4f gradient negative-dx interval fixup for kMirror_TileMode

src/effects/gradients/Sk4fGradientBase.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Sk4fGradientBase.h"
 
 namespace {
@@ skipping 189 matching lines @@
 
     if (shader.fTileMode == SkShader::kClamp_TileMode) {
         // synthetic edge interval: Pn .. +/-inf
         const SkPMColor clamp_color =
             pack_color(shader.fOrigColors[last_index], fColorsArePremul);
         const SkScalar clamp_pos = dx_is_pos ? SK_ScalarMax : SK_ScalarMin;
         fIntervals.emplace_back(clamp_color, last_pos,
                                 clamp_color, clamp_pos,
                                 componentScale);
     } else if (shader.fTileMode == SkShader::kMirror_TileMode) {
+        const int count = fIntervals.count();
         // synthetic flipped intervals in [1 .. 2)
-        for (int i = fIntervals.count() - 1; i >= 0; --i) {
+        for (int i = count - 1; i >= 0; --i) {
             const Interval& interval = fIntervals[i];
             const SkScalar p0 = interval.fP0;
             const SkScalar p1 = interval.fP1;
             Sk4f dc = Sk4f::Load(interval.fDc.fVec);
             Sk4f c = Sk4f::Load(interval.fC0.fVec) + dc * Sk4f(p1 - p0);
             fIntervals.emplace_back(c, dc * Sk4f(-1), 2 - p1, 2 - p0);
         }
+
+        if (!dx_is_pos) {
+            // When dx is negative, our initial invervals are in (1..0] order.
+            // The loop above appends their flipped counterparts, pivoted in 2: (1..0](2..1]
+            // To achieve the expected monotonic interval order, we need to
+            // swap the two halves: (2..1](1..0]
+            // TODO: we can probably avoid this late swap with some additional logic during
+            //       the initial interval buildup.
+            SkASSERT(fIntervals.count() == count * 2)
+            for (int i = 0; i < count; ++i) {
+                SkTSwap(fIntervals[i], fIntervals[count + i]);
+            }
+        }
     }
 
     SkASSERT(fIntervals.count() > 0);
     fCachedInterval = fIntervals.begin();
 }
 
 const SkGradientShaderBase::GradientShaderBase4fContext::Interval*
 SkGradientShaderBase::
 GradientShaderBase4fContext::findInterval(SkScalar fx) const {
     SkASSERT(in_range(fx, fIntervals.front().fP0, fIntervals.back().fP1));
@@ skipping 28 matching lines @@
             } else {
                 SkASSERT(in_range(fx, i->fP1, i1->fP1));
                 i0 = i + 1;
             }
         }
 
         SkASSERT(in_range(fx, i0->fP0, i0->fP1));
         return i0;
     }
 }