Fix handling of radii scaling to force the result to always be less

src/core/SkScaleToSides.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkScaleToSides_DEFINED
+#define SkScaleToSides_DEFINED
+
+#include <cmath>
+#include "SkScalar.h"
+#include "SkTypes.h"
+
+class ScaleToSides {
+public:
+    // This code assumes that a and b fit in in a float, and therefore the resulting smaller value
+    // of a and b will fit in a float. The side of the rectangle may be larger than a float.
+    // Scale must be less than or equal to the ratio limit / (*a + *b).
+    // This code assumes that NaN and Inf are never passed in.
+    static void AdjustRadii(double limit, double scale, SkScalar* a, SkScalar* b) {
+        SkASSERTF(scale < 1.0 && scale > 0.0, "scale: %g", scale);
+
+        *a *= scale;
+        *b *= scale;
+
+        // This check is conservative. (double)*a + (double)*b >= (double)(*a + *b)
+        if ((double)*a + (double)*b > limit) {
+            float* minRadius = a;
+            float* maxRadius = b;
+
+            // Force minRadius to be the smaller of the two.
+            if (*minRadius > *maxRadius) {
+                SkTSwap(minRadius, maxRadius);
+            }
+
+            // newMinRadius must be float in order to give the actual value of the radius.
+            // The newMinRadius will always be smaller than limit. The largest that minRadius can be
+            // is 1/2 the ratio of minRadius : (minRadius + maxRadius), therefore in the resulting
+            // division, minRadius can be no larger than 1/2 limit + ULP.
+            float newMinRadius = *minRadius;
+
+            // Because newMaxRadius is the result of a double to float conversion, it can be larger
+            // than limit, but only by one ULP.
+            float newMaxRadius = (float)(limit - newMinRadius);
+
+            // If newMaxRadius is larger than the same value as a double, then it needs to be
+            // reduced by one ULP to be less than limit - newMinRadius.
+            // Note: nexttowardf is a c99 call and should be std::nexttoward, but this is not
+            // implemented in the ARM compiler.
+            if ((double)newMaxRadius + (double)newMinRadius > limit) {
+                newMaxRadius = nexttowardf(newMaxRadius, 0.0);
+            }
+            *maxRadius = newMaxRadius;
+        }
+
+        SkASSERTF(*a >= 0.0f && *b >= 0.0f, "a: %g, b: %g", *a, *b);
+        SkASSERTF((*a + *b) <= limit, "limit: %g, a: %g, b: %g", limit, *a, *b);
+    }
+};
+#endif // ScaleToSides_DEFINED