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)
I did some minor adjustments to the code, mostly to get it running in pipemode unter zsh (get rid of the ERROR_IF_NOT_ZERO( errno ), it seems NeXTSTEP sets the errno even if there is no error). The enclosed binaries are for NS3.x Intel. I have not tested the compressor much, so it may not work for you at all. (For me it work fine, however your mileage may vary...) Frank Siegert frank@this.net
These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.