The main problem with this explanation is that stream compression doesn't preclude optimization on a per block basis it simply requires that enough buffering be used to accommodate a block at a time, which is what zlib does. NET's implementation is hampered by supporting a stream instead of seeing the whole file.
The other big misconception I've read is that. This has a cost of a three byte header per stored block of up to 64K each, which still results in an upper bound much lower than a 150% ratio.
PKWARE SUCKS ZIP
ZIP container does have a compressed/stored flag in the header for each file, the compressed Deflate stream itself is also composed of blocks such that each block can also be encoded as stored or compressed. ZIP container and not in the compressed stream itself, which is all that the. NET's DeflateStream/GZipStream is unable to take advantage of this because the compression flag is stored in the. I've seen discussions where people said that the. One way to solve this problem is simply to add a single byte for a compression flag. NET Framework 2.0 version of System.IO.Compression.
However, the amount of enlargement required to satisfy this requirement is much lower than the 50%+ enlargement seen in using the. It is provably true that no lossless compression algorithm can consistently compress all incoming streams, and some streams have to enlarge for others to shrink. Thus, Deflate is able to compress not only based on patterns, but also based on some byte values occurring more often than others or not occurring at all. (This fact is rediscovered each time a C programmer sees the forward byte copy loop in the decoder and tries to "fix" it with memmove or "optimize" it with memcpy.) The second layer uses Huffman encoding to compress based on the probability of codes, which include both literal bytes and copy length codes. What is not so obvious is that this layer is also capable of compressing runs of identical byte values or repeated patterns of byte values through self-overlapping runs. The first is LZ77 style sliding window compression, where repeated strings of bytes are replaced by references to the previous instance. Of concern here is only the implementation of the Deflate algorithm itself, the method by which the raw bits are compressed.ĭeflate consists primarily of two layers of compression. ZIP format introduced and made popular by PKWARE's PKZIP tool, which includes multiple compressed files along with directory information. Deflate compressed data can be wrapped in either a minimalistic gzip stream described by RFC 1952, or in the more well known. The compression algorithm itself is called Deflate and is described by RFC 1951. Using System.IO.Compression namespace VCZipTest bytes)", args,įirst, I should probably explain a bit about the gzip/Deflate algorithm and clear up some misconceptions. This is necessary since GZipStream is unfortunately sensitive to write block sizes: using System We'll use a revised version of the C# test program I used the last time, one which feeds an entire buffer to the compressor. This results in much better compression ratios. The compression algorithms for the System.IO.Compression.DeflateStream and System.IO.Compression.GZipStream classes have improved so that data that is already compressed is no longer inflated. NET Framework 4.0, where the changelog includes the following:
NET Framework's System.IO.Compression library, more specifically files getting larger when going through compression using the.
Hash Functions for C++ Unordered ContainersĬruelty Redefined: Undergraduates vs.♪nd I thought my implementation of Deflate was bad, part 2Ī few years ago I posted about poor compression ratios from the.
PKWARE SUCKS WINDOWS
Sapir-Whorf to Dijkstra to Torvalds - Language Bigotry In Our TimeĪrithmetic Coding + Statistical Modeling = Data CompressionĬ++ Generic Programming Meest OOP - std::is_base_ofĭebugging Windows Services Startup Problems Stranger In a Strange Land: A C++ Programmer Learns to Love PHP User input is both solicitied and appreciated! Here to get to the bottom and leave a comment Proper tagging of articles can always benefit from your input, please scroll down or click Amazon Apple C C# C++ Cisco Delphi Google JPEG Java JavaScript Linux MS-DOS Microsoft RS-232 Windows XML algorithms data compression debugging ebooks graphics hardware humor intellectual property mathematics music network programming people scams scripting security snark standards travel video wordplay zip