bet@ecsvax.UUCP (Bennett E. Todd III) (07/07/86)
The responses were overwhelming; everybody wanted this. Herewith: the
documentation and makefile in this posting, and C sources in a second,
to the port of compress(1) to MS-DOS under MSC 3.0. This version should
port to other micros more easily than the original, I would think.
Executables for this can be found in net.micro.pc. Read the C source for
the credits; I had nothing to do with the development of this excellent
piece of software, and credit for this work shouldn't be lost.
#!/bin/sh
# Cut above the preceeding line, or cut here if you must.
# This is a shar archive. Extract with sh, not csh.
# The rest of this file will extract:
# makefile compress.doc
sed 's/^X//' > makefile << '/*EOF'
Xcomsmlpc.exe: compress.c
X msc compress,compress/FPi/Ze/Gs/Ot;
X link compress,comsmlpc/NOI;
X
Xcombigpc.exe: compress.c
X msc compress,compress/DBIG/FPi/Ze/Gs/Ot;
X link compress,combigpc.big/NOI;
X exepack combigpc.big combigpc.exe
X
Xcomsmlat.exe: compress.c
X msc compress,compress/FPi/Ze/Gs/Ot/G2;
X link compress,comsmlat/NOI;
X
X
Xcombigat.exe: compress.c
X msc compress,compress/DBIG/FPi/Ze/Gs/Ot/G2;
X link compress,combigat.big/NOI;
X exepack combigat.big combigat.exe
/*EOF
ls -l makefile
sed 's/^X//' > compress.doc << '/*EOF'
X COMPRESS(1) MS-DOS Programmer's Manual COMPRESS(1)
X
X NAME
X compress, uncompress, zcat - compress or expand data
X
X SYNOPSIS
X compress [-cdfivV] [-b bits] [name ...]
X uncompress [-cfivV] [name ...]
X zcat [-iV] [name ...]
X
X DESCRIPTION
X Compress reduces the size of the named files using
X adaptive Lempel-Ziv coding. Whenever possible, each
X file is replaced by one with the extension .Z or XZ,
X while keeping the same modification times. If no
X files are specified, the standard input is
X compressed to the standard output. Compressed files
X can be restored to their original form using
X uncompress or zcat.
X
X The -c option makes compress/uncompress write to the
X standard output; no files are changed. The
X nondestructive behavior of zcat is identical to that
X of uncompress -c.
X
X The -d (decompress) option makes compress restore
X its input files to their normal form. Uncompress is
X identical to compress with the -d option specified.
X
X The -f option will force compression of "name". This
X is useful for compressing an entire directory, even
X if some of the files do not actually shrink. If -f
X is not given, the user is prompted as to whether an
X existing file should be overwritten.
X
X The -i (image mode) option suppresses the
X transformation of text lines from MS-DOS (CR-LF
X delimited) form to UNIX (LF delimited) form during
X compression, and suppresses the reverse
X transformation during decompression.
X
X The -v (verbose) option causes a message to be
X printed, yielding the percentage of reduction for
X each file compressed.
X
X 1
X COMPRESS(1) MS-DOS Programmer's Manual COMPRESS(1)
X
X
X The -V option causes the current version and compile
X options to be printed on stderr.
X
X Compress uses the modified Lempel-Ziv algorithm
X popularized in "A Technique for High Performance
X Data Compression", Terry A. Welch, IEEE Computer,
X vol. 17, no. 6 (June 1984), pp. 8-19. Common
X substrings in the file are first replaced by 9-bit
X codes 257 and up. When code 512 is reached, the
X algorithm switches to 10-bit codes and continues to
X use more bits until the limit specified by the -b
X flag is reached (default is the maximum for which
X the program was built).
X
X "Bits" must be between 9 and the lesser of 16, and
X the limit imposed at compile-time. The MS-DOS
X version of compress comes in two sizes. One has a
X 12-bit limit, and will run in a machine with 128K
X bytes of available user memory. The other has a
X 16-bit limit, and requires about 450K bytes to run.
X
X After the "bits" limit is attained, compress
X periodically checks the compression ratio. If it is
X increasing, compress continues to use the existing
X code dictionary. However, if the compression ratio
X decreases, compress discards the table of substrings
X and rebuilds it from scratch. This allows the
X algorithm to adapt to the next "block" of the file.
X
X Note that the -b flag is omitted for uncompress,
X since the "bits" parameter specified during
X compression is encoded within the output, along with
X a magic number to ensure that neither decompression
X of random data nor recompression of compressed data
X is attempted.
X
X The amount of compression obtained depends on the
X size of the input, the number of "bits" per code,
X and the distribution of common substrings.
X Typically, text such as source code or English is
X reduced by 50-60%. Compression is generally much
X
X 2
X COMPRESS(1) MS-DOS Programmer's Manual COMPRESS(1)
X
X better than that achieved by Huffman coding (as used
X in SQ), and takes less time to compute.
X
X Exit status is normally 0; if the last file is
X larger after (attempted) compression, the status is
X 2; if an error occurs, exit status is 1.
X
X SEE ALSO
X SQ(1)
X
X DIAGNOSTICS
X Usage: compress [-cdfivV] [-b maxbits] [file ...]
X Invalid options were specified on the command
X line.
X Missing maxbits
X Maxbits must follow -b.
X file: not in compressed format
X The file specified to UNCOMPRESS has not been
X compressed.
X file: compressed with xx bits, can only handle yy bits
X "File" was compressed by a program that could
X deal with more "bits" than the compress code on
X this machine. Recompress the file with smaller
X "bits".
X file: already has xx suffix -- no change
X The file is assumed to be already compressed
X because the last two characters of its extension
X are ".Z" or "XZ". Rename the file and try
X again.
X fn: part of filename extension will be replaced by XZ
X File name, fn, contains at least two characters
X in the "extension" field. The second and third
X will be replaced by "XZ" in the compressed
X file's name.
X fn already exists; do you wish to overwrite fn?
X Respond "y" if you want the output file, fn, to
X be replaced; "n" if not.
X Compression: xx.xx%
X Percentage of the input saved by compression.
X (Relevant only for -v.)
X -- file unchanged
X No savings is achieved by compression. The
X
X 3
X COMPRESS(1) MS-DOS Programmer's Manual COMPRESS(1)
X
X input remains virgin.
X
X BUGS
X Although compressed files are compatible between
X machines with large memory, -b12 should be used for
X file transfer to architectures with a small process
X data space (64KB or less, as exhibited by the DEC
X PDP series, or the small MS-DOS version, for
X example).
X
X MS-DOS version 2 does not permit a program to
X determine the name used to call it. As a result,
X the aliases, uncompress and zcat, cannot be used.
X They can be used under MS-DOS version 3, though the
X actual file name for uncompress will be
X "uncompre.exe".
X
X MS-DOS does not support UNIX-style file links. As a
X result, even though compress, uncompress and zcat
X are all the same program, it (they) will have to be
X stored three times, once under each of the three
X names, in order to use them under MS-DOS version 3.
X As explained in the previous paragraph, this is not
X an option under MS-DOS version 2.
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
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X 4
X
/*EOF
ls -l compress.doc
exit
--
Bennett Todd -- Duke Computation Center, Durham, NC 27706-7756; (919) 684-3695
UUCP: ...{decvax,seismo,philabs,ihnp4,akgua}!mcnc!ecsvax!duccpc!bet