+     * we keep the midpoint the same as it was when read in.  This minimizes VCS churn
+     *
+     * @param aStart Cached start point
+     * @param aMid Cached mid point
+     * @param aEnd Cached end point
+     * @param aCenter Calculated center point using the preceeding three
+     */
+    void SetCachedArcData( const VECTOR2I& aStart, const VECTOR2I& aMid, const VECTOR2I& aEnd, const VECTOR2I& aCenter );
+
     const std::vector<VECTOR2I>& GetBezierPoints() const { return m_bezierPoints; }
 
     /**
@@ -314,6 +334,7 @@ protected:
     VECTOR2I              m_end;               // Line end point or Circle 3 o'clock point
 
     VECTOR2I              m_arcCenter;         // Used only for Arcs: arc end point
+    ARC_MID               m_arcMidData;        // Used to store originating data
 
     VECTOR2I              m_bezierC1;          // Bezier Control Point 1
     VECTOR2I              m_bezierC2;          // Bezier Control Point 2

pcbnew/fp_shape.cpp

@@ -178,6 +178,12 @@ void FP_SHAPE::SetCenter0( const VECTOR2I& aCenter )
 
 VECTOR2I FP_SHAPE::GetArcMid0() const
 {
+    // If none of the input data have changed since we loaded the arc,
+    // keep the original mid point data to minimize churn
+    if( m_arcMidData_0.start == m_start && m_arcMidData_0.end == m_end
+            && m_arcMidData_0.center == m_arcCenter )
+        return m_arcMidData_0.mid;
+
     VECTOR2I mid0 = m_start0;
     RotatePoint( mid0, m_arcCenter0, -GetArcAngle() / 2.0 );
     return mid0;
@@ -202,6 +208,11 @@ void FP_SHAPE::SetArcGeometry0( const VECTOR2I& aStart0, const VECTOR2I& aMid0,
     m_start0 = aStart0;
     m_end0 = aEnd0;
     m_arcCenter0 = CalcArcCenter( aStart0, aMid0, aEnd0 );
+
+    m_arcMidData_0.center = m_arcCenter0;
+    m_arcMidData_0.end = m_end0;
+    m_arcMidData_0.mid = aMid0;
+    m_arcMidData_0.start = m_start0;
 }
 
 

pcbnew/fp_shape.h

@@ -164,6 +164,8 @@ protected:
     VECTOR2I m_arcCenter0;   ///< Center of arc, relative to footprint origin, orient 0.
     VECTOR2I m_bezierC1_0; ///< Bezier Control Point 1, relative to footprint origin, orient 0.
     VECTOR2I m_bezierC2_0;   ///< Bezier Control Point 2, relative to footprint origin, orient 0.
+
+    ARC_MID  m_arcMidData_0; ///< Originating Arc data, orient 0
 };
 
 #endif    // FP_SHAPE_H