Arithmetic Coding Library
-------------------------

Fred Wheeler
Feb 24, 1996

This package was adapted from the program in "Arithmetic Coding for
Data Compression", by Ian H. Witten, Radford M. Neal and John
G. Cleary, Communications of the ACM, June 1987, Vol. 30, No. 6.
(Source at ftp://ftp.cpsc.ucalgary.ca/pub/projects/ar.cod/cacm-87.shar)

This version uses structures for the coder, decoder and data model
instead of global variables.  This object oriented approach allows you
to simultaneously use several arithmetic coders, each streaming bits
to a different file and to have several models for each coder, each
with a possibly different number of symbols and frequency table.

This package does not come with models for character, word and bit
encoding, like the Witten Neal Cleary package.  You must create your
own model (probability table).

Encoders can be given a file to which the bits are stored.  If no file
is given, the encoder will simply count the bits that would have been
stored.  Decoders must be given a file to work.

Models may be initiated with a frequency table, or the table will
default to uniform (all 1's).  Regardless of initiation, the models
can be adaptive or fixed.

Unlike the original version by Witten et. al., symbol indices start at
0, not 1.

The program is written in ANSI C.

I owe credit to Amir Said, who has also written a similar package
based on the Witten Neal Cleary package.

If you find this program useful, please send me an e-mail.

ACDEMO
------

The program ACDEMO is a simple demonstration of the multiple model
arithmetic codec defined in ac.h and ac.c.  ACDEMO encodes a few
random streams and then decodes them, making sure that the original
data is recovered.  To be useful, ac.h and ac.c must be incorporated
into another compression program.

To compile and run ACDEMO with gcc use:
  gcc -Wall -O2 -c ac.c
  gcc -Wall -O2 -c acdemo.c
  gcc -o acdemo ac.o acdemo.o
  ./acdemo

The most common problem encountered is the compiler not finding
random() and srandom(), used in acdemo.c.  See the comments in
acdemo.c to fix this.

Change Log
----------

Sep 10, 1999 by Fred Wheeler
In ac.c removed declaration for main() and declarations for random
functions.

Jan 20, 1999 by Fred Wheeler
In ac.c use "b" in fopen so files are opened in binary mode.  This is
necessary for the code to work on a PC.

Jun 15, 2000 by Fred Wheeler
Use macros for random/rand and srandom/srand and comment their usage

Files
-----
ac.c: source for all arithmetic coding functions
ac.h: header file for any file using the arithmetic coding library
acdemo.c: an example