add more conservative check for wayward divide

src/core/SkStroke.cpp

 /*
  * Copyright 2008 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 "SkStrokerPriv.h"
 #include "SkGeometry.h"
 #include "SkPathPriv.h"
@@ skipping 818 matching lines @@
         // are small, a straight line is good enough
         SkScalar dist1 = pt_to_line(start, end, quadPts->fTangentEnd);
         SkScalar dist2 = pt_to_line(end, start, quadPts->fTangentStart);
         if (SkTMax(dist1, dist2) <= fInvResScaleSquared) {
             return STROKER_RESULT(kDegenerate_ResultType, depth, quadPts,
                     "SkTMax(dist1=%g, dist2=%g) <= fInvResScaleSquared", dist1, dist2);
         }
         return STROKER_RESULT(kSplit_ResultType, depth, quadPts,
                 "(numerA=%g >= 0) == (numerB=%g >= 0)", numerA, numerB);
     }
-    // check to see if the denomerator is teeny relative to the numerator
-    bool validDivide = SkScalarAbs(numerA) * SK_ScalarNearlyZero < SkScalarAbs(denom);
-// the divide check is the same as checking if the scaled denom is nearly zero
-// (commented out because on some platforms the two are not bit-identical)
-//  SkASSERT(!SkScalarNearlyZero(denom / numerA) == validDivide);
+    // check to see if the denominator is teeny relative to the numerator
+    // if the ratio isn't finite, or if the offset by one will be lost, the ratio is too large
+    numerA /= denom;
+    bool validDivide = SkScalarIsFinite(numerA) && numerA != numerA - 1;

[reed1, 2015/11/30 21:24:13]

nit: is "numerA > (numerA - 1)" clearer?
I certainly had to read the comment to understand what this 2nd test was for.
not sure.

     if (validDivide) {
         if (kCtrlPt_RayType == intersectRayType) {
-            numerA /= denom;
             SkPoint* ctrlPt = &quadPts->fQuad[1];
             // the intersection of the tangents need not be on the tangent segment
             // so 0 <= numerA <= 1 is not necessarily true
             ctrlPt->fX = start.fX * (1 - numerA) + quadPts->fTangentStart.fX * numerA;
             ctrlPt->fY = start.fY * (1 - numerA) + quadPts->fTangentStart.fY * numerA;
         }
         return STROKER_RESULT(kQuad_ResultType, depth, quadPts,
                 "(numerA=%g >= 0) != (numerB=%g >= 0)", numerA, numerB);
     }
     // if the lines are parallel, straight line is good enough
@@ skipping 625 matching lines @@
         default:
             break;
     }
 
     if (fWidth < SkMinScalar(rw, rh) && !fDoFill) {
         r = rect;
         r.inset(radius, radius);
         dst->addRect(r, reverse_direction(dir));
     }
 }