More efficient Jpeg skipScanlines for older versions of libjpeg-turbo

src/codec/SkJpegCodec.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodec.h"
 #include "SkMSAN.h"
 #include "SkJpegCodec.h"
@@ skipping 420 matching lines @@
             // use swizzler to sample row
             fSwizzler->swizzle(dst, dstRow);
             dst = SkTAddOffset<JSAMPLE>(dst, dstRowBytes);
         } else {
             dstRow = SkTAddOffset<JSAMPLE>(dstRow, dstRowBytes);
         }
     }
     return count;
 }
 
-#ifndef TURBO_HAS_SKIP
-// TODO (msarett): Avoid reallocating the memory buffer on each call to skip.
-static uint32_t jpeg_skip_scanlines(jpeg_decompress_struct* dinfo, int count) {
-    SkAutoTMalloc<uint8_t> storage(get_row_bytes(dinfo));
-    uint8_t* storagePtr = storage.get();
-    for (int y = 0; y < count; y++) {
-        if (1 != jpeg_read_scanlines(dinfo, &storagePtr, 1)) {
-            return y;
-        }
-    }
-    return count;
-}
-#endif
-
 bool SkJpegCodec::onSkipScanlines(int count) {
     // Set the jump location for libjpeg errors
     if (setjmp(fDecoderMgr->getJmpBuf())) {
         return fDecoderMgr->returnFalse("setjmp");
     }
 
+#ifdef TURBO_HAS_SKIP
     return (uint32_t) count == jpeg_skip_scanlines(fDecoderMgr->dinfo(), count);
+#else
+    if (!fSrcRow) {
+        fStorage.reset(get_row_bytes(fDecoderMgr->dinfo()));
+        fSrcRow = fStorage.get();
+    }
+
+    for (int y = 0; y < count; y++) {
+        if (1 != jpeg_read_scanlines(fDecoderMgr->dinfo(), &fSrcRow, 1)) {
+            return false;
+        }
+    }
+    return true;
+#endif
 }
 
 static bool is_yuv_supported(jpeg_decompress_struct* dinfo) {
     // Scaling is not supported in raw data mode.
     SkASSERT(dinfo->scale_num == dinfo->scale_denom);
 
     // I can't imagine that this would ever change, but we do depend on it.
     static_assert(8 == DCTSIZE, "DCTSIZE (defined in jpeg library) should always be 8.");
 
     if (JCS_YCbCr != dinfo->jpeg_color_space) {
@@ skipping 174 matching lines @@
 
         JDIMENSION linesRead = jpeg_read_raw_data(dinfo, yuv, numRowsPerBlock);
         if (linesRead < remainingRows) {
             // FIXME: Handle incomplete YUV decodes without signalling an error.
             return kInvalidInput;
         }
     }
 
     return kSuccess;
 }