compact.1

早期freebsd实现

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.\"	@(#)compact.1	6.2 (Berkeley) 4/2/94
.\"
.TH COMPACT 1 "April 2, 1994"
.UC 4
.SH NAME
compact, uncompact, ccat \- compress and uncompress files, and cat them
.SH SYNOPSIS
.B compact
[
.B \-v
] [ name ... ]
.br
.B uncompact
[
.B \-v
] [ name ...  ]
.br
.B ccat
[
.B \-v
] [ file ... ]
.SH DESCRIPTION
.I Compact
compresses the named files using an adaptive Huffman code.  If no file
names are given, the standard input is compacted to the standard output.
.I Compact
operates as an on-line algorithm.  Each time a byte is read,
it is encoded immediately according to the current prefix code.
This code is an optimal Huffman code for the set of frequencies seen so far.
It is unnecessary to prepend a decoding tree to the compressed file
since the encoder and the decoder start in the same state and stay
synchronized.  Furthermore,
.I compact
and
.I uncompact
can operate as filters.  In particular,
.sp
	... | compact | uncompact | ...
.br
.sp
operates as a (very slow) no-op.
.PP
When an argument
.I file
is given, it is compacted and the resulting file is placed in
.I file.C;
.I file
is unlinked.  The first two bytes of the compacted file code the
fact that the file is compacted.  This code is used to prohibit recompaction.
.PP
The amount of compression to be expected depends on the type of file being
compressed.  Typical values of compression are:
Text (38%), Pascal Source (43%), C Source (36%) and Binary (19%).
These values are the percentages of file bytes reduced.
.PP
.I Uncompact
restores the original file from a file compressed by
.I compact.
If no file names are given, the standard input is uncompacted to
the standard output.
.PP
.I Ccat
cats the original file from a file compressed by
.I compact,
without uncompressing the file (it is just a shell script which
directs the uncompacted output to the standard output).
.PP
.IR Compact ,
.IR uncompact ,
and
.I ccat
normally do their work silently.  If a
.B \-v
flag is supplied, 
.I compact
will report the compression percentage for each compacted
file while 
.I uncompact
and
.I ccat
will print out the name of each file as they're uncompacted.
.SH RESTRICTION
The last segment of the filename must be short enough
to allow space for the appended '.C'.
.SH FILES
.ta 1i
*.C	compacted file created by compact, removed by uncompact
.SH "SEE ALSO"
Gallager, Robert G., `Variations on a Theme of Huffman',
.I "I.E.E.E.  Transactions on Information Theory,"
vol. IT-24, no. 6, November 1978, pp. 668 - 674.
.SH AUTHOR
Colin L. Mc Master