Implement difference clip rects with window rectangles

src/gpu/GrWindowRectangles.h

Index: src/gpu/GrWindowRectangles.h
diff --git a/src/gpu/GrWindowRectangles.h b/src/gpu/GrWindowRectangles.h
new file mode 100644
index 0000000000000000000000000000000000000000..682af9353d9e4ceb0765cf5ac4f6b005d6b5dfee
--- /dev/null
+++ b/src/gpu/GrWindowRectangles.h
@@ -0,0 +1,110 @@
+/*
+ * 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 GrWindowRectangles_DEFINED
+#define GrWindowRectangles_DEFINED
+
+#include "GrNonAtomicRef.h"
+#include "SkRect.h"
+
+class GrWindowRectangles {
+public:
+    constexpr static int kMaxWindows = 16;
+
+    enum class Mode : bool {
+        kExclusive,
+        kInclusive
+    };
+
+    GrWindowRectangles(Mode mode = Mode::kExclusive) : fMode(mode), fCount(0) {}
+    GrWindowRectangles(const GrWindowRectangles& that) : fCount(0) { *this = that; }
+    ~GrWindowRectangles() { SkSafeUnref(this->rec()); }
+
+    Mode mode() const { return fMode; }
+    uint16_t count() const { return fCount; }
+    bool disabled() const { return Mode::kExclusive == fMode && !fCount; }
+    const SkIRect* data() const;
+
+    void reset(Mode = Mode::kExclusive);
+    GrWindowRectangles& operator=(const GrWindowRectangles&);
+
+    SkIRect& addWindow(const SkIRect& window) { return this->addWindow() = window; }
+    SkIRect& addWindow();
+
+    bool operator!=(const GrWindowRectangles& that) const { return !(*this == that); }
+    bool operator==(const GrWindowRectangles&) const;
+
+private:
+    constexpr static int kNumLocalWindows = 1;
+    struct Rec;
+
+    const Rec* rec() const { return fCount <= kNumLocalWindows ? nullptr : fRec; }
+
+    Mode      fMode;
+    uint8_t   fCount;
+    union {
+        SkIRect   fLocalWindows[kNumLocalWindows]; // If fCount <= kNumLocalWindows.
+        Rec*      fRec;                            // If fCount > kNumLocalWindows.
+    };
+};
+
+struct GrWindowRectangles::Rec : public GrNonAtomicRef<Rec> {
+    Rec(const SkIRect* windows, int numWindows) {
+        SkASSERT(numWindows < kMaxWindows);
+        memcpy(fData, windows, sizeof(SkIRect) * numWindows);
+    }
+
+    SkIRect fData[kMaxWindows];
+};
+
+inline const SkIRect* GrWindowRectangles::data() const {
+    return fCount <= kNumLocalWindows ? fLocalWindows : fRec->fData;
+}
+
+inline void GrWindowRectangles::reset(Mode mode) {
+    SkSafeUnref(this->rec());
+    fMode = mode;
+    fCount = 0;
+}
+
+inline GrWindowRectangles& GrWindowRectangles::operator=(const GrWindowRectangles& that) {
+    SkSafeUnref(this->rec());
+    fMode = that.fMode;
+    fCount = that.fCount;
+    if (fCount <= kNumLocalWindows) {
+        memcpy(fLocalWindows, that.fLocalWindows, fCount * sizeof(SkIRect));
+    } else {
+        fRec = SkRef(that.fRec);
+    }
+    return *this;
+}
+
+inline SkIRect& GrWindowRectangles::addWindow() {
+    SkASSERT(fCount < kMaxWindows);
+    if (fCount < kNumLocalWindows) {
+        return fLocalWindows[fCount++];
+    }
+    if (fCount == kNumLocalWindows) {
+        fRec = new Rec(fLocalWindows, kNumLocalWindows);
+    } else if (!fRec->unique()) { // Simple copy-on-write.
+        fRec->unref();
+        fRec = new Rec(fRec->fData, fCount);
+    }
+    return fRec->fData[fCount++];
+}
+
+inline bool GrWindowRectangles::operator==(const GrWindowRectangles& that) const {
+    if (fMode != that.fMode || fCount != that.fCount) {
+        return false;
+    }
+    if (fCount > kNumLocalWindows && fRec == that.fRec) {
+        return true;
+    }
+    return !fCount || !memcmp(this->data(), that.data(), sizeof(SkIRect) * fCount);
+}
+
+#endif