allow one zero length dash

If the constructed stroke that represents a dash has a
single dash of length zero, and the end cap is square or
round, draw the cap.

The old code initialized the initial dash length to zero,
making it ambiguous whether the first length is zero or
not.

R=robertphillips@google.com
BUG=583299
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1805963002

Committed: https://skia.googlesource.com/skia/+/5e1a24808415df2748822e8082e21a361362cdfe

Review URL: https://codereview.chromium.org/1805963002

7 files changed, 92 insertions, 61 deletions

diff --git a/bench/DashBench.cpp b/bench/DashBench.cpp
index eb964f6bf1..e3d8364013 100644
--- a/bench/DashBench.cpp
+++ b/bench/DashBench.cpp
@@ -72,8 +72,9 @@ protected:
         SkPath path;
         this->makePath(&path);
 
-        paint.setPathEffect(SkDashPathEffect::Create(fIntervals.begin(),
-                                                     fIntervals.count(), 0))->unref();
+        SkAutoTUnref<SkPathEffect> effect(SkDashPathEffect::Create(fIntervals.begin(),
+                                          fIntervals.count(), 0));
+        paint.setPathEffect(effect);
 
         if (fDoClip) {
             SkRect r = path.getBounds();
diff --git a/gm/arcto.cpp b/gm/arcto.cpp
index cd26eef35a..584fca3c4f 100644
--- a/gm/arcto.cpp
+++ b/gm/arcto.cpp
@@ -205,3 +205,24 @@ DEF_SIMPLE_GM(bug593049, canvas, 300, 300) {
 
     canvas->drawPath(p, paint);
 }
+
+#include "SkDashPathEffect.h"
+#include "SkPathMeasure.h"
+
+DEF_SIMPLE_GM(bug583299, canvas, 300, 300) {
+  const char* d="M60,60 A50,50 0 0 0 160,60 A50,50 0 0 0 60,60z";
+  SkPaint p;
+  p.setStyle(SkPaint::kStroke_Style);
+  p.setStrokeWidth(100);
+  p.setAntiAlias(true);
+  p.setColor(0xFF008200);
+  p.setStrokeCap(SkPaint::kSquare_Cap);
+  SkPath path;
+  SkParsePath::FromSVGString(d, &path);
+  SkPathMeasure meas(path, false);
+  SkScalar length = meas.getLength();
+  SkScalar intervals[] = {0, length };
+  int intervalCount = (int) SK_ARRAY_COUNT(intervals);
+  p.setPathEffect(SkDashPathEffect::Create(intervals, intervalCount, 0))->unref();
+  canvas->drawPath(path, p);
+}
diff --git a/gm/dashing.cpp b/gm/dashing.cpp
index dfd1b629c1..c728d15a1a 100644
--- a/gm/dashing.cpp
+++ b/gm/dashing.cpp
@@ -21,7 +21,8 @@ static void drawline(SkCanvas* canvas, int on, int off, const SkPaint& paint,
         SkIntToScalar(off),
     };
 
-    p.setPathEffect(SkDashPathEffect::Create(intervals, 2, phase))->unref();
+    SkAutoTUnref<SkPathEffect> effect(SkDashPathEffect::Create(intervals, 2, phase));
+    p.setPathEffect(effect);
     canvas->drawLine(startX, startY, finalX, finalY, p);
 }
 
diff --git a/src/effects/SkDashPathEffect.cpp b/src/effects/SkDashPathEffect.cpp
index ced0aab69a..3816499915 100644
--- a/src/effects/SkDashPathEffect.cpp
+++ b/src/effects/SkDashPathEffect.cpp
@@ -14,7 +14,7 @@
 
 SkDashPathEffect::SkDashPathEffect(const SkScalar intervals[], int count, SkScalar phase)
         : fPhase(0)
-        , fInitialDashLength(0)
+        , fInitialDashLength(-1)
         , fInitialDashIndex(0)
         , fIntervalLength(0) {
     SkASSERT(intervals);
@@ -23,7 +23,6 @@ SkDashPathEffect::SkDashPathEffect(const SkScalar intervals[], int count, SkScal
     fIntervals = (SkScalar*)sk_malloc_throw(sizeof(SkScalar) * count);
     fCount = count;
     for (int i = 0; i < count; i++) {
-        SkASSERT(intervals[i] >= 0);
         fIntervals[i] = intervals[i];
     }
 
@@ -38,7 +37,7 @@ SkDashPathEffect::~SkDashPathEffect() {
 
 bool SkDashPathEffect::filterPath(SkPath* dst, const SkPath& src,
                               SkStrokeRec* rec, const SkRect* cullRect) const {
-    return SkDashPath::FilterDashPath(dst, src, rec, cullRect, fIntervals, fCount,
+    return SkDashPath::InternalFilter(dst, src, rec, cullRect, fIntervals, fCount,
                                       fInitialDashLength, fInitialDashIndex, fIntervalLength);
 }
 
@@ -162,7 +161,7 @@ bool SkDashPathEffect::asPoints(PointData* results,
                                 const SkMatrix& matrix,
                                 const SkRect* cullRect) const {
     // width < 0 -> fill && width == 0 -> hairline so requiring width > 0 rules both out
-    if (fInitialDashLength < 0 || 0 >= rec.getWidth()) {
+    if (0 >= rec.getWidth()) {
         return false;
     }
 
@@ -388,13 +387,8 @@ void SkDashPathEffect::toString(SkString* str) const {
 //////////////////////////////////////////////////////////////////////////////////////////////////
 
 SkPathEffect* SkDashPathEffect::Create(const SkScalar intervals[], int count, SkScalar phase) {
-    if ((count < 2) || !SkIsAlign2(count)) {
+    if (!SkDashPath::ValidDashPath(phase, intervals, count)) {
         return nullptr;
     }
-    for (int i = 0; i < count; i++) {
-        if (intervals[i] < 0) {
-            return nullptr;
-        }
-    }
     return new SkDashPathEffect(intervals, count, phase);
 }
diff --git a/src/utils/SkDashPath.cpp b/src/utils/SkDashPath.cpp
index 0d2783eba2..d766b0d53e 100644
--- a/src/utils/SkDashPath.cpp
+++ b/src/utils/SkDashPath.cpp
@@ -40,42 +40,35 @@ void SkDashPath::CalcDashParameters(SkScalar phase, const SkScalar intervals[],
         len += intervals[i];
     }
     *intervalLength = len;
-
-    // watch out for values that might make us go out of bounds
-    if ((len > 0) && SkScalarIsFinite(phase) && SkScalarIsFinite(len)) {
-
-        // Adjust phase to be between 0 and len, "flipping" phase if negative.
-        // e.g., if len is 100, then phase of -20 (or -120) is equivalent to 80
-        if (adjustedPhase) {
-            if (phase < 0) {
-                phase = -phase;
-                if (phase > len) {
-                    phase = SkScalarMod(phase, len);
-                }
-                phase = len - phase;
-
-                // Due to finite precision, it's possible that phase == len,
-                // even after the subtract (if len >>> phase), so fix that here.
-                // This fixes http://crbug.com/124652 .
-                SkASSERT(phase <= len);
-                if (phase == len) {
-                    phase = 0;
-                }
-            } else if (phase >= len) {
+    // Adjust phase to be between 0 and len, "flipping" phase if negative.
+    // e.g., if len is 100, then phase of -20 (or -120) is equivalent to 80
+    if (adjustedPhase) {
+        if (phase < 0) {
+            phase = -phase;
+            if (phase > len) {
                 phase = SkScalarMod(phase, len);
             }
-            *adjustedPhase = phase;
+            phase = len - phase;
+
+            // Due to finite precision, it's possible that phase == len,
+            // even after the subtract (if len >>> phase), so fix that here.
+            // This fixes http://crbug.com/124652 .
+            SkASSERT(phase <= len);
+            if (phase == len) {
+                phase = 0;
+            }
+        } else if (phase >= len) {
+            phase = SkScalarMod(phase, len);
         }
-        SkASSERT(phase >= 0 && phase < len);
+        *adjustedPhase = phase;
+    }
+    SkASSERT(phase >= 0 && phase < len);
 
-        *initialDashLength = find_first_interval(intervals, phase,
-                                                initialDashIndex, count);
+    *initialDashLength = find_first_interval(intervals, phase,
+                                            initialDashIndex, count);
 
-        SkASSERT(*initialDashLength >= 0);
-        SkASSERT(*initialDashIndex >= 0 && *initialDashIndex < count);
-    } else {
-        *initialDashLength = -1;    // signal bad dash intervals
-    }
+    SkASSERT(*initialDashLength >= 0);
+    SkASSERT(*initialDashIndex >= 0 && *initialDashIndex < count);
 }
 
 static void outset_for_stroke(SkRect* rect, const SkStrokeRec& rec) {
@@ -220,13 +213,13 @@ private:
 };
 
 
-bool SkDashPath::FilterDashPath(SkPath* dst, const SkPath& src, SkStrokeRec* rec,
+bool SkDashPath::InternalFilter(SkPath* dst, const SkPath& src, SkStrokeRec* rec,
                                 const SkRect* cullRect, const SkScalar aIntervals[],
                                 int32_t count, SkScalar initialDashLength, int32_t initialDashIndex,
                                 SkScalar intervalLength) {
 
-    // we do nothing if the src wants to be filled, or if our dashlength is 0
-    if (rec->isFillStyle() || initialDashLength < 0) {
+    // we do nothing if the src wants to be filled
+    if (rec->isFillStyle()) {
         return false;
     }
 
@@ -306,7 +299,7 @@ bool SkDashPath::FilterDashPath(SkPath* dst, const SkPath& src, SkStrokeRec* rec
 
         // extend if we ended on a segment and we need to join up with the (skipped) initial segment
         if (meas.isClosed() && is_even(initialDashIndex) &&
-            initialDashLength > 0) {
+            initialDashLength >= 0) {
             meas.getSegment(0, initialDashLength, dst, !addedSegment);
             ++segCount;
         }
@@ -321,11 +314,29 @@ bool SkDashPath::FilterDashPath(SkPath* dst, const SkPath& src, SkStrokeRec* rec
 
 bool SkDashPath::FilterDashPath(SkPath* dst, const SkPath& src, SkStrokeRec* rec,
                                 const SkRect* cullRect, const SkPathEffect::DashInfo& info) {
+    if (!ValidDashPath(info.fPhase, info.fIntervals, info.fCount)) {
+        return false;
+    }
     SkScalar initialDashLength = 0;
     int32_t initialDashIndex = 0;
     SkScalar intervalLength = 0;
     CalcDashParameters(info.fPhase, info.fIntervals, info.fCount,
                        &initialDashLength, &initialDashIndex, &intervalLength);
-    return FilterDashPath(dst, src, rec, cullRect, info.fIntervals, info.fCount, initialDashLength,
+    return InternalFilter(dst, src, rec, cullRect, info.fIntervals, info.fCount, initialDashLength,
                           initialDashIndex, intervalLength);
 }
+
+bool SkDashPath::ValidDashPath(SkScalar phase, const SkScalar intervals[], int32_t count) {
+    if (count < 2 || !SkIsAlign2(count)) {
+        return false;
+    }
+    SkScalar length = 0;
+    for (int i = 0; i < count; i++) {
+        if (intervals[i] < 0) {
+            return false;
+        }
+        length += intervals[i];
+    }
+    // watch out for values that might make us go out of bounds
+    return length > 0 && SkScalarIsFinite(phase) && SkScalarIsFinite(length);
+}
diff --git a/src/utils/SkDashPathPriv.h b/src/utils/SkDashPathPriv.h
index 7453dcec82..54bf9a4870 100644
--- a/src/utils/SkDashPathPriv.h
+++ b/src/utils/SkDashPathPriv.h
@@ -16,17 +16,25 @@ namespace SkDashPath {
      * inputed phase and intervals. If adjustedPhase is passed in, then the phase will be
      * adjusted to be between 0 and intervalLength. The result will be stored in adjustedPhase.
      * If adjustedPhase is nullptr then it is assumed phase is already between 0 and intervalLength
+     *
+     * Caller should have already used ValidDashPath to exclude invalid data.
      */
     void CalcDashParameters(SkScalar phase, const SkScalar intervals[], int32_t count,
                             SkScalar* initialDashLength, int32_t* initialDashIndex,
                             SkScalar* intervalLength, SkScalar* adjustedPhase = nullptr);
 
     bool FilterDashPath(SkPath* dst, const SkPath& src, SkStrokeRec*, const SkRect*,
-                        const SkScalar aIntervals[], int32_t count, SkScalar initialDashLength,
-                        int32_t initialDashIndex, SkScalar intervalLength);
-    
-    bool FilterDashPath(SkPath* dst, const SkPath& src, SkStrokeRec*, const SkRect*,
                         const SkPathEffect::DashInfo& info);
+
+    /*
+     * Caller should have already used ValidDashPath to exclude invalid data.
+     */
+    bool InternalFilter(SkPath* dst, const SkPath& src, SkStrokeRec* rec,
+                        const SkRect* cullRect, const SkScalar aIntervals[],
+                        int32_t count, SkScalar initialDashLength, int32_t initialDashIndex,
+                        SkScalar intervalLength);
+
+    bool ValidDashPath(SkScalar phase, const SkScalar intervals[], int32_t count);
 }
 
 #endif
diff --git a/tests/DashPathEffectTest.cpp b/tests/DashPathEffectTest.cpp
index 68fce9a142..fa2395ef75 100644
--- a/tests/DashPathEffectTest.cpp
+++ b/tests/DashPathEffectTest.cpp
@@ -12,20 +12,15 @@
 #include "SkStrokeRec.h"
 
 // crbug.com/348821 was rooted in SkDashPathEffect refusing to flatten and unflatten itself when
-// fInitialDashLength < 0 (a signal the effect is nonsense).  Here we test that it flattens.
+// the effect is nonsense.  Here we test that it fails when passed nonsense parameters.
 
 DEF_TEST(DashPathEffectTest_crbug_348821, r) {
     SkScalar intervals[] = { 1.76934361e+36f, 2.80259693e-45f };  // Values from bug.
     const int count = 2;
-    SkScalar phase = SK_ScalarInfinity;  // Used to force the bad fInitialDashLength = -1 path.
+    SkScalar phase = SK_ScalarInfinity;  // Used to force a nonsense effect.
     SkAutoTUnref<SkPathEffect> dash(SkDashPathEffect::Create(intervals, count, phase));
 
-    // nullptr -> refuses to work with flattening framework.
-    REPORTER_ASSERT(r, dash->getFactory() != nullptr);
-
-    SkWriteBuffer buffer;
-    buffer.writeFlattenable(dash);
-    REPORTER_ASSERT(r, buffer.bytesWritten() > 12);  // We'd write 12 if broken, >=40 if not.
+    REPORTER_ASSERT(r, dash == nullptr);
 }
 
 // Test out the asPoint culling behavior.