dictzip

DICTZIP(1)                                                          DICTZIP(1)



NAME
       dictzip, dictunzip - compress (or expand) files, allowing random access

SYNOPSIS
       dictzip [options] name
       dictunzip [options] name

DESCRIPTION
       dictzip compresses files using the gzip(1) algorithm (LZ77) in a manner
       which is completely compatible with the gzip file format.  An extension
       to the gzip file format (Extra Field, described in 2.3.1.1 of RFC 1952)
       allows extra data to be stored in the header of a compressed file.
       Programs like gzip and zcat will ignore this extra data.  However,
       dictd(8), the DICT protocol dictionary server will make use of this
       data to perform pseudo-random access on the file.  Files in the dictzip
       format should end in ".dz" so that they may be distinguished from
       common gzip files that do not contain the special header information.

       From RFC 1952, the extra field is specified as follows:

              If the FLG.FEXTRA bit is set, an "extra field" is present in the
              header, with total length XLEN bytes.  It consists of a series
              of subfields, each of the form:

              +---+---+---+---+==================================+
              |SI1|SI2|  LEN  |... LEN bytes of subfield data ...|
              +---+---+---+---+==================================+

              SI1 and SI2 provide a subfield ID, typically two ASCII letters
              with some mnemonic value.  Jean-Loup Gailly
              <gzip@prep.ai.mit.edu> is maintaining a registry of subfield
              IDs; please send him any subfield ID you wish to use.  Subfield
              IDs with SI2 = 0 are reserved for future use.

              LEN gives the length of the subfield data, excluding the 4
              initial bytes.

       The dictzip program uses 'R' for SI1, and 'A' for SI2 (i.e., "Random
       Access").  After the LEN field, the data is arranged as follows:

       +---+---+---+---+---+---+===============================+
       |  VER  | CHLEN | CHCNT |  ... CHCNT words of data ...  |
       +---+---+---+---+---+---+===============================+

       As per RFC 1952, all data is stored least-significant byte first.  For
       VER 1 of the data, all values are 16-bits long (2 bytes), and are
       unsigned integers.

       XLEN (which is specified earlier in the header) is a two byte integer,
       so the extra field can be 0xffff bytes long, 2 bytes of which are used
       for the subfield ID (SI1 and SI1), and 2 bytes of which are used for
       the subfield length (LEN).  This leaves 0xfffb bytes (0x7ffd 2-byte
       entries or 0x3ffe 4-byte entries).  Given that the zip output buffer
       must be 10% + 12 bytes larger than the input buffer, we can store 58969
       bytes per entry, or about 1.8GB if the 2-byte entries are used.  If
       this becomes a limiting factor, another format version can be selected
       and defined for 4-byte entries.

       For compression, the file is divided up into "chunks" of data, each
       chunk is less than 64kB, and can be compressed into an area that is
       also less than 64kB long (taking incompressible data into account --
       usually the data is compressed into a block that is much smaller than
       the original).  The CHLEN field specifies the length of a "chunk" of
       data.  The CHCNT field specifies how many chunks are preset, and the
       CHCNT words of data specifies how long each chunk is after compression
       (i.e., in the current compressed file).

       To perform random access on the data, the offset and length of the data
       are provided to library routines.  These routines determine the chunk
       in which the desired data begins, and decompresses that chunk.
       Consecutive chunks are decompressed as necessary.

TRADEOFFS
       Speed  True random file access is not realized, since any access, even
              for a single byte, requires that a 64kB chunk be read and
              decompressed.  This is slower than accessing a flat text file,
              but is much, much faster than performing serial access on a
              fully compressed file.

       Space  For the textual dictionary databases we are working with, the
              use of 64kB chunks and maximal LZ77 compression realizes a file
              which is only about 4% larger than the same file compressed all
              at once.

OPTIONS
       -d or --decompress
              Decompress.  This is the default if the executable is called
              dictunzip.

       -c or --stdout
              Write output on standard output; keep original files unchanged.
              This is only available when decompressing (because parts of the
              header must be updated after a write when compressing).

       -f or --force
              Force compression or decompression even if the output file
              already exists.

       -h or --help
              Display help.

       -k or --keep
              Do not delete the original file.

       -l or --list
              For each compressed file, list the following fields:

                  type: dzip, gzip, or text (includes files in unknown
              formats)
                  crc: CRC checksum
                  date and time: from header
                  chunks: number of chunks in file
                  size: size of each uncompressed chunk
                  compr.: compressed size
                  uncompr.: uncompressed size
                  ratio: compression ratio (0.0% if unknown)
                  name: name of uncompressed file

              Unlike gzip, the compression method is not detected.

       -L or --license
              Display the dictzip license and quit.

       -t or --test
              Check the compressed file integrity.  This option is not
              implemented.  Instead, it will list the header information.

       -v or --verbose
              Verbose. Display extra information during compression.

       -V or --version
              Version. Display the version number and compilation options then
              quit.

       -s start or --start start
              Specify the offer to start decompression, using decimal numbers.
              The default is at the beginning of the file.

       -e size or --size size
              Specify the size of the portion of the file to decompress, using
              decimal numbers.  The default is the whole file.

       -S start or --Start start
              Specify the offer to start decompression, using base64 numbers.
              The default is at the beginning of the file.

       -E size or --Size start
              Specify the size of the portion of the file to decompress, using
              base64 numbers.  The default is the whole file.

       -p prefilter or --pre prefilter
              Specify a shell command to execute as a filter before
              compression or decompression of a chunk.  The pre- and post-
              compression filters can be used to provide additional
              compression or output formatting.  The filters may not increase
              the buffer size significantly.  The pre- and post-compression
              filters were designed to provide the most general interface
              possible.

       -P postfilter or --post postfilter
              Specify a shell command to execute as a filter after compression
              or decompression.

CREDITS
       dictzip was written by Rik Faith (faith@cs.unc.edu) and is distributed
       under the terms of the GNU General Public License.  If you need to
       distribute under other terms, write to the author.

       The main libraries used by this programs (zlib, regex, libmaa) are
       distributed under different terms, so you may be able to use the
       libraries for applications which are incompatible with the GPL --
       please see the copyright notices and license information that come with
       the libraries for more information, and consult with your attorney to
       resolve these issues.

SEE ALSO
       dict(1), dictd(8), gzip(1), gunzip(1), zcat(1)



                                  22 Jun 1997                       DICTZIP(1)