void ReadHeader()
{
// 1. Check the two magic bytes
Crc32 headCRC = new Crc32();
int magic = baseInputStream.ReadByte();
if (magic < 0) {
throw new EndOfStreamException("EOS reading GZIP header");
}
headCRC.Update(magic);
if (magic != (GZipConstants.GZIP_MAGIC >> 8)) {
throw new GZipException("Error GZIP header, first magic byte doesn't match");
}
magic = baseInputStream.ReadByte();
if (magic < 0) {
throw new EndOfStreamException("EOS reading GZIP header");
}
if (magic != (GZipConstants.GZIP_MAGIC & 0xFF)) {
throw new GZipException("Error GZIP header, second magic byte doesn't match");
}
headCRC.Update(magic);
// 2. Check the compression type (must be 8)
int compressionType = baseInputStream.ReadByte();
if ( compressionType < 0 ) {
throw new EndOfStreamException("EOS reading GZIP header");
}
if ( compressionType != 8 ) {
throw new GZipException("Error GZIP header, data not in deflate format");
}
headCRC.Update(compressionType);
// 3. Check the flags
int flags = baseInputStream.ReadByte();
if (flags < 0) {
throw new EndOfStreamException("EOS reading GZIP header");
}
headCRC.Update(flags);
/* This flag byte is divided into individual bits as follows:
bit 0 FTEXT
bit 1 FHCRC
bit 2 FEXTRA
bit 3 FNAME
bit 4 FCOMMENT
bit 5 reserved
bit 6 reserved
bit 7 reserved
*/
// 3.1 Check the reserved bits are zero
if ((flags & 0xE0) != 0) {
throw new GZipException("Reserved flag bits in GZIP header != 0");
}
// 4.-6. Skip the modification time, extra flags, and OS type
for (int i=0; i< 6; i++) {
int readByte = baseInputStream.ReadByte();
if (readByte < 0) {
throw new EndOfStreamException("EOS reading GZIP header");
}
headCRC.Update(readByte);
}
// 7. Read extra field
if ((flags & GZipConstants.FEXTRA) != 0) {
// Skip subfield id
for (int i=0; i< 2; i++) {
int readByte = baseInputStream.ReadByte();
if (readByte < 0) {
throw new EndOfStreamException("EOS reading GZIP header");
}
headCRC.Update(readByte);
}
if (baseInputStream.ReadByte() < 0 || baseInputStream.ReadByte() < 0) {
throw new EndOfStreamException("EOS reading GZIP header");
}
int len1, len2;
len1 = baseInputStream.ReadByte();
len2 = baseInputStream.ReadByte();
if ((len1 < 0) || (len2 < 0)) {
throw new EndOfStreamException("EOS reading GZIP header");
}
headCRC.Update(len1);
headCRC.Update(len2);
int extraLen = (len1 << 8) | len2;
for (int i = 0; i < extraLen;i++) {
int readByte = baseInputStream.ReadByte();
if (readByte < 0)
{
throw new EndOfStreamException("EOS reading GZIP header");
}
headCRC.Update(readByte);
}
}
// 8. Read file name
if ((flags & GZipConstants.FNAME) != 0) {
int readByte;
while ( (readByte = baseInputStream.ReadByte()) > 0) {
headCRC.Update(readByte);
}
if (readByte < 0) {
throw new EndOfStreamException("EOS reading GZIP header");
}
headCRC.Update(readByte);
}
// 9. Read comment
if ((flags & GZipConstants.FCOMMENT) != 0) {
int readByte;
while ( (readByte = baseInputStream.ReadByte()) > 0) {
headCRC.Update(readByte);
}
if (readByte < 0) {
throw new EndOfStreamException("EOS reading GZIP header");
}
headCRC.Update(readByte);
}
// 10. Read header CRC
if ((flags & GZipConstants.FHCRC) != 0) {
int tempByte;
int crcval = baseInputStream.ReadByte();
if (crcval < 0) {
throw new EndOfStreamException("EOS reading GZIP header");
}
tempByte = baseInputStream.ReadByte();
if (tempByte < 0) {
throw new EndOfStreamException("EOS reading GZIP header");
}
crcval = (crcval << 8) | tempByte;
if (crcval != ((int) headCRC.Value & 0xffff)) {
throw new GZipException("Header CRC value mismatch");
}
}
readGZIPHeader = true;
}
/// <summary>Calculates the CRC32 check-sum on specified portion of a buffer.</summary>
/// <param name="data">Data buffer to perform check-sum on.</param>
/// <param name="startIndex">Starts index in data buffer to begin check-sum.</param>
/// <param name="length">Total number of bytes from <paramref name="startIndex">startIndex</paramref> to
/// perform check-sum over.</param>
/// <returns>Computed CRC32 checksum over the specified portion of the buffer.</returns>
public static uint Crc32Checksum(this byte[] data, int startIndex, int length)
{
Crc32 checksum = new Crc32();
checksum.Update(data, startIndex, length);
return checksum.Value;
}
/// <summary>
/// Closes the zip input stream
/// </summary>
public override void Close()
{
internalReader = new ReadDataHandler(ReadingNotAvailable);
crc = null;
entry = null;
base.Close();
}
