/// <summary>
/// Reads format specific information at the start of the
/// file.
/// </summary>
/// <param name="start">
/// A <see cref="long" /> value containing the seek position
/// at which the tags end and the media data begins.
/// </param>
/// <param name="propertiesStyle">
/// A <see cref="ReadStyle" /> value specifying at what level
/// of accuracy to read the media properties, or <see
/// cref="ReadStyle.None" /> to ignore the properties.
/// </param>
/// <remarks>
/// This method only searches for an audio header in the
/// first 16384 bytes of code to avoid searching forever in
/// corrupt files.
/// </remarks>
protected override void ReadStart(long start,
ReadStyle propertiesStyle)
{
// Only check the first 16 bytes so we're not stuck
// reading a bad file forever.
if (propertiesStyle != ReadStyle.None &&
!AudioHeader.Find(out first_header, this,
start, 0x4000))
{
throw new CorruptFileException(
"ADTS audio header not found.");
}
}
/// <summary>
/// Searches for an audio header in a <see cref="TagLib.File"
/// /> starting at a specified position and searching to the
/// end of the file.
/// </summary>
/// <param name="header">
/// A <see cref="AudioHeader" /> object in which the found
/// header will be stored.
/// </param>
/// <param name="file">
/// A <see cref="TagLib.File" /> object to search.
/// </param>
/// <param name="position">
/// A <see cref="long" /> value specifying the seek position
/// in <paramref name="file" /> at which to start searching.
/// </param>
/// <returns>
/// A <see cref="bool" /> value indicating whether or not a
/// header was found.
/// </returns>
/// <remarks>
/// Searching to the end of the file can be very, very slow
/// especially for corrupt or non-MPEG files. It is
/// recommended to use <see
/// cref="Find(AudioHeader,TagLib.File,long,int)" />
/// instead.
/// </remarks>
public static bool Find(out AudioHeader header,
TagLib.File file, long position)
{
return(Find(out header, file, position, -1));
}
/// <summary>
/// Searches for an audio header in a <see cref="TagLib.File"
/// /> starting at a specified position and searching through
/// a specified number of bytes.
/// </summary>
/// <param name="header">
/// A <see cref="AudioHeader" /> object in which the found
/// header will be stored.
/// </param>
/// <param name="file">
/// A <see cref="TagLib.File" /> object to search.
/// </param>
/// <param name="position">
/// A <see cref="long" /> value specifying the seek position
/// in <paramref name="file" /> at which to start searching.
/// </param>
/// <param name="length">
/// A <see cref="int" /> value specifying the maximum number
/// of bytes to search before aborting.
/// </param>
/// <returns>
/// A <see cref="bool" /> value indicating whether or not a
/// header was found.
/// </returns>
/// <exception cref="ArgumentNullException">
/// <paramref name="file" /> is <see langword="null" />.
/// </exception>
public static bool Find(out AudioHeader header,
TagLib.File file, long position, int length)
{
if (file == null)
{
throw new ArgumentNullException("file");
}
long end = position + length;
header = AudioHeader.Unknown;
file.Seek(position);
ByteVector buffer = file.ReadBlock(3);
if (buffer.Count < 3)
{
return(false);
}
do
{
file.Seek(position + 3);
buffer = buffer.Mid(buffer.Count - 3);
buffer.Add(file.ReadBlock(
(int)File.BufferSize));
for (int i = 0; i < buffer.Count - 3 &&
(length < 0 || position + i < end); i++)
{
if (buffer[i] == 0xFF &&
buffer[i + 1] >= 0xF0) // 0xFFF
{
try
{
BitStream bits = new BitStream(buffer.Mid(i, 7).Data);
// 12 bits sync header
bits.ReadInt32(12);
// 1 bit mpeg 2/4
bits.ReadInt32(1);
// 2 bits layer
bits.ReadInt32(2);
// 1 bit protection absent
bits.ReadInt32(1);
// 2 bits profile object type
bits.ReadInt32(2);
// 4 bits sampling frequency index
int samplerateindex = bits.ReadInt32(4);
if (samplerateindex >= sample_rates.Length)
{
return(false);
}
long samplerate = sample_rates[samplerateindex];
// 1 bit private bit
bits.ReadInt32(1);
// 3 bits channel configuration
int channelconfigindex = bits.ReadInt32(3);
if (channelconfigindex >= channels.Length)
{
return(false);
}
// 4 copyright bits
bits.ReadInt32(4);
// 13 bits frame length
long framelength = bits.ReadInt32(13); // double check framelength
if (framelength < 7)
{
return(false);
}
// 11 bits buffer fullness
bits.ReadInt32(11);
// 2 bits number of raw data blocks in frame
int numberofframes = bits.ReadInt32(2) + 1;
long numberofsamples = numberofframes * 1024;
long bitrate = framelength * 8 * samplerate / numberofsamples;
header = new AudioHeader(channels[channelconfigindex],
(int)bitrate,
(int)samplerate,
(int)numberofsamples,
numberofframes);
return(true);
}
catch (CorruptFileException)
{
}
}
}
position += File.BufferSize;
} while (buffer.Count > 3 && (length < 0 || position < end));
return(false);
}