Make SkCodec support peek() and read()

src/codec/SkCodec_wbmp.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 "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkColorTable.h"
+#include "SkData.h"
 #include "SkStream.h"
 #include "SkCodec_wbmp.h"
 
 // Each bit represents a pixel, so width is actually a number of bits.
 // A row will always be stored in bytes, so we round width up to the
 // nearest multiple of 8 to get the number of bits actually in the row.
 // We then divide by 8 to convert to bytes.
 static inline size_t get_src_row_bytes(int width) {
     return SkAlign8(width) >> 3;
 }
@@ skipping 36 matching lines @@
         uint8_t data;
         if (!read_byte(stream, &data) || data != 0) { // unknown type
             return false;
         }
         if (!read_byte(stream, &data) || (data & 0x9F)) { // skip fixed header
             return false;
         }
     }
 
     uint64_t width, height;
     if (!read_mbf(stream, &width) || width > 0xFFFF || !width) {

[msarett, 2015/11/30 14:08:11]

read_header is called by IsWbmp and NewFromStream.

We could consider not even calculating the width and height for IsWbmp.  We
would just need to surround this entire block by:
if (size) {
  // width, height code
}

I think this might be a bad idea because wbmp doesn't have much of a signature. 
There is probably a risk of non-wbmps being considered wbmps.

[scroggo, 2015/11/30 20:02:30]

Yeah, I considered something like that. You're correct; we might consider
non-wbmps to be wbmps, but only initially. We would then call read_header again
in the constructor, at which point we would realize it's not a wbmp. The only
way I think this would really be a problem is if wbmp were not the last one in
the list we check, and any following format might match something we mistakenly
thought was a wbmp. For example, we may add RAW/DNG to the end (that said, it is
possible it should come before wbmp). I don't know whether an image whose first
two bytes look like a wbmp could also look like a RAW/DNG, but the extra checks
make us more certain, and shouldn't take too long.

         return false;
     }
     if (!read_mbf(stream, &height) || height > 0xFFFF || !height) {
         return false;
     }
     if (size) {
         *size = SkISize::Make(SkToS32(width), SkToS32(height));
     }
     return true;
 }
@@ skipping 65 matching lines @@
         if (!this->readRow(src.get())) {
             *rowsDecoded = y;
             return kIncompleteInput;
         }
         swizzler->swizzle(dstRow, src.get());
         dstRow = SkTAddOffset<void>(dstRow, rowBytes);
     }
     return kSuccess;
 }
 
-bool SkWbmpCodec::IsWbmp(SkStream* stream) {
-    return read_header(stream, nullptr);
+bool SkWbmpCodec::IsWbmp(const char* buffer, size_t bytesRead) {
+    SkAutoTUnref<SkData> data(SkData::NewWithoutCopy(buffer, bytesRead));
+    SkMemoryStream stream(data);
+    return read_header(&stream, nullptr);
 }
 
 SkCodec* SkWbmpCodec::NewFromStream(SkStream* stream) {
     SkAutoTDelete<SkStream> streamDeleter(stream);
     SkISize size;
     if (!read_header(stream, &size)) {
         return nullptr;
     }
     SkImageInfo info = SkImageInfo::Make(size.width(), size.height(),
             kGray_8_SkColorType, kOpaque_SkAlphaType);
@@ skipping 34 matching lines @@
     // Initialize the swizzler
     fSwizzler.reset(this->initializeSwizzler(dstInfo, get_color_ptr(fColorTable.get()), options));
     if (nullptr == fSwizzler.get()) {
         return kInvalidConversion;
     }
 
     fSrcBuffer.reset(fSrcRowBytes);
 
     return kSuccess;
 }