
GREETINGS!

   This is the README for BZIP, my block-sorting file compressor,
   version 0.21.  

   BZIP is distributed under the GNU General Public License version 2;
   for details, see the file LICENSE.  Pointers to the algorithms used
   are in ALGORITHMS.  Instructions for use are in bzip.1.preformatted.

   Please read this file carefully.



HOW TO BUILD

   -- for UNIX:

        Type `make'.     (tough, huh? :-)

        This creates binaries "bzip", and "bunzip",
        which is a symbolic link to "bzip".

        It also runs four compress-decompress tests to make sure
        things are working properly.  If all goes well, you should be up &
        running.  Please be sure to read the output from `make'
        just to be sure that the tests went ok.

        To install bzip properly:

           -- Copy the binary "bzip" to a publically visible place,
              possibly /usr/bin, /usr/common/bin or /usr/local/bin.

           -- In that directory, make "bunzip" be a symbolic link
              to "bzip".

           -- Copy the manual page, bzip.1, to the relevant place.
              Probably the right place is /usr/man/man1/.
   
   -- for Windows 95 and NT: 

        For a start, do you *really* want to recompile bzip?  
        The standard distribution includes a pre-compiled version
        for Windows 95 and NT, `BZIP.EXE'.

        Assuming you do, compilation is less straightforward than for
        Unix platforms.  You can compile either with Microsoft Visual C++ 2.0 
        or later, or with Borland C++ 5.0 or later.  

        NOTE [THIS IS IMPORTANT] that it would *appear* that
        MS VC++ 2.0's optimising compiler has a bug which, at maximum
        optimisation, gives an executable which produces garbage 
        compressed files.  Proceed with caution.  I do not know whether
        or not this happens with later versions of VC++.

        Edit the defines starting at line 86 of bzip.c to select your
        platform/compiler combination, and then compile.  Then check that
        the resulting executable (assumed to be called BZIP.EXE) works
        correctly, using the SELFTEST.BAT file.  Bearing in mind the 
        previous paragraph, the self-test is important.

   A manual page is supplied, unformatted (bzip.1),
   preformatted (bzip.1.preformatted), and preformatted
   and sanitised for MS-DOS (bzip1.txt).

   

COMPILATION NOTES

   bzip should work on any 32-bit machine.  It is known to work
   [meaning: it has compiled and passed self-tests] on the 
   following platform-os combinations:

      Intel i386/i486        running Linux 1.2.13 and Linux 2.0.0
      Sun Sparcs (various)   running SunOS 4.1.3 and Solaris 2.5
      SGI Indy R3000         running Irix 5.3
      HP 9000/700            running HPUX 9.03
      HP 9000/300            running NetBSD 1.1
      Acorn R260             running RISC iX (a BSD 4.? derivative)

      Intel i386/i486        running Windows 95

   I have also heard, but have not myself verified, that bzip works
   on the following machines:

      Intel i486             running Windows NT 3.51
      IBM 3090 clone         running OSF/1
      Dec Alpha              running ?????

   The #defines starting at around line 86 of bzip.c supply some
   degree of platform-independance.  If you configure bzip for some
   new far-out platform, please send me the relevant definitions.

   I recommend GNU C for compilation.  The code is standard ANSI C,
   except for the Unix-specific file handling, so any ANSI C compiler
   should work.  Note however that the many routines marked INLINE
   should be inlined by your compiler, else performance will be very
   poor.  Asking your compiler to unroll loops might give some
   small improvement too; for gcc, the relevant flag is
   -funroll-loops.

   On a 386/486 machines, I'd recommend giving gcc the
   -fomit-frame-pointer flag; this liberates another register for
   allocation, which measurably improves performance.

   On SPARCs (and, I guess, on many low-range RISC machines) there is no
   hardware implementation of integer multiply and divide.  This can
   mean poor decompression performance.  It also means it is important
   to generate code for the version of the SPARC instruction set you
   intend to use.  gcc -mcypress (for older sparcs) and gcc
   -msupersparc (for newer ones) give binaries which run at strikingly
   different speeds on different flavours of SPARCs.  If you are
   interested in performance figures, try both.

   If you compile bzip on a new platform or with a new compiler,
   please be sure to run the four compress-decompress tests, either
   using the Makefile, or with the test.bat (MSDOS) or test.cmd (OS/2)
   files.  Some compilers have been seen to introduce subtle bugs
   when optimising, so this check is important.  Ideally you should
   then go on to test bzip on a file several megabytes or even
   tens of megabytes long, just to be 110% sure.  ``Professional
   programmers are paranoid programmers.'' (anon).



MAKING IT GO FASTER

   After 0.15 was released, various people asked whether it would
   be possible to make it compress faster.  The answer falls in
   three parts:

   1.  Yes, and 0.21 compresses substantially faster than 0.15.

   2.  You can probably compress somewhat faster, even, than 0.21,
       by tinkering with the sorting algorithms.  However, it is
       easy to fall into the trap of speeding up the average
       case a little whilst at the same time imposing a giant
       (25 times) slowdown on the worst-but-not-uncommon case,
       files which are highly repetitive.  Beware!

   3.  Are you solving the right problem?  In many situations,
       it is the *de*compression speed which is the limiting factor
       on overall usefulness of bzip.  If you want to do some
       serious hacking on bzip, your hacking could be useful if
       you could speed up decompression.

       I appreciate that the arithmetic-coding back end imposes a 
       fairly serious restriction on decompression speed.  A possible
       future option would be to make a variant of bzip which
       used Huffman-coding (or some such) instead; this would reduce
       the compression ratio but greatly accelerate decompression.
       Experimental results welcomed!



VALIDATION

   Correct operation, in the sense that a compressed file can always be
   decompressed to reproduce the original, is obviously of paramount
   importance.  To validate bzip, I used a modified version of 
   Mark Nelson's churn program.  Churn is an automated test driver
   which recursively traverses a directory structure, using bzip to
   compress and then decompress each file it encounters, and checking
   that the decompressed data is the same as the original.  As test 
   material, I used the entirety of my Linux filesystem, constituting
   390 megabytes in 20,440 files.  The largest file was about seventeen
   megabytes long.  Included in this filesystem was a directory containing
   39 specially constructed test files, designed to break the sorting
   phase of compression, the most elaborate part of the machinery.
   This included files of zero length, various long, highly repetitive 
   files, and some files which generate blocks with all values the same.

   Validation of version 0.15
   ~~~~~~~~~~~~~~~~~~~~~~~~~~
   There were actually six test runs on this filesystem, taking about
   50 CPU hours on an Intel 486DX4-100 machine:

      One with the block size set to 900k (ie, with the -9 flag, the default).

      One with the block size set to 500k (ie, with -5).

      One with the block size set to 100k (ie, with -1).

      One where the parameters for the arithmetic coder were
      set to smallB == 14 and smallF == 11, rather than the
      usual values of 26 and 18.  This was intended to expose 
      possible boundary-case problems with the arithmetic coder;
      in particular, setting smallB == 14 keeps the coding values
      all below or equal to 8192.  Doing this, I hoped that the
      values actually would hit their endpoints from time to time,
      so I'd see problems if any lurked.  With smallB = 26, the 
      range of values goes up to 2^26 (64 million), which makes
      potential bugs associated with endpoint effects vastly less
      likely to be detected.

      One where the block size was set to a trivial value, 173,
      so as to invoke the blocking/unblocking machinery tens of
      thousands of times over the run, and expose any potential
      problem there.

      One with normal settings, the block size set 900k, but
      compiled with the symbol DEBUG set to 1, which turns on
      many assertion-checks in the compressor.

   None of these test runs exposed any problems.

   In addition, earlier versions of bzip have been in informal use
   for a while without difficulties.  The largest file I have tried
   so far is a log file from a chip-simulator, 52 megabytes long, 
   and that decompressed correctly.
   
   The distribution does four tests after building bzip.  These tests
   include test decompressions of pre-supplied compressed files, so
   they not only test that bzip works correctly on the machine it was
   built on, but can also decompress files compressed on a different
   machine.  This guards against unforseen interoperability problems.

   Validation of version 0.21
   ~~~~~~~~~~~~~~~~~~~~~~~~~~
   0.21 differs radically from 0.15 in the sorting phase which 
   constitutes the bulk of the work during compression, and in
   several other non-cosmetic ways, so there was considerable
   emphasis on trying to break it before release.  100% compatibility
   with 0.15 was also an issue.  On the other hand, the arithmetic
   coder is unchanged, so I didn't put special effort into trying
   to break that.  Testing was done on two filesystems, a Linux
   filesystem with about 21000 files in 400 megabytes, and a 
   Windows 95 filesystem with 14900 files in about 610 megabytes.
   The test runs were:

      Linux FS, blocksize = 900k, 0.15 compressing, 0.21 decompressing
      Linux FS, blocksize = 900k, 0.21 compressing, 0.15 decompressing

      Linux FS, blocksize = 900k, -DDEBUG=1
      Linux FS, blocksize = 500k, -DDEBUG=1
      Linux FS, blocksize = 100k, -DDEBUG=1

      Linux FS, blocksize = 900k

      Win95 FS, blocksize = 900k

      A single text file 186 megabytes long.

      My Win95 disk read by Linux as a single entity -- 425 Megabytes.

      Misc other anecdotal tests, incl some on a Sparc box (as a check
      for endian issues), covering another 140 megabytes of new data.

      Misc tests with Purify 3.0.1 snooping on the proceedings,
      to check for subscript range errors, &c.

   Overall, the quantity of original files in this validation
   run is about 1760 megabytes.  Not Bad.


Please read and be aware of the following:

WARNING:

   This program (attempts to) compress data by performing several
   non-trivial transformations on it.  Unless you are 100% familiar
   with *all* the algorithms contained herein, and with the
   consequences of modifying them, you should NOT meddle with the
   compression or decompression machinery.  Incorrect changes can and
   very likely *will* lead to disastrous loss of data.


DISCLAIMER:

   I TAKE NO RESPONSIBILITY FOR ANY LOSS OF DATA ARISING FROM THE
   USE OF THIS PROGRAM, HOWSOEVER CAUSED.

   Every compression of a file implies an assumption that the
   compressed file can be decompressed to reproduce the original.
   Great efforts in design, coding and testing have been made to
   ensure that this program works correctly.  However, the complexity
   of the algorithms, and, in particular, the presence of various
   special cases in the code which occur with very low but non-zero
   probability make it impossible to rule out the possibility of bugs
   remaining in the program.  DO NOT COMPRESS ANY DATA WITH THIS
   PROGRAM UNLESS YOU ARE PREPARED TO ACCEPT THE POSSIBILITY, HOWEVER
   SMALL, THAT THE DATA WILL NOT BE RECOVERABLE.

   That is not to say this program is inherently unreliable.  Indeed,
   I very much hope the opposite is true.  BZIP has been carefully
   constructed and extensively tested.

End of nasty legalities.


I hope you find bzip useful.  Feel free to contact me at
   sewardj@cs.man.ac.uk
if you have any suggestions or queries.  Many people mailed me with
comments, suggestions and patches after the release of 0.15, and the
changes in 0.21 are largely a result of this feedback.

Julian Seward
Manchester, UK
18 July 1996 (version 0.15)
25 August 1996 (version 0.21)

