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In computing , Deflate is a lossless data compression algorithm and associated file format that uses a combination of the LZ77 algorithm and Huffman coding. The file format was later specified in RFC The original algorithm as designed by Katz was patented as U. A Deflate stream consists of a series of blocks. Each block is preceded by a 3- bit header:. The stored block option adds minimal overhead, and is used for data that is incompressible. Most compressible data will end up being encoded using method 10 , the dynamic Huffman encoding, which produces an optimised Huffman tree customised for each block of data individually.
Instructions to generate the necessary Huffman tree immediately follow the block header. The static Huffman option is used for short messages, where the fixed saving gained by omitting the tree outweighs the percentage compression loss due to using a non-optimal thus, not technically Huffman code. Within compressed blocks, if a duplicate series of bytes is spotted a repeated string , then a back- reference is inserted, linking to the previous location of that identical string instead. An encoded match to an earlier string consists of an 8-bit length 3— bytes and a bit distance 1—32, bytes to the beginning of the duplicate.
If the distance is less than the length, the duplicate overlaps itself, indicating repetition. For example, a run of 10 identical bytes can be encoded as one byte, followed by a duplicate of length 9 beginning 1 byte ago. The second compression stage consists of replacing commonly used symbols with shorter representations and less commonly used symbols with longer representations. The method used is Huffman coding which creates an unprefixed tree of non-overlapping intervals, where the length of each sequence is inversely proportional to the probability of that symbol needing to be encoded.
The more likely a symbol has to be encoded, the shorter its bit-sequence will be. A match length code will always be followed by a distance code.
Based on the distance code read, further "extra" bits may be read in order to produce the final distance. The distance tree contains space for 32 symbols:.
The code is itself a canonical Huffman code sent by giving the bit length of the code for each symbol. The bit lengths are themselves run-length encoded to produce as compact a representation as possible.
As an alternative to including the tree representation, the "static tree" option provides a standard fixed Huffman tree. The compressed size using the static tree can be computed using the same statistics the number of times each symbol appears as are used to generate the dynamic tree, so it is easy for a compressor to choose whichever is smaller. During the compression stage, it is the encoder that chooses the amount of time spent looking for matching strings.
Other Deflate encoders have been produced, all of which will also produce a compatible bitstream capable of being decompressed by any existing Deflate decoder. Differing implementations will likely produce variations on the final encoded bit-stream produced.
The focus with non-zlib versions of an encoder has normally been to produce a more efficiently compressed and smaller encoded stream. The fundamental mechanisms remain the same. Implementations of Deflate are freely available in many languages. C programs typically use the zlib library licensed under the zlib License , which allows use with both free and proprietary software.
Java includes support as part of the standard library in java. Inflate is the decoding process that takes a Deflate bit stream for decompression and correctly produces the original full-size data or file.
The normal intent with an alternative Inflate implementation is highly optimised decoding speed, or extremely predictable RAM usage for micro-controller embedded systems. From Wikipedia, the free encyclopedia. For other uses, see Deflation disambiguation. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.