Universal Lossless Coding of Sources with Large and Unbounded Alphabets
A multilevel arithmetic coding algorithm is proposed to encode data sequences with large or unbounded source alphabets. The algorithm first converts the source alphabet into a dynamic tree, and then represents each symbol in the input sequence by its path in the tree and its index in the corresponding leaf. Encoding of the input sequence is then accomplished by encoding the path sequence and the index sequence conditionally. It is shown that the proposed algorithm is universal in the sense that it can achieve asymptotically the entropy rate of any independently and identically distributed integer source with a finite or infinite alphabet, as long as the mean value is finite. The advantages of the proposed algorithm over the traditional adaptive arithmetic coding algorithm are two folds: (1) the proposed algorithm can be used to encode any data sequence no matter whether the corresponding source alphabet is finite or infinite, while the traditional adaptive arithmetic coding algorithm can work only for data sequences with bounded, small alphabets; (2) in the situation in which the traditional adaptive arithmetic coding algorithm can work, the proposed algorithm can reduce coding complexity and improve compression performance. The proposed algorithm is then used to implement the recent Multilevel Pattern Matching(MPM) algorithms. Simulation results show that for a variety of files, the combination of the proposed algorithm with the MPM algorithms results in compression performance better than that afforded by the UNIX Compress algorithm, which is based on the LZ78 algorithm. Other applications of the proposed algorithm are also discussed.
KeywordsInput Sequence Compression Rate Binary Search Tree Integer Sequence Arithmetic Code
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