private static FrameHeader findFrame(BufferedBinaryReader source, ref VBRData oVBR, SizeInfo sizeInfo)
{
byte[] headerData = new byte[4];
FrameHeader result = new FrameHeader();
source.Read(headerData, 0, 4);
result.Found = isValidFrameHeader(headerData);
/*
* Many things can actually be found before a proper MP3 header :
* - Padding with 0x55, 0xAA and even 0xFF bytes
* - RIFF header declaring either MP3 or WAVE data
* - Xing encoder-specific frame
* - One of the above with a few "parasite" bytes before their own header
*
* The most solid way to deal with all of them is to "scan" the file until proper MP3 header is found.
* This method may not the be fastest, but ensures audio data is actually detected, whatever garbage lies before
*/
if (!result.Found)
{
// "Quick win" for files starting with padding bytes
// 4 identical bytes => MP3 starts with padding bytes => Skip padding
if ((headerData[0] == headerData[1]) && (headerData[1] == headerData[2]) && (headerData[2] == headerData[3]))
{
// Scan the whole padding until it stops
while (headerData[0] == source.ReadByte())
{
;
}
source.Seek(-1, SeekOrigin.Current);
// If padding uses 0xFF bytes, take one step back in case MP3 header lies there
if (0xFF == headerData[0])
{
source.Seek(-1, SeekOrigin.Current);
}
source.Read(headerData, 0, 4);
result.Found = isValidFrameHeader(headerData);
}
// Blindly look for the MP3 header
if (!result.Found)
{
source.Seek(-4, SeekOrigin.Current);
long limit = sizeInfo.ID3v2Size + (long)Math.Round((source.Length - sizeInfo.ID3v2Size) * 0.3);
// Look for the beginning of the MP3 header (2nd byte is variable, so it cannot be searched that way)
while (!result.Found && source.Position < limit)
{
while (0xFF != source.ReadByte() && source.Position < limit)
{
;
}
source.Seek(-1, SeekOrigin.Current);
source.Read(headerData, 0, 4);
result.Found = isValidFrameHeader(headerData);
// Valid header candidate found
// => let's see if it is a legit MP3 header by using its Size descriptor to find the next header
if (result.Found)
{
result.LoadFromByteArray(headerData);
result.Position = source.Position - 4;
result.Size = getFrameSize(result);
byte[] nextHeaderData = new byte[4];
source.Seek(result.Position + result.Size, SeekOrigin.Begin);
source.Read(nextHeaderData, 0, 4);
result.Found = isValidFrameHeader(nextHeaderData);
if (result.Found)
{
source.Seek(result.Position + 4, SeekOrigin.Begin); // Go back to header candidate position
break;
}
else
{
// Restart looking for a candidate
source.Seek(result.Position + 1, SeekOrigin.Begin);
}
}
else
{
source.Seek(-3, SeekOrigin.Current);
}
}
}
}
if (result.Found)
{
result.LoadFromByteArray(headerData);
result.Position = source.Position - 4;
// result.Xing = isXing(i + 4, Data); // Will look into it when encoder ID is needed by upper interfaces
// Look for VBR signature
oVBR = findVBR(source, result.Position + getVBRDeviation(result));
}
return(result);
}
