DEFLATE using a preset dictionary with SharpZipLib (and the equivalent zlib code)

_.md

I came across a deflate stream I needed to inflate from C# and figured I would need to compile in zlib since there didn't appear to be any API for using raw deflate with a preset dictionary. As it turns out, ICSharpCode.SharpZipLib can do this, you just have to write a fake zlib header to make its Inflater ask for a dictionary.

DictionaryExample.cs

using System;
using System.IO;
using ICSharpCode.SharpZipLib.Zip.Compression;
using ICSharpCode.SharpZipLib.Checksums;

public class DictionaryExample
{
	Inflater m_inflater;

	void InitInflater()
	{
		if (m_inflater != null)
		{
			return;
		}
		m_inflater = new Inflater(false);
		// Read dictionary from embedded resource
		byte[] dict;
		using (var dictStream = Assembly.GetExecutingAssembly().GetManifestResourceStream("packet.dict"))
		using (var ms = new MemoryStream())
		{
			dictStream.CopyTo(ms);
			dict = ms.GetBuffer();
		}
		// Calculate checksum of dictionary
		var adler = new Adler32();
		adler.Update(dict);
		long dictAdler = adler.Value;
		// Write fake zlib header
		byte[] header = new byte[6];
		// 7 = 32k Window, 8 = DEFLATE, b = FDICT | FLEVEL_DEFAULT, b = checksum
		ushort hword = 0x78bb;
		header[0] = (byte)(hword >> 8);
		header[1] = (byte)(hword & 0xff);
		header[2] = (byte)(dictAdler >> 24);
		header[3] = (byte)(dictAdler >> 16);
		header[4] = (byte)(dictAdler >> 8);
		header[5] = (byte)(dictAdler);
		// Let inflater read our header
		m_inflater.SetInput(header);
		byte[] dummy = new byte[1];
		int hdWritten = m_inflater.Inflate(dummy);
		if (hdWritten != 0)
		{
			throw new Exception("Expected inflating header to decode 0 bytes");
		}
		// Now inflater can read the dictionary
		m_inflater.SetDictionary(dict);
	}

	public void Inflate(byte[] dst, byte[] src, int srcIndex, int srcCount)
	{
		InitInflater();
		m_inflater.SetInput(src, srcIndex, srcCount);
		int dstWritten = m_inflater.Inflate(dst);
		if (dstWritten != dst.Length)
		{
			throw new Exception("inflate didn't decode enough bytes");
		}
		// To discard any SYNC_FLUSH stuff, we have to read another block
		byte[] dummy = new byte[1];
		while (!m_inflater.IsNeedingInput)
		{
			m_inflater.Inflate(dummy);
		}
	}
}

example.c

#include <zlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
	int ret, dictLen, len, flush;
	FILE *dictFile, *packetFile;
	char *dict, *packet;
	z_stream strm;
	char dst[0x8000];

	dictFile = fopen("packet.dict", "rb");
	fseek(dictFile, 0, SEEK_END);
	dictLen = ftell(dictFile);
	dict = (char *)malloc(dictLen);
	fseek(dictFile, 0, SEEK_SET);
	fread(dict, 1, dictLen, dictFile);

	packetFile = fopen("packet.zlib", "rb");
	fseek(packetFile, 0, SEEK_END);
	len = ftell(packetFile);
	packet = (char *)malloc(len);
	fseek(packetFile, 0, SEEK_SET);
	fread(packet, 1, len, packetFile);

	strm.zalloc = Z_NULL;
	strm.zfree = Z_NULL;
	strm.opaque = Z_NULL;
	ret = inflateInit2(&strm, -15);
	if (ret != Z_OK) {
		printf("inflateInit2 failed: %d\n", ret);
		return ret;
	}

	ret = inflateSetDictionary(&strm, dict, dictLen);
	if (ret != Z_OK) {
		printf("inflateSetDictionary failed: %d\n", ret);
		return ret;
	}

	strm.next_in = packet;
	strm.avail_in = len;
	strm.next_out = dst;
	strm.avail_out = 0x8000;
	ret = inflate(&strm, 1);
	if (ret != Z_OK) {
		printf("inflate failed: %d\n", ret);
		return ret;
	}

	fwrite(dst, 1, strm.total_out, stdout);
	return 0;
}