Create a scanline decoder without creating a codec

include/codec/SkScanlineDecoder.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 SkScanlineDecoder_DEFINED
 #define SkScanlineDecoder_DEFINED
 
 #include "SkTypes.h"
 #include "SkCodec.h"
 #include "SkTemplates.h"
 #include "SkImageInfo.h"
 
 class SkScanlineDecoder : public SkNoncopyable {
 public:
     /**
+     *  If this stream represents an encoded image that we know how to decode
+     *  in scanlines, return an SkScanlineDecoder that can decode it. Otherwise
+     *  return NULL.
+     *
+     *  start() must be called in order to decode any scanlines.
+     *
+     *  If NULL is returned, the stream is deleted immediately. Otherwise, the
+     *  SkScanlineDecoder takes ownership of it, and will delete it when done
+     *  with it.
+     */
+    static SkScanlineDecoder* NewFromStream(SkStream*);
+
+    /**
+     *  Similar to NewFromStream, but reads from an SkData.
+     *
+     *  Will take a ref if it returns a scanline decoder, else will not affect
+     *  the data.
+     */
+    static SkScanlineDecoder* NewFromData(SkData*);
+
+    /**
      *  Clean up after reading/skipping scanlines.
      *
      *  It is possible that not all scanlines will have been read/skipped.  In
      *  fact, in the case of subset decodes, it is likely that there will be
      *  scanlines at the bottom of the image that have been ignored.
      */
     virtual ~SkScanlineDecoder() {}
 
     /**
+     *  Returns the default info, corresponding to the encoded data.
+     */
+    const SkImageInfo& getInfo() { return fSrcInfo; }
+
+    /**
+     *  Initialize on the first scanline, with the specified options.
+     *
+     *  This must be called in order to call getScanlnies or skipScanlines. If
+     *  it has been called before, this will require rewinding the stream.
+     *
+     *  @param dstInfo Info of the destination. If the dimensions do not match
+     *      those of getInfo, this implies a scale.
+     *  @param options Contains decoding options, including if memory is zero
+     *      initialized.
+     *  @param ctable A pointer to a color table.  When dstInfo.colorType() is
+     *      kIndex8, this should be non-NULL and have enough storage for 256
+     *      colors.  The color table will be populated after decoding the palette.
+     *  @param ctableCount A pointer to the size of the color table.  When
+     *      dstInfo.colorType() is kIndex8, this should be non-NULL.  It will
+     *      be modified to the true size of the color table (<= 256) after
+     *      decoding the palette.
+     *  @return Enum representing success or reason for failure.
+     */
+    SkCodec::Result start(const SkImageInfo& dstInfo, const SkCodec::Options* options,
+                          SkPMColor ctable[], int* ctableCount);
+
+    /**
+     *  Simplified version of start() that asserts that info is NOT
+     *  kIndex8_SkColorType and uses the default Options.
+     */
+    SkCodec::Result start(const SkImageInfo& dstInfo);
+
+    /**
      *  Write the next countLines scanlines into dst.
      *
+     *  Not valid to call before calling start().
+     *
      *  @param dst Must be non-null, and large enough to hold countLines
      *      scanlines of size rowBytes.
      *  @param countLines Number of lines to write.
      *  @param rowBytes Number of bytes per row. Must be large enough to hold
      *      a scanline based on the SkImageInfo used to create this object.
      */
     SkCodec::Result getScanlines(void* dst, int countLines, size_t rowBytes) {
+        SkASSERT(!fDstInfo.isEmpty());
         if ((rowBytes < fDstInfo.minRowBytes() && countLines > 1 ) || countLines <= 0
                 || fCurrScanline + countLines > fDstInfo.height()) {
             return SkCodec::kInvalidParameters;
         }
         const SkCodec::Result result = this->onGetScanlines(dst, countLines, rowBytes);
         fCurrScanline += countLines;
         return result;
     }
 
     /**
      *  Skip count scanlines.
      *
+     *  Not valid to call before calling start().
+     *
      *  The default version just calls onGetScanlines and discards the dst.
      *  NOTE: If skipped lines are the only lines with alpha, this default
      *  will make reallyHasAlpha return true, when it could have returned
      *  false.
      */
     SkCodec::Result skipScanlines(int countLines) {
+        SkASSERT(!fDstInfo.isEmpty());
         if (fCurrScanline + countLines > fDstInfo.height()) {
             // Arguably, we could just skip the scanlines which are remaining,
             // and return kSuccess. We choose to return invalid so the client
             // can catch their bug.
             return SkCodec::kInvalidParameters;
         }
         const SkCodec::Result result = this->onSkipScanlines(countLines);
         fCurrScanline += countLines;
         return result;
     }
 
     /**
      *  Some images may initially report that they have alpha due to the format
      *  of the encoded data, but then never use any colors which have alpha
      *  less than 100%. This function can be called *after* decoding to
      *  determine if such an image truly had alpha. Calling it before decoding
      *  is undefined.
      *  FIXME: see skbug.com/3582.
      */
     bool reallyHasAlpha() const {
         return this->onReallyHasAlpha();
     }
 
 protected:
-    SkScanlineDecoder(const SkImageInfo& requested)
-        : fDstInfo(requested)
+    SkScanlineDecoder(const SkImageInfo& srcInfo)
+        : fSrcInfo(srcInfo)
+        , fDstInfo()
         , fCurrScanline(0) {}
 
     virtual bool onReallyHasAlpha() const { return false; }
 
     const SkImageInfo& dstInfo() const { return fDstInfo; }
 
 private:
-    const SkImageInfo   fDstInfo;
+    const SkImageInfo   fSrcInfo;
+    SkImageInfo         fDstInfo;
     int                 fCurrScanline;
 
+    virtual SkCodec::Result onStart(const SkImageInfo& dstInfo,
+                                    const SkCodec::Options& options,
+                                    SkPMColor ctable[], int* ctableCount) = 0;
+
     // Naive default version just calls onGetScanlines on temp memory.
     virtual SkCodec::Result onSkipScanlines(int countLines) {
         SkAutoMalloc storage(fDstInfo.minRowBytes());
         // Note that we pass 0 to rowBytes so we continue to use the same memory.
         // Also note that while getScanlines checks that rowBytes is big enough,
         // onGetScanlines bypasses that check.
         // Calling the virtual method also means we do not double count
         // countLines.
         return this->onGetScanlines(storage.get(), countLines, 0);
     }
 
     virtual SkCodec::Result onGetScanlines(void* dst, int countLines,
                                                     size_t rowBytes) = 0;
 
 };
 #endif // SkScanlineDecoder_DEFINED