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Bit recycling aims at improving the rates achieved by compression techniques, such as LZ77, that suffer from the redundancy caused by the multiplicity of the encodings. The performance of bit recycling depends crucially on the recycling codes that it uses. A recipe for the construction of optimal recycling codes has been mentioned in previous work. However, no efficient algorithm and proof of optimality were given. We present both here.

There exist lossless compression techniques, such as LZ77, that have the particularity that some original file may be compressed in more than one way, e.g. by choosing other matches than the closest longest ones only. The existence of multiple encodings per original file causes redundancy, i.e. it tends to make compressed files longer than necessary, on average. Recently, a technique called bit recycling was introduced to help reduce the redundancy caused by the multiplicity of encodings. It has been used to improve LZ77 compression. It exploits the fact that there often exists more than one longest match and it is called longest-match bit recycling. This work presents a more general, and more powerful, bit recycling technique that exploits shorter matches also. We call the technique all-match bit recycling. Our experiments demonstrate that at least 1 bit out of 11 results from the multiplicity of encodings, in LZ77 compression.

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