(Note that the mechanism reported by Stuiver and Moffat is not currently included.)
Source code: arith_coder-3.tar.gz, February 1999.
Source code: Versions 1 (March 1995) and 2 (October 1996).
A suite of minimum-redundancy coding routines is also available, see http://www.csse.unimelb.edu.au/~alistair/mr_coder/.
You could also consider Managing Gigabytes: Compressing and Indexing Documents and Images by Ian H. Witten, Alistair Moffat, and Timothy C. Bell, Morgan Kaufmann, San Francisco, 1999.
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