/// <summary>
/// Parse ID3 tags from stream and calculate where the audio must be
/// </summary>
/// <remarks>
/// if this throws an exception, the File is not usable.
/// </remarks>
private void Initialise()
{
// create an empty framemodel, to use if we don't parse anything better
FrameModel tagModel = new FrameModel();
// clear out any previous taghandler
_tagHandler = null;
// don't know how big the audio is until we've parsed the tags
_audio = null;
UInt32 audioNumBytes;
using (FileStream sourceStream = _sourceFileInfo.Open(FileMode.Open, FileAccess.Read, FileShare.Read))
{
// all the header calculations use UInt32;
// this guarantees all the file offsets we have to deal with fit in a UInt32
if (sourceStream.Length > UInt32.MaxValue)
{
throw new InvalidAudioFrameException("MP3 file can't be bigger than 4gb");
}
// in the absence of any recognised tags,
// audio starts at the start
_audioStart = 0;
// audio is entire file length
audioNumBytes = (UInt32)sourceStream.Length;
// try to read an ID3v1 block.
// If ID3v2 block exists, its values overwrite these
// Otherwise, if ID3V1 block exists, its values are used
// The audio is anything that's left after all the tags are excluded.
try
{
ID3v1 id3v1 = new ID3v1();
id3v1.Deserialize(sourceStream);
// fill in ID3v2 block from the ID3v1 data
tagModel = id3v1.FrameModel;
// audio is shorter by the length of the id3v1 tag
audioNumBytes -= ID3v1.TagLength;
}
catch (TagNotFoundException)
{
// ignore "no ID3v1 block"
// everything else isn't caught here, and throws out to the caller
}
try
{
sourceStream.Seek(0, SeekOrigin.Begin);
tagModel = FrameManager.Deserialize(sourceStream);
// audio starts after the tag
_audioStart = (uint)sourceStream.Position;
// audio is shorter by the length of the id3v2 tag
audioNumBytes -= _audioStart;
}
catch (TagNotFoundException)
{
// ignore "no ID3v2 block"
// everything else isn't caught here, and throws out to the caller
}
// create a taghandler to hold the tagmodel we've parsed, if any
_tagHandler = new TagHandler(tagModel);
} // closes sourceStream
// save the location of the audio in the original file
// passing in audio size and id3 length tag (if any) to help with bitrate calculations
_audio = new AudioFile(_sourceFileInfo, _audioStart, audioNumBytes, _tagHandler.Length);
_audioReplaced = false;
}