Don't modify the pts array in SkOpEdgeBuilder::preFetch

Change-Id: I2596986cb203e00b1d6d24c0086e98eb6373142b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/288178
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>

src/pathops/SkOpEdgeBuilder.cpp

@@ -17,21 +17,23 @@ void SkOpEdgeBuilder::init() {
 }
 
 // very tiny points cause numerical instability : don't allow them
-static void force_small_to_zero(SkPoint* pt) {
-    if (SkScalarAbs(pt->fX) < FLT_EPSILON_ORDERABLE_ERR) {
-        pt->fX = 0;
+static SkPoint force_small_to_zero(const SkPoint& pt) {
+    SkPoint ret = pt;
+    if (SkScalarAbs(ret.fX) < FLT_EPSILON_ORDERABLE_ERR) {
+        ret.fX = 0;
     }
-    if (SkScalarAbs(pt->fY) < FLT_EPSILON_ORDERABLE_ERR) {
-        pt->fY = 0;
+    if (SkScalarAbs(ret.fY) < FLT_EPSILON_ORDERABLE_ERR) {
+        ret.fY = 0;
     }
+    return ret;
 }
 
 static bool can_add_curve(SkPath::Verb verb, SkPoint* curve) {
     if (SkPath::kMove_Verb == verb) {
         return false;
     }
     for (int index = 0; index <= SkPathOpsVerbToPoints(verb); ++index) {
-        force_small_to_zero(&curve[index]);
+        curve[index] = force_small_to_zero(curve[index]);
     }
     return SkPath::kLine_Verb != verb || !SkDPoint::ApproximatelyEqual(curve[0], curve[1]);
 }
@@ -95,51 +97,44 @@ int SkOpEdgeBuilder::preFetch() {
                     closeContour(curve[0], curveStart);
                 }
                 *fPathVerbs.append() = verb;
-                force_small_to_zero(&pts[0]);
-                *fPathPts.append() = pts[0];
-                curveStart = curve[0] = pts[0];
+                curve[0] = force_small_to_zero(pts[0]);
+                *fPathPts.append() = curve[0];
+                curveStart = curve[0];
                 lastCurve = false;
                 continue;
             case SkPath::kLine_Verb:
-                force_small_to_zero(&pts[1]);
-                if (SkDPoint::ApproximatelyEqual(curve[0], pts[1])) {
+                curve[1] = force_small_to_zero(pts[1]);
+                if (SkDPoint::ApproximatelyEqual(curve[0], curve[1])) {
                     uint8_t lastVerb = fPathVerbs.top();
                     if (lastVerb != SkPath::kLine_Verb && lastVerb != SkPath::kMove_Verb) {
-                        fPathPts.top() = curve[0] = pts[1];
+                        fPathPts.top() = curve[0] = curve[1];
                     }
                     continue;  // skip degenerate points
                 }
                 break;
             case SkPath::kQuad_Verb:
-                force_small_to_zero(&pts[1]);
-                force_small_to_zero(&pts[2]);
-                curve[1] = pts[1];
-                curve[2] = pts[2];
-                verb = SkReduceOrder::Quad(curve, pts);
+                curve[1] = force_small_to_zero(pts[1]);
+                curve[2] = force_small_to_zero(pts[2]);
+                verb = SkReduceOrder::Quad(curve, curve);
                 if (verb == SkPath::kMove_Verb) {
                     continue;  // skip degenerate points
                 }
                 break;
             case SkPath::kConic_Verb:
-                force_small_to_zero(&pts[1]);
-                force_small_to_zero(&pts[2]);
-                curve[1] = pts[1];
-                curve[2] = pts[2];
-                verb = SkReduceOrder::Quad(curve, pts);
+                curve[1] = force_small_to_zero(pts[1]);
+                curve[2] = force_small_to_zero(pts[2]);
+                verb = SkReduceOrder::Quad(curve, curve);
                 if (SkPath::kQuad_Verb == verb && 1 != iter.conicWeight()) {
                   verb = SkPath::kConic_Verb;
                 } else if (verb == SkPath::kMove_Verb) {
                     continue;  // skip degenerate points
                 }
                 break;
             case SkPath::kCubic_Verb:
-                force_small_to_zero(&pts[1]);
-                force_small_to_zero(&pts[2]);
-                force_small_to_zero(&pts[3]);
-                curve[1] = pts[1];
-                curve[2] = pts[2];
-                curve[3] = pts[3];
-                verb = SkReduceOrder::Cubic(curve, pts);
+                curve[1] = force_small_to_zero(pts[1]);
+                curve[2] = force_small_to_zero(pts[2]);
+                curve[3] = force_small_to_zero(pts[3]);
+                verb = SkReduceOrder::Cubic(curve, curve);
                 if (verb == SkPath::kMove_Verb) {
                     continue;  // skip degenerate points
                 }
@@ -153,11 +148,11 @@ int SkOpEdgeBuilder::preFetch() {
         }
         *fPathVerbs.append() = verb;
         int ptCount = SkPathOpsVerbToPoints(verb);
-        fPathPts.append(ptCount, &pts[1]);
+        fPathPts.append(ptCount, &curve[1]);
         if (verb == SkPath::kConic_Verb) {
             *fWeights.append() = iter.conicWeight();
         }
-        curve[0] = pts[ptCount];
+        curve[0] = curve[ptCount];
         lastCurve = true;
     } while (verb != SkPath::kDone_Verb);
     if (!fAllowOpenContours && lastCurve) {
@@ -218,7 +213,7 @@ bool SkOpEdgeBuilder::walk() {
                             return false;
                         }
                         for (unsigned index = 0; index < SK_ARRAY_COUNT(pair); ++index) {
-                            force_small_to_zero(&pair[index]);
+                            pair[index] = force_small_to_zero(pair[index]);
                         }
                         SkPoint cStorage[2][2];
                         SkPath::Verb v1 = SkReduceOrder::Quad(&pair[0], cStorage[0]);