word.1

本程序可以对文件进行算术编码处理 解码处理等

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

.SH SYNOPSIS
.ll +8
.B word 
[\fB\-e\fR]
[\fB\-d\fR]
[\fB\-v\fR]
[\fB\-m\fR \fImbytes\fP]
[\fB\-f\fR \fIfbits\fP]
[\fIfname\fP]
.ll -8
.SH DESCRIPTION
.I Word 
is a compression program that uses arithmetic 
coding and an adaptive zero-order
word-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.
Binary files (for example, bit-map images)
will be handled correctly, but
compression is unlikely to be as good as the rates
obtained by tailor-made compression
regimes.

If no compression option is selected, 
.I word
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.
.ll -8
.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.

.ll -8
.SH NOTES
Memory range given with the -m option must be between 1 and 65535.
When memory is exhausted the data structure will be purged and
the collection of words restarted.

.SH BUGS
Compressed files may not be portable across architectures with different 
alignment restrictions. This is due to memory purging occuring at different
stages.
 
.SH REFERENCE
``Arithmetic Coding Revisited'',
Alistair Moffat, Radford Neal, Ian H. Witten,
Proc. 1995 IEEE Data Compression Conference,
Snowbird, Utah, March 1995.
Contact [e-mail] 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, [e-mail].  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.