Fix bug where SkPath will convert an arc to an oval and change the starting point.

src/core/SkPath.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
+#include <cmath>
 #include "SkBuffer.h"
 #include "SkCubicClipper.h"
 #include "SkErrorInternals.h"
 #include "SkGeometry.h"
 #include "SkMath.h"
 #include "SkPathPriv.h"
 #include "SkPathRef.h"
 #include "SkRRect.h"
 
 ////////////////////////////////////////////////////////////////////////////
@@ skipping 1396 matching lines @@
 }
 
 void SkPath::addArc(const SkRect& oval, SkScalar startAngle, SkScalar sweepAngle) {
     if (oval.isEmpty() || 0 == sweepAngle) {
         return;
     }
 
     const SkScalar kFullCircleAngle = SkIntToScalar(360);
 
     if (sweepAngle >= kFullCircleAngle || sweepAngle <= -kFullCircleAngle) {
-        this->addOval(oval, sweepAngle > 0 ? kCW_Direction : kCCW_Direction);
-    } else {
-        this->arcTo(oval, startAngle, sweepAngle, true);
+        // We can treat the arc as an oval if it begins at one of our legal starting positions.
+        // See SkPath::addOval() docs.
+        SkScalar startOver90 = startAngle / 90.f;
+        SkScalar startOver90I = SkScalarRoundToScalar(startOver90);
+        SkScalar error = startOver90 - startOver90I;
+        if (SkScalarNearlyEqual(error, 0)) {
+            // Index 1 is at startAngle == 0.
+            SkScalar startIndex = std::fmod(startOver90I + 1.f, 4.f);
+            startIndex = startIndex < 0 ? startIndex + 4.f : startIndex;
+            this->addOval(oval, sweepAngle > 0 ? kCW_Direction : kCCW_Direction,
+                          (unsigned) startIndex);
+            return;
+        }
     }
+    this->arcTo(oval, startAngle, sweepAngle, true);
 }
 
 /*
     Need to handle the case when the angle is sharp, and our computed end-points
     for the arc go behind pt1 and/or p2...
 */
 void SkPath::arcTo(SkScalar x1, SkScalar y1, SkScalar x2, SkScalar y2, SkScalar radius) {
     if (radius == 0) {
         this->lineTo(x1, y1);
         return;
@@ skipping 1788 matching lines @@
         }
     } while (!done);
     return SkToBool(tangents.count()) ^ isInverse;
 }
 
 int SkPath::ConvertConicToQuads(const SkPoint& p0, const SkPoint& p1, const SkPoint& p2,
                                 SkScalar w, SkPoint pts[], int pow2) {
     const SkConic conic(p0, p1, p2, w);
     return conic.chopIntoQuadsPOW2(pts, pow2);
 }