compress: import Plan9 compress

Add #define USED(x)... boilerplate

compress: import Plan9 manpage.

man/man1/compress.1

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+.TH COMPRESS 1
+.SH NAME
+compress, uncompress, zcat \- compress and expand data
+.SH SYNOPSIS
+.B compress
+[
+.B \-f
+] [
+.B \-v
+] [
+.B \-c
+] [
+.B \-V
+] [
+.B \-b
+.I bits
+] [
+.I "name \&..."
+]
+.PP
+.B uncompress
+[
+.B \-f
+] [
+.B \-v
+] [
+.B \-c
+] [
+.B \-V
+] [
+.I "name \&..."
+]
+.PP
+.B zcat
+[
+.B \-V
+] [
+.I "name \&..."
+]
+.SH DESCRIPTION
+.I Compress
+reduces the size of the named files using adaptive Lempel-Ziv coding.
+Whenever possible,
+each file is replaced by one with the extension
+.B "\&.Z,"
+while keeping the same ownership modes, access and modification times.
+If no files are specified, the standard input is compressed to the
+standard output.
+Compressed files can be restored to their original form using
+.I uncompress
+or
+.I zcat.
+.PP
+The
+.B \-f
+option will force compression of
+.I name.
+This is useful for compressing an entire directory,
+even if some of the files do not actually shrink.
+If
+.B \-f
+is not given and
+.I compress
+is run in the foreground,
+the user is prompted as to whether an existing file should be overwritten.
+.PP
+The
+.B \-c
+option makes
+.I compress/uncompress
+write to the standard output; no files are changed.
+The nondestructive behavior of
+.I zcat
+is identical to that of
+.I uncompress
+.B \-c.
+.PP
+.I Compress
+uses the modified Lempel-Ziv algorithm popularized in
+"A Technique for High Performance Data Compression",
+Terry A. Welch,
+.I "IEEE Computer,"
+vol. 17, no. 6 (June 1984), pp. 8-19.
+Common substrings in the file are first replaced by 9-bit codes 257 and up.
+When code 512 is reached, the algorithm switches to 10-bit codes and
+continues to use more bits until the
+limit specified by the
+.B \-b
+flag is reached (default 16).
+.I Bits
+must be between 9 and 16.  The default can be changed in the source to allow
+.I compress
+to be run on a smaller machine.
+.PP
+After the
+.I bits
+limit is attained,
+.I compress
+periodically checks the compression ratio.  If it is increasing,
+.I compress
+continues to use the existing code dictionary.  However,
+if the compression ratio decreases,
+.I compress
+discards the table of substrings and rebuilds it from scratch.  This allows
+the algorithm to adapt to the next "block" of the file.
+.PP
+Note that the
+.B \-b
+flag is omitted for
+.I uncompress,
+since the 
+.I bits
+parameter specified during compression
+is encoded within the output, along with
+a magic number to ensure that neither decompression of random data nor
+recompression of compressed data is attempted. 
+.PP
+.ne 8
+The amount of compression obtained depends on the size of the
+input, the number of
+.I bits
+per code, and the distribution of common substrings.
+Typically, text such as source code or English
+is reduced by 50\-60%.
+Compression is generally much better than that achieved by
+Huffman coding (as used in
+.IR pack ),
+or adaptive Huffman coding
+.RI ( compact ),
+and takes less time to compute.
+.PP
+Under the
+.B \-v
+option,
+a message is printed yielding the percentage of
+reduction for each file compressed.
+.PP
+If the
+.B \-V
+option is specified, the current version and compile options are printed on
+stderr.
+.PP
+Exit status is normally 0;
+if the last file is larger after (attempted) compression, the status is 2;
+if an error occurs, exit status is 1.
+.SH "SEE ALSO"
+pack(1), compact(1)
+.SH "DIAGNOSTICS"
+Usage: compress [\-dfvcV] [\-b maxbits] [file ...]
+.in +8
+Invalid options were specified on the command line.
+.in -8
+Missing maxbits
+.in +8
+Maxbits must follow
+.BR \-b \.
+.in -8
+.IR file :
+not in compressed format
+.in +8
+The file specified to
+.I uncompress
+has not been compressed.
+.in -8
+.IR file :
+compressed with 
+.I xx
+bits, can only handle 
+.I yy
+bits
+.in +8
+.I File
+was compressed by a program that could deal with
+more 
+.I bits
+than the compress code on this machine.
+Recompress the file with smaller
+.IR bits \.
+.in -8
+.IR file :
+already has .Z suffix -- no change
+.in +8
+The file is assumed to be already compressed.
+Rename the file and try again.
+.in -8
+.IR file :
+filename too long to tack on .Z
+.in +8
+The file cannot be compressed because its name is longer than
+12 characters.
+Rename and try again.
+This message does not occur on BSD systems.
+.in -8
+.I file
+already exists; do you wish to overwrite (y or n)?
+.in +8
+Respond "y" if you want the output file to be replaced; "n" if not.
+.in -8
+uncompress: corrupt input
+.in +8
+A SIGSEGV violation was detected which usually means that the input file has
+been corrupted.
+.in -8
+Compression: 
+.I "xx.xx%"
+.in +8
+Percentage of the input saved by compression.
+(Relevant only for
+.BR \-v \.)
+.in -8
+-- not a regular file: unchanged
+.in +8
+When the input file is not a regular file,
+(e.g. a directory), it is
+left unaltered.
+.in -8
+-- has 
+.I xx 
+other links: unchanged
+.in +8
+The input file has links; it is left unchanged.  See
+.IR ln "(1)"
+for more information.
+.in -8
+-- file unchanged
+.in +8
+No savings is achieved by
+compression.  The input remains virgin.
+.in -8
+.SH SOURCE
+.B \*9/src/cmd/compress/compress.c
+.SH "BUGS"
+Although compressed files are compatible between machines with large memory,
+.BR \-b \12
+should be used for file transfer to architectures with 
+a small process data space (64KB or less, as exhibited by the DEC PDP
+series, the Intel 80286, etc.)

src/cmd/compress/mkfile

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+<$PLAN9/src/mkhdr
+
+TARG=\
+	compress \
+	zcat \
+	uncompress
+
+$O.uncompress:Q: $O.compress
+	cp $prereq $target
+
+$O.zcat:Q: $O.compress
+	cp $prereq $target
+
+<$PLAN9/src/mkmany
+