First pass at a blitter for R11 EAC alpha masks. This shaves 10ms off

src/utils/SkTextureCompressor.h

Index: src/utils/SkTextureCompressor.h
diff --git a/src/utils/SkTextureCompressor.h b/src/utils/SkTextureCompressor.h
index ec6153ace9d945c38544633335dde8f9f3f85ce0..ea57934c642d8a1cbe8cf5a5aefe0e3c24f73535 100644
--- a/src/utils/SkTextureCompressor.h
+++ b/src/utils/SkTextureCompressor.h
@@ -9,6 +9,7 @@
 #define SkTextureCompressor_DEFINED
 
 #include "SkImageInfo.h"
+#include "SkBlitter.h"
 
 class SkBitmap;
 class SkData;
@@ -42,6 +43,167 @@ namespace SkTextureCompressor {
     // allows SIMD optimized compression functions to be implemented.
     typedef bool (*CompressionProc)(uint8_t* dst, const uint8_t* src,
                                     int width, int height, int rowBytes);
+
+    // This class implements a blitter that blits directly into a buffer that will
+    // be used as an R11 EAC compressed texture. We compute this buffer by
+    // buffering four scan lines and then outputting them all at once. This blitter
+    // is only expected to be used with alpha masks, i.e. kAlpha8_SkColorType.
+    class R11_EACBlitter : public SkBlitter {
+    public:
+        R11_EACBlitter(int width, int height, void *compressedBuffer);
+        virtual ~R11_EACBlitter() { this->flushRuns(); }
+
+        // Blit a horizontal run of one or more pixels.
+        virtual void blitH(int x, int y, int width) SK_OVERRIDE {
+            // This function is intended to be called from any standard RGB
+            // buffer, so we should never encounter it. However, if some code
+            // path does end up here, then this needs to be investigated.
+            SkFAIL("Not implemented!");
+        }
+
+        /// Blit a horizontal run of antialiased pixels; runs[] is a *sparse*
+        /// zero-terminated run-length encoding of spans of constant alpha values.
+        virtual void blitAntiH(int x, int y,
+                               const SkAlpha antialias[],
+                               const int16_t runs[]) SK_OVERRIDE;
+
+        // Blit a vertical run of pixels with a constant alpha value.
+        virtual void blitV(int x, int y, int height, SkAlpha alpha) SK_OVERRIDE {
+            // This function is currently not implemented. It is not explicitly
+            // required by the contract, but if at some time a code path runs into
+            // this function (which is entirely possible), it needs to be implemented.
+            //
+            // TODO (krajcevski):
+            // This function will be most easily implemented in one of two ways:
+            // 1. Buffer each vertical column value and then construct a list
+            //    of alpha values and output all of the blocks at once. This only
+            //    requires a write to the compressed buffer
+            // 2. Replace the indices of each block with the proper indices based
+            //    on the alpha value. This requires a read and write of the compressed
+            //    buffer, but much less overhead.
+            SkFAIL("Not implemented!");
+        }
+
+        // Blit a solid rectangle one or more pixels wide.
+        virtual void blitRect(int x, int y, int width, int height) SK_OVERRIDE {
+            // Analogous to blitRow, this function is intended for RGB targets
+            // and should never be called by this blitter. Any calls to this function
+            // are probably a bug and should be investigated.
+            SkFAIL("Not implemented!");
+        }
+
+        // Blit a rectangle with one alpha-blended column on the left,
+        // width (zero or more) opaque pixels, and one alpha-blended column
+        // on the right. The result will always be at least two pixels wide.
+        virtual void blitAntiRect(int x, int y, int width, int height,
+                                  SkAlpha leftAlpha, SkAlpha rightAlpha) SK_OVERRIDE {
+            // This function is currently not implemented. It is not explicitly
+            // required by the contract, but if at some time a code path runs into
+            // this function (which is entirely possible), it needs to be implemented.
+            //
+            // TODO (krajcevski):
+            // This function will be most easily implemented as follows:
+            // 1. If width/height are smaller than a block, then update the
+            //    indices of the affected blocks.
+            // 2. If width/height are larger than a block, then construct a 9-patch
+            //    of block encodings that represent the rectangle, and write them
+            //    to the compressed buffer as necessary. Whether or not the blocks
+            //    are overwritten by zeros or just their indices are updated is up
+            //    to debate.
+            SkFAIL("Not implemented!");
+        }
+
+        // Blit a pattern of pixels defined by a rectangle-clipped mask;
+        // typically used for text.
+        virtual void blitMask(const SkMask&, const SkIRect& clip) SK_OVERRIDE {
+            // This function is currently not implemented. It is not explicitly
+            // required by the contract, but if at some time a code path runs into
+            // this function (which is entirely possible), it needs to be implemented.
+            //
+            // TODO (krajcevski):
+            // This function will be most easily implemented in the same way as
+            // blitAntiRect above.
+            SkFAIL("Not implemented!");
+        }
+
+        // If the blitter just sets a single value for each pixel, return the
+        // bitmap it draws into, and assign value. If not, return NULL and ignore
+        // the value parameter.
+        virtual const SkBitmap* justAnOpaqueColor(uint32_t* value) SK_OVERRIDE {
+            return NULL;
+        }
+
+        /**
+         * Compressed texture blitters only really work correctly if they get
+         * four blocks at a time. That being said, this blitter tries it's best
+         * to preserve semantics if blitAntiH doesn't get called in too many
+         * weird ways...
+         */
+        virtual int requestRowsPreserved() const { return kR11_EACBlockSz; }
+
+    protected:
+        virtual void onNotifyFinished() { this->flushRuns(); }
+
+    private:
+        static const int kR11_EACBlockSz = 4;
+        static const int kPixelsPerBlock = kR11_EACBlockSz * kR11_EACBlockSz;
+
+        // The longest possible run of pixels that this blitter will receive.
+        // This is initialized in the constructor to 0x7FFE, which is one less
+        // than the largest positive 16-bit integer. We make sure that it's one
+        // less for debugging purposes. We also don't make this variable static
+        // in order to make sure that we can construct a valid pointer to it.
+        const int16_t kLongestRun;
+
+        // Usually used in conjunction with kLongestRun. This is initialized to
+        // zero.
+        const SkAlpha kZeroAlpha;
+
+        // This is the information that we buffer whenever we're asked to blit
+        // a row with this blitter.
+        struct BufferedRun {
+            const SkAlpha* fAlphas;
+            const int16_t* fRuns;
+            int fX, fY;
+        } fBufferedRuns[kR11_EACBlockSz];
+
+        // The next row (0-3) that we need to blit. This value should never exceed
+        // the number of rows that we have (kR11_EACBlockSz)
+        int fNextRun;
+
+        // The width and height of the image that we're blitting
+        const int fWidth;
+        const int fHeight;
+
+        // The R11 EAC buffer that we're blitting into. It is assumed that the buffer
+        // is large enough to store a compressed image of size fWidth*fHeight.
+        uint64_t* const fBuffer;
+
+        // Various utility functions
+        int blocksWide() const { return fWidth / kR11_EACBlockSz; }
+        int blocksTall() const { return fHeight / kR11_EACBlockSz; }
+        int totalBlocks() const { return (fWidth * fHeight) / kPixelsPerBlock; }
+
+        // Returns the block index for the block containing pixel (x, y). Block
+        // indices start at zero and proceed in raster order.
+        int getBlockOffset(int x, int y) const {
+            SkASSERT(x < fWidth);
+            SkASSERT(y < fHeight);
+            const int blockCol = x / kR11_EACBlockSz;
+            const int blockRow = y / kR11_EACBlockSz;
+            return blockRow * this->blocksWide() + blockCol;
+        }
+
+        // Returns a pointer to the block containing pixel (x, y)
+        uint64_t *getBlock(int x, int y) const {
+            return fBuffer + this->getBlockOffset(x, y);
+        }
+
+        // The following function writes the buffered runs to compressed blocks.
+        // If fNextRun < 4, then we fill the runs that we haven't buffered with
+        // the constant zero buffer.
+        void flushRuns();
+    };
 }
 
 #endif