Avoid regenerating cached textblobs on integer scrolls

src/gpu/GrTextBlobCache.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrTextBlobCache_DEFINED
 #define GrTextBlobCache_DEFINED
 
 #include "GrAtlasTextContext.h"
-#include "SkTInternalLList.h"
+#include "SkTDynamicHash.h"
+#include "SkTextBlob.h"
 
 class GrTextBlobCache {
 public:
     typedef GrAtlasTextContext::BitmapTextBlob BitmapTextBlob;
 
+    GrTextBlobCache() : fPool(2 << 17, 2 << 17), fBudget(2 << 22) {}
+    ~GrTextBlobCache();
+
+    // creates an uncached blob
     BitmapTextBlob* createBlob(int glyphCount, int runCount, size_t maxVASize);
+    BitmapTextBlob* createBlob(const SkTextBlob* blob, size_t maxVAStride) {
+        int glyphCount = 0;
+        int runCount = 0;
+        BlobGlyphCount(&glyphCount, &runCount, blob);
+        BitmapTextBlob* cacheBlob = this->createBlob(glyphCount, runCount, maxVAStride);
+        return cacheBlob;
+    }
+
+    BitmapTextBlob* createCachedBlob(const SkTextBlob* blob, size_t maxVAStride) {
+        int glyphCount = 0;
+        int runCount = 0;
+        BlobGlyphCount(&glyphCount, &runCount, blob);
+        BitmapTextBlob* cacheBlob = this->createBlob(glyphCount, runCount, maxVAStride);
+        cacheBlob->fUniqueID = blob->uniqueID();
+        this->add(cacheBlob);
+        return cacheBlob;
+    }
+
+    BitmapTextBlob* find(uint32_t uniqueID) {
+        return fCache.find(uniqueID);
+    }
+
+    void remove(BitmapTextBlob* blob) {
+        fCache.remove(blob->fUniqueID);
+        fBlobList.remove(blob);
+        blob->unref();
+    }
+
+    void add(BitmapTextBlob* blob) {
+        fCache.add(blob);
+        fBlobList.addToHead(blob);
+
+        // If we are overbudget, then unref until we are below budget again
+        if (fPool.size() > fBudget) {
+            BitmapBlobList::Iter iter;
+            iter.init(fBlobList, BitmapBlobList::Iter::kTail_IterStart);
+            BitmapTextBlob* lruBlob = iter.get();
+            SkASSERT(lruBlob);
+            do {
+                fCache.remove(lruBlob->fUniqueID);
+                fBlobList.remove(lruBlob);
+                lruBlob->unref();
+                iter.prev();
+            } while (fPool.size() > fBudget && (lruBlob = iter.get()));
+        }
+    }
+
+    void makeMRU(BitmapTextBlob* blob) {
+        if (fBlobList.head() == blob) {
+            return;
+        }
+
+        fBlobList.remove(blob);
+        fBlobList.addToHead(blob);
+    }
 
 private:
+    // TODO move to SkTextBlob
+    void BlobGlyphCount(int* glyphCount, int* runCount, const SkTextBlob* blob) {
+        SkTextBlob::RunIterator itCounter(blob);
+        for (; !itCounter.done(); itCounter.next(), (*runCount)++) {
+            *glyphCount += itCounter.glyphCount();
+        }
+    }
+
     typedef SkTInternalLList<BitmapTextBlob> BitmapBlobList;
     BitmapBlobList fBlobList;
     SkTDynamicHash<BitmapTextBlob, uint32_t> fCache;
     GrMemoryPool fPool;
     size_t fBudget;
-    size_t fCurrentUsage;
 };
 
 #endif