Add SkPaint::getHash().

BUG=skia:

Review URL: https://codereview.chromium.org/637583002

include/core/SkPaint.h

@@ -62,6 +62,11 @@ public:
         return !(a == b);
     }
 
+    /** getHash() is a shallow hash, with the same limitations as operator==.
+     *  If operator== returns true for two paints, getHash() returns the same value for each.
+     */
+    uint32_t getHash() const;
+
     void flatten(SkWriteBuffer&) const;
     void unflatten(SkReadBuffer&);
 

src/core/SkPaint.cpp

@@ -8,6 +8,7 @@
 #include "SkPaint.h"
 #include "SkAnnotation.h"
 #include "SkAutoKern.h"
+#include "SkChecksum.h"
 #include "SkColorFilter.h"
 #include "SkData.h"
 #include "SkDeviceProperties.h"
@@ -2460,3 +2461,11 @@ bool SkPaint::nothingToDraw() const {
     return false;
 }
 
+uint32_t SkPaint::getHash() const {
+    // We're going to hash 10 pointers and 7 32-bit values, finishing up with fBitfields,
+    // so fBitfields should be 10 pointers and 6 32-bit values from the start.
+    SK_COMPILE_ASSERT(offsetof(SkPaint, fBitfields) == 10 * sizeof(void*) + 6 * sizeof(uint32_t),
+                      SkPaint_notPackedTightly);
+    return SkChecksum::Murmur3(reinterpret_cast<const uint32_t*>(this),
+                               offsetof(SkPaint, fBitfields) + sizeof(fBitfields));
+}

tests/PaintTest.cpp

@@ -343,3 +343,29 @@ DEF_TEST(Paint_MoreFlattening, r) {
     ASSERT(paint.getXfermode()->asMode(&paintMode));
     ASSERT(otherMode == paintMode);
 }
+
+DEF_TEST(Paint_getHash, r) {
+    // Try not to inspect the actual hash values in here.
+    // We might want to change the hash function.
+
+    SkPaint paint;
+    const uint32_t defaultHash = paint.getHash();
+
+    // Check that some arbitrary field affects the hash.
+    paint.setColor(0xFF00FF00);
+    REPORTER_ASSERT(r, paint.getHash() != defaultHash);
+    paint.setColor(SK_ColorBLACK);  // Reset to default value.
+    REPORTER_ASSERT(r, paint.getHash() == defaultHash);
+
+    // SkTypeface is the first field we hash, so test it specially.
+    paint.setTypeface(SkTypeface::RefDefault())->unref();
+    REPORTER_ASSERT(r, paint.getHash() != defaultHash);
+    paint.setTypeface(NULL);
+    REPORTER_ASSERT(r, paint.getHash() == defaultHash);
+
+    // This is part of fBitfields, the last field we hash.
+    paint.setHinting(SkPaint::kSlight_Hinting);
+    REPORTER_ASSERT(r, paint.getHash() != defaultHash);
+    paint.setHinting(SkPaint::kNormal_Hinting);
+    REPORTER_ASSERT(r, paint.getHash() == defaultHash);
+}