Simplify checking for MP3 format

media/base/container_names.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "media/base/container_names.h"
 
 #include <stddef.h>
 
 #include <cctype>
 #include <limits>
@@ skipping 1066 matching lines @@
     else
       bitrate = kBitRateTableV2L23[bitrate_index];
   }
   if (layer == LAYER_1)
     *framesize = ((12000 * bitrate) / sampling_rate + padding) * 4;
   else
     *framesize = (144000 * bitrate) / sampling_rate + padding;
   return (bitrate > 0 && sampling_rate > 0);
 }
 
-// Extract a size encoded the MP3 way.
-static int GetMp3HeaderSize(const uint8_t* buffer, int buffer_size) {
-  DCHECK_GE(buffer_size, 9);
-  int size = ((buffer[6] & 0x7f) << 21) + ((buffer[7] & 0x7f) << 14) +
-             ((buffer[8] & 0x7f) << 7) + (buffer[9] & 0x7f) + 10;
-  if (buffer[5] & 0x10)  // Footer added?
-    size += 10;
-  return size;
-}
-
 // Additional checks for a MP3 container.
-static bool CheckMp3(const uint8_t* buffer, int buffer_size, bool seenHeader) {
-  RCHECK(buffer_size >= 10);  // Must be enough to read the initial header.
-
-  int framesize;
+static bool CheckMp3(const uint8_t* buffer, int buffer_size) {
+  // This function assumes that the ID3 header is not present in the file and
+  // simply checks for several valid MPEG audio buffers after skipping any
+  // optional padding characters.
   int numSeen = 0;
   int offset = 0;
-  if (seenHeader) {
-    offset = GetMp3HeaderSize(buffer, buffer_size);
-  } else {
-    // Skip over leading 0's.
-    while (offset < buffer_size && buffer[offset] == 0)
-      ++offset;
-  }
+
+  // Skip over any padding (0's).
+  while (offset < buffer_size && buffer[offset] == 0)
+    ++offset;
 
   while (offset + 3 < buffer_size) {
+    int framesize;
     RCHECK(ValidMpegAudioFrameHeader(
         buffer + offset, buffer_size - offset, &framesize));
 
     // Have we seen enough valid headers?
     if (++numSeen > 10)
       return true;
     offset += framesize;
   }
   // Off the end of the buffer, return success if a few valid headers seen.
   return numSeen > 2;
@@ skipping 463 matching lines @@
   }
 
   // Now try a few different ones that look at something other
   // than the first 4 bytes.
   uint32_t first3 = first4 & 0xffffff00;
   switch (first3) {
     case TAG('C','W','S',0):
     case TAG('F','W','S',0):
       return CONTAINER_SWF;
 
     case TAG('I','D','3',0):

[DaleCurtis, 2017/04/13 23:25:42]

Ah, I think this accidentally completely dropped non-id3 mp3 files.

-      if (CheckMp3(buffer, buffer_size, true))
-        return CONTAINER_MP3;
-      break;
+      return CONTAINER_MP3;
   }
 
   // Maybe the first 2 characters are something we can use.
   uint32_t first2 = Read16(buffer);
   switch (first2) {
     case kAc3SyncWord:
       if (CheckAc3(buffer, buffer_size))
         return CONTAINER_AC3;
       if (CheckEac3(buffer, buffer_size))
         return CONTAINER_EAC3;
       break;
 
     case 0xfff0:
     case 0xfff1:
     case 0xfff8:
     case 0xfff9:
       if (CheckAac(buffer, buffer_size))
         return CONTAINER_AAC;
       break;
   }
 
-  // Check if the file is in MP3 format without the header.
-  if (CheckMp3(buffer, buffer_size, false))
+  // Check if the file is in MP3 format without the ID3 header.
+  if (CheckMp3(buffer, buffer_size))
     return CONTAINER_MP3;
 
   return CONTAINER_UNKNOWN;
 }
 
 // Attempt to determine the container name from the buffer provided.
 MediaContainerName DetermineContainer(const uint8_t* buffer, int buffer_size) {
   DCHECK(buffer);
 
   // Since MOV/QuickTime/MPEG4 streams are common, check for them first.
@@ skipping 39 matching lines @@
     if (CheckEac3(buffer + offset, buffer_size - offset))
       return CONTAINER_EAC3;
   }
 
   return CONTAINER_UNKNOWN;
 }
 
 }  // namespace container_names
 
 }  // namespace media