/// <summary>
/// Reads the extended header.
/// </summary>
/// <param name="streamBuffer">The stream buffer.</param>
/// <param name="tag">The tag.</param>
/// <param name="crcData">The CRC data.</param>
/// <returns>
/// The extended header if used; otherwise, null.
/// </returns>
/// <exception cref="System.IO.InvalidDataException">Thrown if the extended header size does not match the amount of bytes read for the extended header.</exception>
private static Id3v2ExtendedHeader ReadExtendedHeader(StreamBuffer streamBuffer, Id3v2Tag tag, out int crcData)
{
crcData = 0;
if (!tag.UseExtendedHeader)
return null;
// The extended header contains information that can provide further insight in the structure of the tag,
// but is not vital to the correct parsing of the tag information; hence the extended header is optional.
int extendedHeaderSize = 0;
long startPosition = streamBuffer.Position;
Id3v2ExtendedHeader extendedHeader = null;
if ((tag.Version >= Id3v2Version.Id3v230) && (tag.Version < Id3v2Version.Id3v240))
{
// Where the 'Extended header size', currently 6 or 10 bytes, excludes itself.
extendedHeaderSize = streamBuffer.ReadBigEndianInt32() + 4;
int extendedFlags = streamBuffer.ReadBigEndianInt16();
extendedHeader = Id3v2ExtendedHeader.InitExtendedHeader(tag.Version, extendedFlags);
extendedHeader.PaddingSize = streamBuffer.ReadBigEndianInt32();
if (extendedHeader.CrcDataPresent)
crcData = streamBuffer.ReadBigEndianInt32();
}
else if (tag.Version >= Id3v2Version.Id3v240)
{
// Where the 'Extended header size' is the size of the whole extended header, stored as a 32 bit synchsafe integer.
// An extended header can thus never have a size of fewer than six bytes.
extendedHeaderSize = streamBuffer.ReadBigEndianInt32();
extendedHeaderSize = Id3v2Tag.GetUnsynchedValue(extendedHeaderSize);
int extendedFlagsFieldLength = streamBuffer.ReadByte();
// The extended flags field, with its size described by 'number of flag bytes', is defined as: %0bcd0000
int extendedFlags = streamBuffer.ReadInt(extendedFlagsFieldLength);
extendedHeader = Id3v2ExtendedHeader.InitExtendedHeader(tag.Version, extendedFlags);
extendedHeader.SetExtendedFlagsFieldLength(extendedFlagsFieldLength);
// Each flag that is set in the extended header has data attached,
// which comes in the order in which the flags are encountered (i.e. the data for flag 'b' comes before the data for flag 'c').
// Unset flags cannot have any attached data. All unknown flags MUST be unset and their corresponding data removed when a tag is modified.
//
// Every set flag's data starts with a length byte, which contains a value between 0 and 127 ($00 - $7f),
// followed by data that has the field length indicated by the length byte.
// If a flag has no attached data, the value $00 is used as length byte.
// If this flag is set, the present tag is an update of a tag found earlier in the present file or stream.
// If frames defined as unique are found in the present tag, they are to override any corresponding ones found in the earlier tag.
// This flag has no corresponding data.
if (extendedHeader.TagIsUpdate)
{
// If a flag has no attached data, the value $00 is used as length byte.
streamBuffer.ReadByte();
}
// If this flag is set, a CRC-32 [ISO-3309] data is included in the extended header.
// The CRC is calculated on all the data between the header and footer as indicated by the header's tag length field, minus the extended header.
// Note that this includes the padding (if there is any), but excludes the footer.
// The CRC-32 is stored as an 35 bit synchsafe integer, leaving the upper four bits always zeroed.
if (extendedHeader.CrcDataPresent)
{
long crcBytes = streamBuffer.ReadBigEndianInt64(5);
crcData = (int)Id3v2Tag.GetUnsynchedValue(crcBytes, 5);
}
// For some applications it might be desired to restrict a tag in more ways than imposed by the ID3v2 specification.
// Note that the presence of these restrictions does not affect how the tag is decoded, merely how it was restricted before encoding.
// If this flag is set the tag is restricted as follows:
if (extendedHeader.TagIsRestricted)
{
// Restrictions: %ppqrrstt
byte tagRestrictions = (byte)streamBuffer.ReadByte();
extendedHeader.TagRestrictions = new Id3v2TagRestrictions
{
// p - Tag size restrictions
TagSizeRestriction = (Id3v2TagSizeRestriction)(tagRestrictions & Id3v2TagRestrictions.TagSizeRestrictionFlags),
// q - Text encoding restrictions
TextEncodingRestriction = (Id3v2TextEncodingRestriction)(tagRestrictions & Id3v2TagRestrictions.TextEncodingRestrictionFlags),
// r - Text fields size restrictions
TextFieldsSizeRestriction = (Id3v2TextFieldsSizeRestriction)(tagRestrictions & Id3v2TagRestrictions.TextFieldsSizeRestrictionFlags),
// s - Image encoding restrictions
ImageEncodingRestriction = (Id3v2ImageEncodingRestriction)(tagRestrictions & Id3v2TagRestrictions.ImageEncodingRestrictionFlags),
// t - Image size restrictions
ImageSizeRestriction = (Id3v2ImageSizeRestriction)(tagRestrictions & Id3v2TagRestrictions.ImageSizeRestrictionFlags)
};
}
}
if ((streamBuffer.Position - startPosition) != extendedHeaderSize)
{
throw new InvalidDataException(
string.Format(
"ExtendedHeaderSize does not match amount of bytes read: expected {0} but got {1} bytes.",
extendedHeaderSize,
streamBuffer.Position - startPosition));
}
return extendedHeader;
}
public void ReadBigEndianInt64Test7Bytes()
{
const long Value = 0x12345678901234;
const long Expected = 0x34129078563412;
StreamBuffer target = new StreamBuffer(BitConverter.GetBytes(Value)) { Position = 0 };
long actual = target.ReadBigEndianInt64(7);
Assert.AreEqual(Expected, actual);
}
