Add support for raw lossless bitstream in decoder.

Previously, it used to assume any raw bitstream is a VP8 one.

Also,
- Factor out VP8CheckSignature() & VP8LCheckSignature().
- Use a local var for *data_ptr in ParseVP8Header() for
readability.

Change-Id: I0fa8aa177dad7865e00c8898f7e7ce76a9db19d5

src/dec/vp8.c

@@ -82,13 +82,18 @@ int VP8SetError(VP8Decoder* const dec,
 
 //------------------------------------------------------------------------------
 
+int VP8CheckSignature(const uint8_t* const data, size_t data_size) {
+  return (data_size >= 3 &&
+          data[0] == 0x9d && data[1] == 0x01 && data[2] == 0x2a);
+}
+
 int VP8GetInfo(const uint8_t* data, size_t data_size, size_t chunk_size,
                int* width, int* height) {
   if (data == NULL || data_size < VP8_FRAME_HEADER_SIZE) {
     return 0;         // not enough data
   }
   // check signature
-  if (data[3] != 0x9d || data[4] != 0x01 || data[5] != 0x2a) {
+  if (!VP8CheckSignature(data + 3, data_size - 3)) {
     return 0;         // Wrong signature.
   } else {
     const uint32_t bits = data[0] | (data[1] << 8) | (data[2] << 16);
@@ -316,7 +321,7 @@ int VP8GetHeaders(VP8Decoder* const dec, VP8Io* const io) {
       return VP8SetError(dec, VP8_STATUS_NOT_ENOUGH_DATA,
                          "cannot parse picture header");
     }
-    if (buf[0] != 0x9d || buf[1] != 0x01 || buf[2] != 0x2a) {
+    if (!VP8CheckSignature(buf, buf_size)) {
       return VP8SetError(dec, VP8_STATUS_BITSTREAM_ERROR,
                          "Bad code word");
     }

src/dec/vp8i.h

@@ -290,6 +290,9 @@ struct VP8Decoder {
 int VP8SetError(VP8Decoder* const dec,
                 VP8StatusCode error, const char* const msg);
 
+// Returns true if the next 3 bytes in data contain the VP8 signature.
+int VP8CheckSignature(const uint8_t* const data, size_t data_size);
+
 // Validates the VP8 data-header and retrieves basic header information viz
 // width and height. Returns 0 in case of formatting error. *width/*height
 // can be passed NULL.

src/dec/vp8l.c

@@ -78,11 +78,15 @@ static int DecodeImageStream(int xsize, int ysize,
 
 //------------------------------------------------------------------------------
 
+int VP8LCheckSignature(const uint8_t* const data, size_t size) {
+  return (size >= 1) &&
+         (data[0] == VP8L_MAGIC_BYTE || data[0] == VP8L_MAGIC_BYTE_RSVD);
+}
+
 static int ReadImageSize(VP8LBitReader* const br,
                          int* const width, int* const height) {
-  const int signature = VP8LReadBits(br, 8);
-  if (signature != VP8L_MAGIC_BYTE &&
-      signature != VP8L_MAGIC_BYTE_RSVD) {
+  const uint8_t signature = VP8LReadBits(br, 8);
+  if (!VP8LCheckSignature(&signature, 1)) {
     return 0;
   }
   *width = VP8LReadBits(br, VP8L_IMAGE_SIZE_BITS) + 1;

src/dec/vp8li.h

@@ -87,6 +87,9 @@ typedef struct {
 
 // in vp8l.c
 
+// Returns true if the next byte(s) in data is a VP8L signature.
+int VP8LCheckSignature(const uint8_t* const data, size_t size);
+
 // Validates the VP8L data-header and retrieves basic header information viz
 // width and height. Returns 0 in case of formatting error. width/height
 // can be passed NULL.

src/dec/webp.c

@@ -187,43 +187,49 @@ static VP8StatusCode ParseOptionalChunks(const uint8_t** data,
   }
 }
 
-// Validates the VP8 Header ("VP8 nnnn" or "VP8L nnnn") and skips over it.
+// Validates the VP8/VP8L Header ("VP8 nnnn" or "VP8L nnnn") and skips over it.
 // Returns VP8_STATUS_BITSTREAM_ERROR for invalid (chunk larger than
-//         riff_size) VP8 header,
+//         riff_size) VP8/VP8L header,
 //         VP8_STATUS_NOT_ENOUGH_DATA in case of insufficient data, and
 //         VP8_STATUS_OK otherwise.
 // If a VP8/VP8L chunk is found, chunk_size is set to the total number of bytes
 // extracted from the VP8/VP8L chunk header.
-// The flag 'is_lossless' is set to 1 in case of VP8L chunk.
-static VP8StatusCode ParseVP8Header(const uint8_t** data, size_t* data_size,
+// The flag 'is_lossless' is set to 1 in case of VP8L chunk / raw VP8L data.
+static VP8StatusCode ParseVP8Header(const uint8_t** data_ptr, size_t* data_size,
                                     size_t riff_size,
                                     size_t* chunk_size, int* is_lossless) {
-  const int is_vp8 = !memcmp(*data, "VP8 ", TAG_SIZE);
-  const int is_vp8l = !memcmp(*data, "VP8L", TAG_SIZE);
+  const uint8_t* const data = *data_ptr;
+  const int is_vp8 = !memcmp(data, "VP8 ", TAG_SIZE);
+  const int is_vp8l = !memcmp(data, "VP8L", TAG_SIZE);
   const uint32_t minimal_size =
-      TAG_SIZE + CHUNK_HEADER_SIZE;  // "WEBP" + "VP8 nnnn"
+      TAG_SIZE + CHUNK_HEADER_SIZE;  // "WEBP" + "VP8 nnnn" OR
+                                     // "WEBP" + "VP8Lnnnn"
   assert(data);
   assert(data_size);
   assert(chunk_size);
   assert(is_lossless);
 
-  *chunk_size = *data_size;   // default: raw data
-
   if (*data_size < CHUNK_HEADER_SIZE) {
     return VP8_STATUS_NOT_ENOUGH_DATA;  // Insufficient data.
   }
 
   if (is_vp8 || is_vp8l) {
-    const uint32_t size = get_le32(*data + TAG_SIZE);
+    // Bitstream contains VP8/VP8L header.
+    const uint32_t size = get_le32(data + TAG_SIZE);
     if ((riff_size >= minimal_size) && (size > riff_size - minimal_size)) {
       return VP8_STATUS_BITSTREAM_ERROR;  // Inconsistent size information.
     }
     // Skip over CHUNK_HEADER_SIZE bytes from VP8/VP8L Header.
     *chunk_size = size;
-    *data += CHUNK_HEADER_SIZE;
+    *data_ptr += CHUNK_HEADER_SIZE;
     *data_size -= CHUNK_HEADER_SIZE;
     *is_lossless = is_vp8l;
+  } else {
+    // Raw VP8/VP8L bitstream (no header).
+    *is_lossless = VP8LCheckSignature(data, *data_size);
+    *chunk_size = *data_size;
   }
+
   return VP8_STATUS_OK;
 }