Generalize compressed blitter into its own templated class

src/utils/SkTextureCompressor_Blitter.h

Index: src/utils/SkTextureCompressor_Blitter.h
diff --git a/src/utils/SkTextureCompressor_Blitter.h b/src/utils/SkTextureCompressor_Blitter.h
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index 0000000000000000000000000000000000000000..23265a4d184c6645d72c6ea38544f3d75730d57d
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+++ b/src/utils/SkTextureCompressor_Blitter.h
@@ -0,0 +1,405 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkTextureCompressor_Blitter_DEFINED
+#define SkTextureCompressor_Blitter_DEFINED
+
+#include "SkTypes.h"
+#include "SkBlitter.h"
+
+namespace SkTextureCompressor {
+
+// The function used to compress an A8 block. This function is expected to be
+// used as a template argument to SkCompressedAlphaBlitter. The layout of the
+// block is also expected to be in column-major order.
+typedef void (*CompressA8Proc)(uint8_t* dst, const uint8_t block[]);
+
+// This class implements a blitter that blits directly into a buffer that will
+// be used as an compressed alpha texture. We compute this buffer by
+// buffering scan lines and then outputting them all at once. The number of
+// scan lines buffered is controlled by kBlockSize
+template<int BlockDim, int EncodedBlockSize, CompressA8Proc CompressionProc>
+class SkTCompressedAlphaBlitter : public SkBlitter {
+public:
+    SkTCompressedAlphaBlitter(int width, int height, void *compressedBuffer)
+        // 0x7FFE is one minus the largest positive 16-bit int. We use it for
+        // debugging to make sure that we're properly setting the nextX distance
+        // in flushRuns(). 
+        : kLongestRun(0x7FFE), kZeroAlpha(0)
+        , fNextRun(0)
+        , fWidth(width)
+        , fHeight(height)
+        , fBuffer(compressedBuffer)
+        {
+            SkASSERT((width % BlockDim) == 0);
+            SkASSERT((height % BlockDim) == 0);
+        }
+
+    virtual ~SkTCompressedAlphaBlitter() { 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 {
+        // Make sure that the new row to blit is either the first
+        // row that we're blitting, or it's exactly the next scan row
+        // since the last row that we blit. This is to ensure that when
+        // we go to flush the runs, that they are all the same four
+        // runs.
+        if (fNextRun > 0 &&
+            ((x != fBufferedRuns[fNextRun-1].fX) ||
+             (y-1 != fBufferedRuns[fNextRun-1].fY))) {
+            this->flushRuns();
+        }
+
+        // Align the rows to a block boundary. If we receive rows that
+        // are not on a block boundary, then fill in the preceding runs
+        // with zeros. We do this by producing a single RLE that says
+        // that we have 0x7FFE pixels of zero (0x7FFE = 32766).
+        const int row = BlockDim * (y / BlockDim);
+        while ((row + fNextRun) < y) {
+            fBufferedRuns[fNextRun].fAlphas = &kZeroAlpha;
+            fBufferedRuns[fNextRun].fRuns = &kLongestRun;
+            fBufferedRuns[fNextRun].fX = 0;
+            fBufferedRuns[fNextRun].fY = row + fNextRun;
+            ++fNextRun;
+        }
+
+        // Make sure that our assumptions aren't violated...
+        SkASSERT(fNextRun == (y % BlockDim));
+        SkASSERT(fNextRun == 0 || fBufferedRuns[fNextRun - 1].fY < y);
+
+        // Set the values of the next run
+        fBufferedRuns[fNextRun].fAlphas = antialias;
+        fBufferedRuns[fNextRun].fRuns = runs;
+        fBufferedRuns[fNextRun].fX = x;
+        fBufferedRuns[fNextRun].fY = y;
+
+        // If we've output a block of scanlines in a row that don't violate our
+        // assumptions, then it's time to flush them...
+        if (BlockDim == ++fNextRun) {
+            this->flushRuns();
+        }
+    }
+    
+    // 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
+     * BlockDim rows 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 BlockDim; }
+
+private:
+    static const int kPixelsPerBlock = BlockDim * BlockDim;
+
+    // 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[BlockDim];
+
+    // The next row [0, BlockDim) that we need to blit.
+    int fNextRun;
+
+    // The width and height of the image that we're blitting
+    const int fWidth;
+    const int fHeight;
+
+    // The compressed buffer that we're blitting into. It is assumed that the buffer
+    // is large enough to store a compressed image of size fWidth*fHeight.
+    void* const fBuffer;
+
+    // Various utility functions
+    int blocksWide() const { return fWidth / BlockDim; }
+    int blocksTall() const { return fHeight / BlockDim; }
+    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 / BlockDim;
+        const int blockRow = y / BlockDim;
+        return blockRow * this->blocksWide() + blockCol;
+    }
+
+    // Returns a pointer to the block containing pixel (x, y)
+    uint8_t *getBlock(int x, int y) const {
+        uint8_t* ptr = reinterpret_cast<uint8_t*>(fBuffer);
+        return ptr + EncodedBlockSize*this->getBlockOffset(x, y);
+    }
+
+    // Updates the block whose columns are stored in block. curAlphai is expected
+    // to store the alpha values that will be placed within each of the columns in
+    // the range [col, col+colsLeft).
+    typedef uint32_t Column[BlockDim/4];
+    typedef uint32_t Block[BlockDim][BlockDim/4];
+    inline void updateBlockColumns(Block block, const int col,
+                                   const int colsLeft, const Column curAlphai) {
+        SkASSERT(NULL != block);
+        SkASSERT(col + colsLeft <= 4);
+
+        for (int i = col; i < (col + colsLeft); ++i) {
+            memcpy(block[i], curAlphai, sizeof(Column));
+        }
+    }
+
+    // The following function writes the buffered runs to compressed blocks.
+    // If fNextRun < BlockDim, then we fill the runs that we haven't buffered with
+    // the constant zero buffer.
+    void flushRuns() {
+        // If we don't have any runs, then just return.
+        if (0 == fNextRun) {
+            return;
+        }
+
+#ifndef NDEBUG
+        // Make sure that if we have any runs, they all match
+        for (int i = 1; i < fNextRun; ++i) {
+            SkASSERT(fBufferedRuns[i].fY == fBufferedRuns[i-1].fY + 1);
+            SkASSERT(fBufferedRuns[i].fX == fBufferedRuns[i-1].fX);
+        }
+#endif
+
+        // If we don't have as many runs as we have rows, fill in the remaining
+        // runs with constant zeros.
+        for (int i = fNextRun; i < BlockDim; ++i) {
+            fBufferedRuns[i].fY = fBufferedRuns[0].fY + i;
+            fBufferedRuns[i].fX = fBufferedRuns[0].fX;
+            fBufferedRuns[i].fAlphas = &kZeroAlpha;
+            fBufferedRuns[i].fRuns = &kLongestRun;
+        }
+
+        // Make sure that our assumptions aren't violated.
+        SkASSERT(fNextRun > 0 && fNextRun <= BlockDim);
+        SkASSERT((fBufferedRuns[0].fY % BlockDim) == 0);
+
+        // The following logic walks BlockDim rows at a time and outputs compressed
+        // blocks to the buffer passed into the constructor.
+        // We do the following:
+        //
+        //      c1 c2 c3 c4
+        // -----------------------------------------------------------------------
+        // ... |  |  |  |  |  ----> fBufferedRuns[0]
+        // -----------------------------------------------------------------------
+        // ... |  |  |  |  |  ----> fBufferedRuns[1]
+        // -----------------------------------------------------------------------
+        // ... |  |  |  |  |  ----> fBufferedRuns[2]
+        // -----------------------------------------------------------------------
+        // ... |  |  |  |  |  ----> fBufferedRuns[3]
+        // -----------------------------------------------------------------------
+        // 
+        // curX -- the macro X value that we've gotten to.
+        // c[BlockDim] -- the buffers that represent the columns of the current block
+        //                  that we're operating on
+        // curAlphaColumn -- buffer containing the column of alpha values from fBufferedRuns.
+        // nextX -- for each run, the next point at which we need to update curAlphaColumn
+        //          after the value of curX.
+        // finalX -- the minimum of all the nextX values.
+        //
+        // curX advances to finalX outputting any blocks that it passes along
+        // the way. Since finalX will not change when we reach the end of a
+        // run, the termination criteria will be whenever curX == finalX at the
+        // end of a loop.
+
+        // Setup:
+        Block block;
+        sk_bzero(block, sizeof(block));
+
+        Column curAlphaColumn;
+        sk_bzero(curAlphaColumn, sizeof(curAlphaColumn));
+
+        SkAlpha *curAlpha = reinterpret_cast<SkAlpha*>(&curAlphaColumn);
+
+        int nextX[BlockDim];
+        for (int i = 0; i < BlockDim; ++i) {
+            nextX[i] = 0x7FFFFF;
+        }
+
+        uint8_t* outPtr = this->getBlock(fBufferedRuns[0].fX, fBufferedRuns[0].fY);
+
+        // Populate the first set of runs and figure out how far we need to
+        // advance on the first step
+        int curX = 0;
+        int finalX = 0xFFFFF;
+        for (int i = 0; i < BlockDim; ++i) {
+            nextX[i] = *(fBufferedRuns[i].fRuns);
+            curAlpha[i] = *(fBufferedRuns[i].fAlphas);
+
+            finalX = SkMin32(nextX[i], finalX);
+        }
+
+        // Make sure that we have a valid right-bound X value
+        SkASSERT(finalX < 0xFFFFF);
+
+        // Run the blitter...
+        while (curX != finalX) {
+            SkASSERT(finalX >= curX);
+
+            // Do we need to populate the rest of the block?
+            if ((finalX - (BlockDim*(curX / BlockDim))) >= BlockDim) {
+                const int col = curX % BlockDim;
+                const int colsLeft = BlockDim - col;
+                SkASSERT(curX + colsLeft <= finalX);
+
+                this->updateBlockColumns(block, col, colsLeft, curAlphaColumn);
+
+                // Write this block
+                CompressionProc(outPtr, reinterpret_cast<uint8_t*>(block));
+                outPtr += EncodedBlockSize;
+                curX += colsLeft;
+            }
+
+            // If we can advance even further, then just keep memsetting the block
+            if ((finalX - curX) >= BlockDim) {
+                SkASSERT((curX % BlockDim) == 0);
+
+                const int col = 0;
+                const int colsLeft = BlockDim;
+
+                this->updateBlockColumns(block, col, colsLeft, curAlphaColumn);
+
+                // While we can keep advancing, just keep writing the block.
+                uint8_t lastBlock[EncodedBlockSize];
+                CompressionProc(lastBlock, reinterpret_cast<uint8_t*>(block));
+                while((finalX - curX) >= BlockDim) {
+                    memcpy(outPtr, lastBlock, EncodedBlockSize);
+                    outPtr += EncodedBlockSize;
+                    curX += BlockDim;
+                }
+            }
+
+            // If we haven't advanced within the block then do so.
+            if (curX < finalX) {
+                const int col = curX % BlockDim;
+                const int colsLeft = finalX - curX;
+
+                this->updateBlockColumns(block, col, colsLeft, curAlphaColumn);
+                curX += colsLeft;
+            }
+
+            SkASSERT(curX == finalX);
+
+            // Figure out what the next advancement is...
+            for (int i = 0; i < BlockDim; ++i) {
+                if (nextX[i] == finalX) {
+                    const int16_t run = *(fBufferedRuns[i].fRuns);
+                    fBufferedRuns[i].fRuns += run;
+                    fBufferedRuns[i].fAlphas += run;
+                    curAlpha[i] = *(fBufferedRuns[i].fAlphas);
+                    nextX[i] += *(fBufferedRuns[i].fRuns);
+                }
+            }
+
+            finalX = 0xFFFFF;
+            for (int i = 0; i < BlockDim; ++i) {
+                finalX = SkMin32(nextX[i], finalX);
+            }
+        }
+
+        // If we didn't land on a block boundary, output the block...
+        if ((curX % BlockDim) > 1) {
+            CompressionProc(outPtr, reinterpret_cast<uint8_t*>(block));
+        }
+
+        fNextRun = 0;
+    }
+};
+
+}  // namespace SkTextureCompressor
+
+#endif  // SkTextureCompressor_Blitter_DEFINED