Improve MP3 detection slightly more.

Looking too much result in increase of false-positive and vice-versa. So only look first 128 bytes of the file (after the ID3 tags). Also reject known audio file signature such as WAV, Ogg (Theora/Vorbis), and FLAC.

Fixes #1939

src/modules/sound/lullaby/MP3Decoder.cpp

@@ -30,24 +30,14 @@ namespace sound
 namespace lullaby
 {
 
-// Copied from dr_mp3 function drmp3_hdr_valid()
-static bool isMP3HeaderValid(const uint8 *h)
-{
- return
- // Sync bits
- h[0] == 0xff &&
- ((h[1] & 0xF0) == 0xf0 || (h[1] & 0xFE) == 0xe2) &&
- // Check layer
- (DRMP3_HDR_GET_LAYER(h) != 0) &&
- // Check bitrate
- (DRMP3_HDR_GET_BITRATE(h) != 15) &&
- // Check sample rate
- (DRMP3_HDR_GET_SAMPLE_RATE(h) != 3);
-}
-
+// dr_mp3 looks too far, but it can lead to false-positive. dr_mp3 also doesn't recognize ID3.
+// Implement our own "MP3 detection" heuristics that also help dr_mp3 skip ID3 tags.
 static int64 findFirstValidHeader(Stream* stream)
 {
- constexpr size_t LOOKUP_SIZE = 16384;
+ // Tweaking this variable has trade-off between false-positive and false-negative. Lesser value
+ // means lesser false-positive and false-negative. Larger value means more false-positive AND
+ // false-negative.
+ constexpr size_t LOOKUP_SIZE = 128;
 
  std::vector<uint8> data(LOOKUP_SIZE);
  uint8 header[10];
@@ -58,6 +48,14 @@ static int64 findFirstValidHeader(Stream* stream)
  if (stream->read(header, 10) < 10)
  return -1;
 
+ // Test for known audio formats which are definitely not MP3.
+ if (memcmp(header, "RIFF", 4) == 0)
+ return -1;
+ if (memcmp(header, "OggS", 4) == 0)
+ return -1;
+ if (memcmp(header, "fLaC", 4) == 0)
+ return -1;
+
  if (memcmp(header, "TAG", 3) == 0)
  {
  // ID3v1 tag is always 128 bytes long
@@ -93,14 +91,14 @@ static int64 findFirstValidHeader(Stream* stream)
  // Look for mp3 data
  for (int i = 0; i < buffer - 4; i++, offset++)
  {
- if (isMP3HeaderValid(dataPtr++))
+ if (drmp3_hdr_valid(dataPtr++))
  {
  stream->seek(offset, Stream::SEEKORIGIN_BEGIN);
  return offset;
  }
  }
 
- // No valid MP3 frame found in first 16KB data
+ // No valid MP3 frame found in first few bytes of the data.
  return -1;
 }