.PU
.TH bits 1 local
.SH NAME
bits \- data compression using arithmetic coding 

.SH SYNOPSIS
.ll +8
.B bits 
[\fB\-e\fR]
[\fB\-d\fR]
[\fB\-v\fR]
[\fB\-m\fR \fImbytes\fP]
[\fB\-f\fR \fIfbits\fP]
[\fB\-c\fR \fIcbits\fP]
[\fIfname\fP]

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.SH DESCRIPTION
.I Bits 
is a compression program that uses arithmetic 
coding and an adaptive fixed order
bit-based model for compressing data.
The specified uncompressed file 
.I fname
is compressed to standard output if
.B \-e
is specified. Similarly, 
a compressed file
.I fname
is decompressed to standard output if
.B \-d
is specified.
If no
.I fname
is given, input is read from stdin.

The level of compression obtained depends upon the size of the file, and the
amount of redundancy. The program uses an arithmetic coding 
scheme which allows frequency counts of up to 2^30.
The number of bits used for frequency counts is specified by
.I fbits.
The model encodes each
bit of the input using probabilities conditioned by the previous 
.I cbits
bits of the file.
The program potentially allocates up to 2^\fIcbits\fR contexts requiring
12 bytes each. If memory is exceeded when allocating new contexts,
existing contexts are re-used.

If no compression option is selected, 
.I bits
detects whether the input file is compressed. If not, it will compress
the file to standard output. A magic number is encoded in compressed
files to allow detection of compressed files.
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.SH OPTIONS 
.TP
\f4\-e\fP
Force compression.
.TP
\f4\-d\fP
Force decompression.
.TP
\f4\-v\fP
Verbose mode. Display details about compression performance.
.TP
\f4\-m\fP
Limit memory usage (for data structures) to
.I mbytes
megabytes.
Default value is 1.
.TP
\f4\-f\fP
Use
.I fbits
bits for frequency counts. Larger values allow
more accurate modelling of large files and
faster compression and decompression if
the SHIFT_ADD option (in the Makefile) is used.
However larger values of
.I fbits
may also reduce overall compression efficiency because of
truncation effects at the coding stage, and because the recency
effect that is a by-product of count scaling is reduced.
.I Fbits
must be between 10 and 30. Default is 27.
.TP
\f4\-c\fP
Use
.I cbits
previous bits
as a context for each compressed bit. Larger values
provide more accurate modelling and better compression.
Note, however, that in the absence of any memory limit
memory use is proportional to 2^\fIcbits\fP.
Default is 16.

 
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.SH REFERENCE
``Arithmetic Coding Revisited'',
Alistair Moffat, Radford Neal, Ian H. Witten,
Proc. 1995 IEEE Data Compression Conference,
Snowbird, Utah, March 1995.
Contact alistair@cs.mu.oz.au for a copy.

.SH CONDITIONS
These programs are supplied free of charge for research purposes only,
and may not sold or incorporated into any commercial product.  There is
ABSOLUTELY NO WARRANTY of any sort, nor any undertaking that they are
fit for ANY PURPOSE WHATSOEVER.  Use them at your own risk.  If you do
happen to find a bug, or have modifications to suggest, please report
the same to Alistair Moffat, alistair@cs.mu.oz.au.  The copyright
notice included in the source code
and this statement of conditions must remain an integral
part of each and every copy made of these files.