This is just a quick post to describe how to format compressed data with the .NET DeflateStream class so that it can be read back in a C or C++ program using zlib. Compressing data with DeflateStream is extremely simple. The original code I was using looked like this:
using (var compressor = new DeflateStream(data, CompressionMode.Compress))
compressor.Write(bytes, 0, bytes.Length);
data is a MemoryStream that I used to serialize the compressed data to disk.
zlib refused to read it. It kept throwing a “zlib error” exception.
It turns out that although they use the same compression algorithm, zlib expects a header in front of the data and an adler-32 checksum at the end. The code is now:
data.WriteByte(0x58);
data.WriteByte(0x85);
using (var compressor = new DeflateStream(data, CompressionMode.Compress, true))
compressor.Write(bytes, 0, bytes.Length);
data.Write(BitConverter.GetBytes(IPAddress.HostToNetworkOrder(Adler32(bytes))), 0, sizeof(uint));
the adler-32 code:
// naive implementation of adler-32 checksum
int Adler32(byte[] bytes) {
const uint a32mod = 65521;
uint s1 = 1, s2 = 0;
foreach (byte b in bytes) {
s1 = (s1 + b) % a32mod;
s2 = (s2 + s1) % a32mod;
}
return unchecked((int) ((s2 << 16) + s1));
}
and zlib is happy again.
If you’re wondering what the magic numbers are, take a look at RFC 1950.
