2002-04-29 Robert Collins <rbtcollins@hotmail.com>

[cygwin-apps/setup.git] / bz2lib / manual.texi

\input texinfo  @c                                  -*- Texinfo -*-
@setfilename bzip2.info

@ignore
This file documents bzip2 version 1.0.2, and associated library
libbzip2, written by Julian Seward (jseward@acm.org).

Copyright (C) 1996-2002 Julian R Seward

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for verbatim copies.
@end ignore

@ifinfo
@format
START-INFO-DIR-ENTRY
* Bzip2: (bzip2).		A program and library for data compression.
END-INFO-DIR-ENTRY
@end format

@end ifinfo

@iftex
@c @finalout
@settitle bzip2 and libbzip2
@titlepage
@title bzip2 and libbzip2
@subtitle a program and library for data compression
@subtitle copyright (C) 1996-2002 Julian Seward
@subtitle version 1.0.2 of 30 December 2001
@author Julian Seward

@end titlepage

@parindent 0mm
@parskip 2mm

@end iftex
@node Top,,, (dir)

The following text is the License for this software.  You should
find it identical to that contained in the file LICENSE in the 
source distribution.

@bf{------------------ START OF THE LICENSE ------------------}

This program, @code{bzip2}, 
and associated library @code{libbzip2}, are
Copyright (C) 1996-2002 Julian R Seward.  All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
@itemize @bullet
@item
   Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.
@item
   The origin of this software must not be misrepresented; you must 
   not claim that you wrote the original software.  If you use this 
   software in a product, an acknowledgment in the product 
   documentation would be appreciated but is not required.
@item
   Altered source versions must be plainly marked as such, and must
   not be misrepresented as being the original software.
@item
   The name of the author may not be used to endorse or promote 
   products derived from this software without specific prior written 
   permission.
@end itemize
THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS
OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Julian Seward, Cambridge, UK.

@code{jseward@@acm.org}

@code{bzip2}/@code{libbzip2} version 1.0.2 of 30 December 2001.

@bf{------------------ END OF THE LICENSE ------------------}

Web sites:

@code{http://sources.redhat.com/bzip2}

@code{http://www.cacheprof.org}

PATENTS: To the best of my knowledge, @code{bzip2} does not use any patented
algorithms.  However, I do not have the resources available to carry out
a full patent search.  Therefore I cannot give any guarantee of the
above statement.







@chapter Introduction

@code{bzip2}  compresses  files  using the Burrows-Wheeler 
block-sorting text compression algorithm,  and  Huffman  coding.
Compression  is  generally  considerably  better than that
achieved by more conventional LZ77/LZ78-based compressors,
and  approaches  the performance of the PPM family of statistical compressors.

@code{bzip2} is built on top of @code{libbzip2}, a flexible library
for handling compressed data in the @code{bzip2} format.  This manual
describes both how to use the program and 
how to work with the library interface.  Most of the
manual is devoted to this library, not the program, 
which is good news if your interest is only in the program.

Chapter 2 describes how to use @code{bzip2}; this is the only part 
you need to read if you just want to know how to operate the program.
Chapter 3 describes the programming interfaces in detail, and
Chapter 4 records some miscellaneous notes which I thought
ought to be recorded somewhere.


@chapter How to use @code{bzip2}

This chapter contains a copy of the @code{bzip2} man page,
and nothing else.

@quotation

@unnumberedsubsubsec NAME
@itemize
@item @code{bzip2}, @code{bunzip2}
- a block-sorting file compressor, v1.0.2
@item @code{bzcat} 
- decompresses files to stdout
@item @code{bzip2recover}
- recovers data from damaged bzip2 files
@end itemize

@unnumberedsubsubsec SYNOPSIS
@itemize
@item @code{bzip2} [ -cdfkqstvzVL123456789 ] [ filenames ...  ]
@item @code{bunzip2} [ -fkvsVL ] [ filenames ...  ]
@item @code{bzcat} [ -s ] [ filenames ...  ]
@item @code{bzip2recover} filename
@end itemize

@unnumberedsubsubsec DESCRIPTION

@code{bzip2} compresses files using the Burrows-Wheeler block sorting
text compression algorithm, and Huffman coding.  Compression is
generally considerably better than that achieved by more conventional
LZ77/LZ78-based compressors, and approaches the performance of the PPM
family of statistical compressors.

The command-line options are deliberately very similar to those of GNU
@code{gzip}, but they are not identical.

@code{bzip2} expects a list of file names to accompany the command-line
flags.  Each file is replaced by a compressed version of itself, with
the name @code{original_name.bz2}.  Each compressed file has the same
modification date, permissions, and, when possible, ownership as the
corresponding original, so that these properties can be correctly
restored at decompression time.  File name handling is naive in the
sense that there is no mechanism for preserving original file names,
permissions, ownerships or dates in filesystems which lack these
concepts, or have serious file name length restrictions, such as MS-DOS.

@code{bzip2} and @code{bunzip2} will by default not overwrite existing
files.  If you want this to happen, specify the @code{-f} flag.

If no file names are specified, @code{bzip2} compresses from standard
input to standard output.  In this case, @code{bzip2} will decline to
write compressed output to a terminal, as this would be entirely
incomprehensible and therefore pointless.

@code{bunzip2} (or @code{bzip2 -d}) decompresses all
specified files.  Files which were not created by @code{bzip2}
will be detected and ignored, and a warning issued.  
@code{bzip2} attempts to guess the filename for the decompressed file 
from that of the compressed file as follows:
@itemize
@item @code{filename.bz2 } becomes @code{filename}
@item @code{filename.bz  } becomes @code{filename}
@item @code{filename.tbz2} becomes @code{filename.tar}
@item @code{filename.tbz } becomes @code{filename.tar}
@item @code{anyothername } becomes @code{anyothername.out}
@end itemize
If the file does not end in one of the recognised endings, 
@code{.bz2}, @code{.bz}, 
@code{.tbz2} or @code{.tbz}, @code{bzip2} complains that it cannot
guess the name of the original file, and uses the original name
with @code{.out} appended.

As with compression, supplying no
filenames causes decompression from standard input to standard output.

@code{bunzip2} will correctly decompress a file which is the
concatenation of two or more compressed files.  The result is the
concatenation of the corresponding uncompressed files.  Integrity
testing (@code{-t}) of concatenated compressed files is also supported.

You can also compress or decompress files to the standard output by
giving the @code{-c} flag.  Multiple files may be compressed and
decompressed like this.  The resulting outputs are fed sequentially to
stdout.  Compression of multiple files in this manner generates a stream
containing multiple compressed file representations.  Such a stream
can be decompressed correctly only by @code{bzip2} version 0.9.0 or
later.  Earlier versions of @code{bzip2} will stop after decompressing
the first file in the stream.

@code{bzcat} (or @code{bzip2 -dc}) decompresses all specified files to
the standard output.

@code{bzip2} will read arguments from the environment variables
@code{BZIP2} and @code{BZIP}, in that order, and will process them
before any arguments read from the command line.  This gives a 
convenient way to supply default arguments.

Compression is always performed, even if the compressed file is slightly
larger than the original.  Files of less than about one hundred bytes
tend to get larger, since the compression mechanism has a constant
overhead in the region of 50 bytes.  Random data (including the output
of most file compressors) is coded at about 8.05 bits per byte, giving
an expansion of around 0.5%.

As a self-check for your protection, @code{bzip2} uses 32-bit CRCs to
make sure that the decompressed version of a file is identical to the
original.  This guards against corruption of the compressed data, and
against undetected bugs in @code{bzip2} (hopefully very unlikely).  The
chances of data corruption going undetected is microscopic, about one
chance in four billion for each file processed.  Be aware, though, that
the check occurs upon decompression, so it can only tell you that
something is wrong.  It can't help you recover the original uncompressed
data.  You can use @code{bzip2recover} to try to recover data from
damaged files.

Return values: 0 for a normal exit, 1 for environmental problems (file
not found, invalid flags, I/O errors, &c), 2 to indicate a corrupt
compressed file, 3 for an internal consistency error (eg, bug) which
caused @code{bzip2} to panic.


@unnumberedsubsubsec OPTIONS
@table @code
@item -c  --stdout
Compress or decompress to standard output.
@item -d  --decompress
Force decompression.  @code{bzip2}, @code{bunzip2} and @code{bzcat} are
really the same program, and the decision about what actions to take is
done on the basis of which name is used.  This flag overrides that
mechanism, and forces bzip2 to decompress.
@item -z --compress
The complement to @code{-d}: forces compression, regardless of the
invokation name.
@item -t --test
Check integrity of the specified file(s), but don't decompress them.
This really performs a trial decompression and throws away the result.
@item -f --force
Force overwrite of output files.  Normally, @code{bzip2} will not overwrite
existing output files.  Also forces @code{bzip2} to break hard links
to files, which it otherwise wouldn't do.

@code{bzip2} normally declines to decompress files which don't have the
correct magic header bytes.  If forced (@code{-f}), however, it will
pass such files through unmodified.  This is how GNU @code{gzip}
behaves.
@item -k --keep
Keep (don't delete) input files during compression
or decompression.
@item -s --small
Reduce memory usage, for compression, decompression and testing.  Files
are decompressed and tested using a modified algorithm which only
requires 2.5 bytes per block byte.  This means any file can be
decompressed in 2300k of memory, albeit at about half the normal speed.

During compression, @code{-s} selects a block size of 200k, which limits
memory use to around the same figure, at the expense of your compression
ratio.  In short, if your machine is low on memory (8 megabytes or
less), use -s for everything.  See MEMORY MANAGEMENT below.
@item -q --quiet
Suppress non-essential warning messages.  Messages pertaining to
I/O errors and other critical events will not be suppressed.
@item -v --verbose
Verbose mode -- show the compression ratio for each file processed.
Further @code{-v}'s increase the verbosity level, spewing out lots of
information which is primarily of interest for diagnostic purposes.
@item -L --license -V --version
Display the software version, license terms and conditions.
@item -1 (or --fast) to -9 (or --best)
Set the block size to 100 k, 200 k ..  900 k when compressing.  Has no
effect when decompressing.  See MEMORY MANAGEMENT below.
The @code{--fast} and @code{--best} aliases are primarily for GNU
@code{gzip} compatibility.  In particular, @code{--fast} doesn't make
things significantly faster.  And @code{--best} merely selects the
default behaviour.
@item --
Treats all subsequent arguments as file names, even if they start
with a dash.  This is so you can handle files with names beginning
with a dash, for example: @code{bzip2 -- -myfilename}.
@item --repetitive-fast 
@item --repetitive-best
These flags are redundant in versions 0.9.5 and above.  They provided
some coarse control over the behaviour of the sorting algorithm in
earlier versions, which was sometimes useful.  0.9.5 and above have an
improved algorithm which renders these flags irrelevant.
@end table


@unnumberedsubsubsec MEMORY MANAGEMENT

@code{bzip2} compresses large files in blocks.  The block size affects
both the compression ratio achieved, and the amount of memory needed for
compression and decompression.  The flags @code{-1} through @code{-9}
specify the block size to be 100,000 bytes through 900,000 bytes (the
default) respectively.  At decompression time, the block size used for
compression is read from the header of the compressed file, and
@code{bunzip2} then allocates itself just enough memory to decompress
the file.  Since block sizes are stored in compressed files, it follows
that the flags @code{-1} to @code{-9} are irrelevant to and so ignored
during decompression.

Compression and decompression requirements, in bytes, can be estimated
as:
@example
     Compression:   400k + ( 8 x block size )

     Decompression: 100k + ( 4 x block size ), or
                    100k + ( 2.5 x block size )
@end example
Larger block sizes give rapidly diminishing marginal returns.  Most of
the compression comes from the first two or three hundred k of block
size, a fact worth bearing in mind when using @code{bzip2} on small machines.
It is also important to appreciate that the decompression memory
requirement is set at compression time by the choice of block size.

For files compressed with the default 900k block size, @code{bunzip2}
will require about 3700 kbytes to decompress.  To support decompression
of any file on a 4 megabyte machine, @code{bunzip2} has an option to
decompress using approximately half this amount of memory, about 2300
kbytes.  Decompression speed is also halved, so you should use this
option only where necessary.  The relevant flag is @code{-s}.

In general, try and use the largest block size memory constraints allow,
since that maximises the compression achieved.  Compression and
decompression speed are virtually unaffected by block size.

Another significant point applies to files which fit in a single block
-- that means most files you'd encounter using a large block size.  The
amount of real memory touched is proportional to the size of the file,
since the file is smaller than a block.  For example, compressing a file
20,000 bytes long with the flag @code{-9} will cause the compressor to
allocate around 7600k of memory, but only touch 400k + 20000 * 8 = 560
kbytes of it.  Similarly, the decompressor will allocate 3700k but only
touch 100k + 20000 * 4 = 180 kbytes.

Here is a table which summarises the maximum memory usage for different
block sizes.  Also recorded is the total compressed size for 14 files of
the Calgary Text Compression Corpus totalling 3,141,622 bytes.  This
column gives some feel for how compression varies with block size.
These figures tend to understate the advantage of larger block sizes for
larger files, since the Corpus is dominated by smaller files.
@example
          Compress   Decompress   Decompress   Corpus
   Flag     usage      usage       -s usage     Size

    -1      1200k       500k         350k      914704
    -2      2000k       900k         600k      877703
    -3      2800k      1300k         850k      860338
    -4      3600k      1700k        1100k      846899
    -5      4400k      2100k        1350k      845160
    -6      5200k      2500k        1600k      838626
    -7      6100k      2900k        1850k      834096
    -8      6800k      3300k        2100k      828642
    -9      7600k      3700k        2350k      828642
@end example

@unnumberedsubsubsec RECOVERING DATA FROM DAMAGED FILES

@code{bzip2} compresses files in blocks, usually 900kbytes long.  Each
block is handled independently.  If a media or transmission error causes
a multi-block @code{.bz2} file to become damaged, it may be possible to
recover data from the undamaged blocks in the file.

The compressed representation of each block is delimited by a 48-bit
pattern, which makes it possible to find the block boundaries with
reasonable certainty.  Each block also carries its own 32-bit CRC, so
damaged blocks can be distinguished from undamaged ones.

@code{bzip2recover} is a simple program whose purpose is to search for
blocks in @code{.bz2} files, and write each block out into its own
@code{.bz2} file.  You can then use @code{bzip2 -t} to test the
integrity of the resulting files, and decompress those which are
undamaged.

@code{bzip2recover} 
takes a single argument, the name of the damaged file, and writes a
number of files @code{rec00001file.bz2}, @code{rec00002file.bz2}, etc,
containing the extracted blocks.  The output filenames are designed so
that the use of wildcards in subsequent processing -- for example,
@code{bzip2 -dc rec*file.bz2 > recovered_data} -- processes the files in
the correct order.

@code{bzip2recover} should be of most use dealing with large @code{.bz2}
files, as these will contain many blocks.  It is clearly futile to use
it on damaged single-block files, since a damaged block cannot be
recovered.  If you wish to minimise any potential data loss through
media or transmission errors, you might consider compressing with a
smaller block size.


@unnumberedsubsubsec PERFORMANCE NOTES

The sorting phase of compression gathers together similar strings in the
file.  Because of this, files containing very long runs of repeated
symbols, like "aabaabaabaab ..."  (repeated several hundred times) may
compress more slowly than normal.  Versions 0.9.5 and above fare much
better than previous versions in this respect.  The ratio between
worst-case and average-case compression time is in the region of 10:1.
For previous versions, this figure was more like 100:1.  You can use the
@code{-vvvv} option to monitor progress in great detail, if you want.

Decompression speed is unaffected by these phenomena.

@code{bzip2} usually allocates several megabytes of memory to operate
in, and then charges all over it in a fairly random fashion.  This means
that performance, both for compressing and decompressing, is largely
determined by the speed at which your machine can service cache misses.
Because of this, small changes to the code to reduce the miss rate have
been observed to give disproportionately large performance improvements.
I imagine @code{bzip2} will perform best on machines with very large
caches.


@unnumberedsubsubsec CAVEATS

I/O error messages are not as helpful as they could be.  @code{bzip2}
tries hard to detect I/O errors and exit cleanly, but the details of
what the problem is sometimes seem rather misleading.

This manual page pertains to version 1.0.2 of @code{bzip2}.  Compressed
data created by this version is entirely forwards and backwards
compatible with the previous public releases, versions 0.1pl2, 0.9.0,
0.9.5, 1.0.0 and 1.0.1, but with the following exception: 0.9.0 and
above can correctly decompress multiple concatenated compressed files.
0.1pl2 cannot do this; it will stop after decompressing just the first
file in the stream.

@code{bzip2recover} versions prior to this one, 1.0.2, used 32-bit
integers to represent bit positions in compressed files, so it could not
handle compressed files more than 512 megabytes long.  Version 1.0.2 and
above uses 64-bit ints on some platforms which support them (GNU
supported targets, and Windows).  To establish whether or not
@code{bzip2recover} was built with such a limitation, run it without
arguments.  In any event you can build yourself an unlimited version if
you can recompile it with @code{MaybeUInt64} set to be an unsigned
64-bit integer.



@unnumberedsubsubsec AUTHOR
Julian Seward, @code{jseward@@acm.org}.

@code{http://sources.redhat.com/bzip2}

The ideas embodied in @code{bzip2} are due to (at least) the following
people: Michael Burrows and David Wheeler (for the block sorting
transformation), David Wheeler (again, for the Huffman coder), Peter
Fenwick (for the structured coding model in the original @code{bzip},
and many refinements), and Alistair Moffat, Radford Neal and Ian Witten
(for the arithmetic coder in the original @code{bzip}).  I am much
indebted for their help, support and advice.  See the manual in the
source distribution for pointers to sources of documentation.  Christian
von Roques encouraged me to look for faster sorting algorithms, so as to
speed up compression.  Bela Lubkin encouraged me to improve the
worst-case compression performance.  The @code{bz*} scripts are derived
from those of GNU @code{gzip}.  Many people sent patches, helped with
portability problems, lent machines, gave advice and were generally
helpful.

@end quotation




@chapter Programming with @code{libbzip2}

This chapter describes the programming interface to @code{libbzip2}.

For general background information, particularly about memory
use and performance aspects, you'd be well advised to read Chapter 2
as well.

@section Top-level structure

@code{libbzip2} is a flexible library for compressing and decompressing
data in the @code{bzip2} data format.  Although packaged as a single
entity, it helps to regard the library as three separate parts: the low
level interface, and the high level interface, and some utility
functions.

The structure of @code{libbzip2}'s interfaces is similar to
that of Jean-loup Gailly's and Mark Adler's excellent @code{zlib} 
library.

All externally visible symbols have names beginning @code{BZ2_}.
This is new in version 1.0.  The intention is to minimise pollution
of the namespaces of library clients.

@subsection Low-level summary

This interface provides services for compressing and decompressing
data in memory.  There's no provision for dealing with files, streams
or any other I/O mechanisms, just straight memory-to-memory work.
In fact, this part of the library can be compiled without inclusion
of @code{stdio.h}, which may be helpful for embedded applications.

The low-level part of the library has no global variables and
is therefore thread-safe.

Six routines make up the low level interface: 
@code{BZ2_bzCompressInit}, @code{BZ2_bzCompress}, and @* @code{BZ2_bzCompressEnd}
for compression,
and a corresponding trio @code{BZ2_bzDecompressInit}, @* @code{BZ2_bzDecompress}
and @code{BZ2_bzDecompressEnd} for decompression.  
The @code{*Init} functions allocate
memory for compression/decompression and do other
initialisations, whilst the @code{*End} functions close down operations
and release memory.

The real work is done by @code{BZ2_bzCompress} and @code{BZ2_bzDecompress}.  
These compress and decompress data from a user-supplied input buffer
to a user-supplied output buffer.  These buffers can be any size;
arbitrary quantities of data are handled by making repeated calls
to these functions.  This is a flexible mechanism allowing a 
consumer-pull style of activity, or producer-push, or a mixture of
both.



@subsection High-level summary

This interface provides some handy wrappers around the low-level
interface to facilitate reading and writing @code{bzip2} format
files (@code{.bz2} files).  The routines provide hooks to facilitate
reading files in which the @code{bzip2} data stream is embedded 
within some larger-scale file structure, or where there are
multiple @code{bzip2} data streams concatenated end-to-end.

For reading files, @code{BZ2_bzReadOpen}, @code{BZ2_bzRead},
@code{BZ2_bzReadClose} and @* @code{BZ2_bzReadGetUnused} are supplied.  For
writing files, @code{BZ2_bzWriteOpen}, @code{BZ2_bzWrite} and
@code{BZ2_bzWriteFinish} are available.

As with the low-level library, no global variables are used
so the library is per se thread-safe.  However, if I/O errors
occur whilst reading or writing the underlying compressed files,
you may have to consult @code{errno} to determine the cause of
the error.  In that case, you'd need a C library which correctly
supports @code{errno} in a multithreaded environment.

To make the library a little simpler and more portable,
@code{BZ2_bzReadOpen} and @code{BZ2_bzWriteOpen} require you to pass them file
handles (@code{FILE*}s) which have previously been opened for reading or
writing respectively.  That avoids portability problems associated with
file operations and file attributes, whilst not being much of an
imposition on the programmer.



@subsection Utility functions summary
For very simple needs, @code{BZ2_bzBuffToBuffCompress} and
@code{BZ2_bzBuffToBuffDecompress} are provided.  These compress
data in memory from one buffer to another buffer in a single
function call.  You should assess whether these functions
fulfill your memory-to-memory compression/decompression
requirements before investing effort in understanding the more
general but more complex low-level interface.

Yoshioka Tsuneo (@code{QWF00133@@niftyserve.or.jp} /
@code{tsuneo-y@@is.aist-nara.ac.jp}) has contributed some functions to
give better @code{zlib} compatibility.  These functions are
@code{BZ2_bzopen}, @code{BZ2_bzread}, @code{BZ2_bzwrite}, @code{BZ2_bzflush},
@code{BZ2_bzclose},
@code{BZ2_bzerror} and @code{BZ2_bzlibVersion}.  You may find these functions
more convenient for simple file reading and writing, than those in the
high-level interface.  These functions are not (yet) officially part of
the library, and are minimally documented here.  If they break, you
get to keep all the pieces.  I hope to document them properly when time
permits.

Yoshioka also contributed modifications to allow the library to be
built as a Windows DLL.


@section Error handling

The library is designed to recover cleanly in all situations, including
the worst-case situation of decompressing random data.  I'm not 
100% sure that it can always do this, so you might want to add
a signal handler to catch segmentation violations during decompression
if you are feeling especially paranoid.  I would be interested in
hearing more about the robustness of the library to corrupted
compressed data.

Version 1.0 is much more robust in this respect than
0.9.0 or 0.9.5.  Investigations with Checker (a tool for 
detecting problems with memory management, similar to Purify)
indicate that, at least for the few files I tested, all single-bit
errors in the decompressed data are caught properly, with no
segmentation faults, no reads of uninitialised data and no 
out of range reads or writes.  So it's certainly much improved,
although I wouldn't claim it to be totally bombproof.

The file @code{bzlib.h} contains all definitions needed to use
the library.  In particular, you should definitely not include
@code{bzlib_private.h}.

In @code{bzlib.h}, the various return values are defined.  The following
list is not intended as an exhaustive description of the circumstances 
in which a given value may be returned -- those descriptions are given
later.  Rather, it is intended to convey the rough meaning of each
return value.  The first five actions are normal and not intended to 
denote an error situation.
@table @code
@item BZ_OK
The requested action was completed successfully.
@item BZ_RUN_OK
@itemx BZ_FLUSH_OK
@itemx BZ_FINISH_OK
In @code{BZ2_bzCompress}, the requested flush/finish/nothing-special action
was completed successfully.
@item BZ_STREAM_END
Compression of data was completed, or the logical stream end was
detected during decompression.
@end table

The following return values indicate an error of some kind.
@table @code
@item BZ_CONFIG_ERROR
Indicates that the library has been improperly compiled on your
platform -- a major configuration error.  Specifically, it means
that @code{sizeof(char)}, @code{sizeof(short)} and @code{sizeof(int)}
are not 1, 2 and 4 respectively, as they should be.  Note that the 
library should still work properly on 64-bit platforms which follow
the LP64 programming model -- that is, where @code{sizeof(long)}
and @code{sizeof(void*)} are 8.  Under LP64, @code{sizeof(int)} is
still 4, so @code{libbzip2}, which doesn't use the @code{long} type,
is OK.
@item BZ_SEQUENCE_ERROR
When using the library, it is important to call the functions in the
correct sequence and with data structures (buffers etc) in the correct
states.  @code{libbzip2} checks as much as it can to ensure this is
happening, and returns @code{BZ_SEQUENCE_ERROR} if not.  Code which
complies precisely with the function semantics, as detailed below,
should never receive this value; such an event denotes buggy code
which you should investigate.
@item BZ_PARAM_ERROR
Returned when a parameter to a function call is out of range 
or otherwise manifestly incorrect.  As with @code{BZ_SEQUENCE_ERROR},
this denotes a bug in the client code.  The distinction between
@code{BZ_PARAM_ERROR} and @code{BZ_SEQUENCE_ERROR} is a bit hazy, but still worth
making.
@item BZ_MEM_ERROR
Returned when a request to allocate memory failed.  Note that the
quantity of memory needed to decompress a stream cannot be determined
until the stream's header has been read.  So @code{BZ2_bzDecompress} and
@code{BZ2_bzRead} may return @code{BZ_MEM_ERROR} even though some of
the compressed data has been read.  The same is not true for
compression; once @code{BZ2_bzCompressInit} or @code{BZ2_bzWriteOpen} have
successfully completed, @code{BZ_MEM_ERROR} cannot occur.
@item BZ_DATA_ERROR
Returned when a data integrity error is detected during decompression.
Most importantly, this means when stored and computed CRCs for the
data do not match.  This value is also returned upon detection of any
other anomaly in the compressed data.
@item BZ_DATA_ERROR_MAGIC
As a special case of @code{BZ_DATA_ERROR}, it is sometimes useful to
know when the compressed stream does not start with the correct
magic bytes (@code{'B' 'Z' 'h'}).  
@item BZ_IO_ERROR
Returned by @code{BZ2_bzRead} and @code{BZ2_bzWrite} when there is an error
reading or writing in the compressed file, and by @code{BZ2_bzReadOpen}
and @code{BZ2_bzWriteOpen} for attempts to use a file for which the
error indicator (viz, @code{ferror(f)}) is set.
On receipt of @code{BZ_IO_ERROR}, the caller should consult
@code{errno} and/or @code{perror} to acquire operating-system
specific information about the problem.
@item BZ_UNEXPECTED_EOF
Returned by @code{BZ2_bzRead} when the compressed file finishes
before the logical end of stream is detected.
@item BZ_OUTBUFF_FULL
Returned by @code{BZ2_bzBuffToBuffCompress} and
@code{BZ2_bzBuffToBuffDecompress} to indicate that the output data
will not fit into the output buffer provided.
@end table



@section Low-level interface

@subsection @code{BZ2_bzCompressInit}
@example
typedef 
   struct @{
      char *next_in;
      unsigned int avail_in;
      unsigned int total_in_lo32;
      unsigned int total_in_hi32;

      char *next_out;
      unsigned int avail_out;
      unsigned int total_out_lo32;
      unsigned int total_out_hi32;

      void *state;

      void *(*bzalloc)(void *,int,int);
      void (*bzfree)(void *,void *);
      void *opaque;
   @} 
   bz_stream;

int BZ2_bzCompressInit ( bz_stream *strm, 
                         int blockSize100k, 
                         int verbosity,
                         int workFactor );

@end example

Prepares for compression.  The @code{bz_stream} structure
holds all data pertaining to the compression activity.  
A @code{bz_stream} structure should be allocated and initialised
prior to the call.
The fields of @code{bz_stream}
comprise the entirety of the user-visible data.  @code{state}
is a pointer to the private data structures required for compression.

Custom memory allocators are supported, via fields @code{bzalloc}, 
@code{bzfree},
and @code{opaque}.  The value 
@code{opaque} is passed to as the first argument to
all calls to @code{bzalloc} and @code{bzfree}, but is 
otherwise ignored by the library.
The call @code{bzalloc ( opaque, n, m )} is expected to return a 
pointer @code{p} to
@code{n * m} bytes of memory, and @code{bzfree ( opaque, p )} 
should free
that memory.

If you don't want to use a custom memory allocator, set @code{bzalloc}, 
@code{bzfree} and
@code{opaque} to @code{NULL}, 
and the library will then use the standard @code{malloc}/@code{free}
routines.

Before calling @code{BZ2_bzCompressInit}, fields @code{bzalloc}, 
@code{bzfree} and @code{opaque} should
be filled appropriately, as just described.  Upon return, the internal
state will have been allocated and initialised, and @code{total_in_lo32}, 
@code{total_in_hi32}, @code{total_out_lo32} and 
@code{total_out_hi32} will have been set to zero.  
These four fields are used by the library
to inform the caller of the total amount of data passed into and out of
the library, respectively.  You should not try to change them.
As of version 1.0, 64-bit counts are maintained, even on 32-bit
platforms, using the @code{_hi32} fields to store the upper 32 bits
of the count.  So, for example, the total amount of data in
is @code{(total_in_hi32 << 32) + total_in_lo32}.

Parameter @code{blockSize100k} specifies the block size to be used for
compression.  It should be a value between 1 and 9 inclusive, and the
actual block size used is 100000 x this figure.  9 gives the best
compression but takes most memory.

Parameter @code{verbosity} should be set to a number between 0 and 4
inclusive.  0 is silent, and greater numbers give increasingly verbose
monitoring/debugging output.  If the library has been compiled with
@code{-DBZ_NO_STDIO}, no such output will appear for any verbosity
setting.

Parameter @code{workFactor} controls how the compression phase behaves
when presented with worst case, highly repetitive, input data.  If
compression runs into difficulties caused by repetitive data, the
library switches from the standard sorting algorithm to a fallback
algorithm.  The fallback is slower than the standard algorithm by
perhaps a factor of three, but always behaves reasonably, no matter how
bad the input.

Lower values of @code{workFactor} reduce the amount of effort the
standard algorithm will expend before resorting to the fallback.  You
should set this parameter carefully; too low, and many inputs will be
handled by the fallback algorithm and so compress rather slowly, too
high, and your average-to-worst case compression times can become very
large.  The default value of 30 gives reasonable behaviour over a wide
range of circumstances.

Allowable values range from 0 to 250 inclusive.  0 is a special case,
equivalent to using the default value of 30.

Note that the compressed output generated is the same regardless of
whether or not the fallback algorithm is used.

Be aware also that this parameter may disappear entirely in future
versions of the library.  In principle it should be possible to devise a
good way to automatically choose which algorithm to use.  Such a
mechanism would render the parameter obsolete.

Possible return values:
@display
      @code{BZ_CONFIG_ERROR}
         if the library has been mis-compiled
      @code{BZ_PARAM_ERROR} 
         if @code{strm} is @code{NULL} 
         or @code{blockSize} < 1 or @code{blockSize} > 9
         or @code{verbosity} < 0 or @code{verbosity} > 4
         or @code{workFactor} < 0 or @code{workFactor} > 250
      @code{BZ_MEM_ERROR} 
         if not enough memory is available
      @code{BZ_OK} 
         otherwise
@end display
Allowable next actions:
@display
      @code{BZ2_bzCompress} 
         if @code{BZ_OK} is returned
      no specific action needed in case of error
@end display

@subsection @code{BZ2_bzCompress}
@example
   int BZ2_bzCompress ( bz_stream *strm, int action );
@end example
Provides more input and/or output buffer space for the library.  The
caller maintains input and output buffers, and calls @code{BZ2_bzCompress} to
transfer data between them.

Before each call to @code{BZ2_bzCompress}, @code{next_in} should point at
the data to be compressed, and @code{avail_in} should indicate how many
bytes the library may read.  @code{BZ2_bzCompress} updates @code{next_in},
@code{avail_in} and @code{total_in} to reflect the number of bytes it
has read.

Similarly, @code{next_out} should point to a buffer in which the
compressed data is to be placed, with @code{avail_out} indicating how
much output space is available.  @code{BZ2_bzCompress} updates
@code{next_out}, @code{avail_out} and @code{total_out} to reflect the
number of bytes output.

You may provide and remove as little or as much data as you like on each
call of @code{BZ2_bzCompress}.  In the limit, it is acceptable to supply and
remove data one byte at a time, although this would be terribly
inefficient.  You should always ensure that at least one byte of output
space is available at each call.

A second purpose of @code{BZ2_bzCompress} is to request a change of mode of the
compressed stream.  

Conceptually, a compressed stream can be in one of four states: IDLE,
RUNNING, FLUSHING and FINISHING.  Before initialisation
(@code{BZ2_bzCompressInit}) and after termination (@code{BZ2_bzCompressEnd}), a
stream is regarded as IDLE.

Upon initialisation (@code{BZ2_bzCompressInit}), the stream is placed in the
RUNNING state.  Subsequent calls to @code{BZ2_bzCompress} should pass
@code{BZ_RUN} as the requested action; other actions are illegal and
will result in @code{BZ_SEQUENCE_ERROR}.

At some point, the calling program will have provided all the input data
it wants to.  It will then want to finish up -- in effect, asking the
library to process any data it might have buffered internally.  In this
state, @code{BZ2_bzCompress} will no longer attempt to read data from
@code{next_in}, but it will want to write data to @code{next_out}.
Because the output buffer supplied by the user can be arbitrarily small,
the finishing-up operation cannot necessarily be done with a single call
of @code{BZ2_bzCompress}.

Instead, the calling program passes @code{BZ_FINISH} as an action to
@code{BZ2_bzCompress}.  This changes the stream's state to FINISHING.  Any
remaining input (ie, @code{next_in[0 .. avail_in-1]}) is compressed and
transferred to the output buffer.  To do this, @code{BZ2_bzCompress} must be
called repeatedly until all the output has been consumed.  At that
point, @code{BZ2_bzCompress} returns @code{BZ_STREAM_END}, and the stream's
state is set back to IDLE.  @code{BZ2_bzCompressEnd} should then be
called.

Just to make sure the calling program does not cheat, the library makes
a note of @code{avail_in} at the time of the first call to
@code{BZ2_bzCompress} which has @code{BZ_FINISH} as an action (ie, at the
time the program has announced its intention to not supply any more
input).  By comparing this value with that of @code{avail_in} over
subsequent calls to @code{BZ2_bzCompress}, the library can detect any
attempts to slip in more data to compress.  Any calls for which this is
detected will return @code{BZ_SEQUENCE_ERROR}.  This indicates a
programming mistake which should be corrected.

Instead of asking to finish, the calling program may ask
@code{BZ2_bzCompress} to take all the remaining input, compress it and
terminate the current (Burrows-Wheeler) compression block.  This could
be useful for error control purposes.  The mechanism is analogous to
that for finishing: call @code{BZ2_bzCompress} with an action of
@code{BZ_FLUSH}, remove output data, and persist with the
@code{BZ_FLUSH} action until the value @code{BZ_RUN} is returned.  As
with finishing, @code{BZ2_bzCompress} detects any attempt to provide more
input data once the flush has begun.

Once the flush is complete, the stream returns to the normal RUNNING
state.

This all sounds pretty complex, but isn't really.  Here's a table
which shows which actions are allowable in each state, what action
will be taken, what the next state is, and what the non-error return
values are.  Note that you can't explicitly ask what state the
stream is in, but nor do you need to -- it can be inferred from the
values returned by @code{BZ2_bzCompress}.
@display
IDLE/@code{any}           
      Illegal.  IDLE state only exists after @code{BZ2_bzCompressEnd} or
      before @code{BZ2_bzCompressInit}.
      Return value = @code{BZ_SEQUENCE_ERROR}

RUNNING/@code{BZ_RUN}     
      Compress from @code{next_in} to @code{next_out} as much as possible.
      Next state = RUNNING
      Return value = @code{BZ_RUN_OK}

RUNNING/@code{BZ_FLUSH}   
      Remember current value of @code{next_in}.  Compress from @code{next_in}
      to @code{next_out} as much as possible, but do not accept any more input.  
      Next state = FLUSHING
      Return value = @code{BZ_FLUSH_OK}

RUNNING/@code{BZ_FINISH}  
      Remember current value of @code{next_in}.  Compress from @code{next_in}
      to @code{next_out} as much as possible, but do not accept any more input.
      Next state = FINISHING
      Return value = @code{BZ_FINISH_OK}

FLUSHING/@code{BZ_FLUSH}  
      Compress from @code{next_in} to @code{next_out} as much as possible, 
      but do not accept any more input.  
      If all the existing input has been used up and all compressed
      output has been removed
         Next state = RUNNING; Return value = @code{BZ_RUN_OK}
      else
         Next state = FLUSHING; Return value = @code{BZ_FLUSH_OK}

FLUSHING/other     
      Illegal.
      Return value = @code{BZ_SEQUENCE_ERROR}

FINISHING/@code{BZ_FINISH}  
      Compress from @code{next_in} to @code{next_out} as much as possible,
      but to not accept any more input.  
      If all the existing input has been used up and all compressed
      output has been removed
         Next state = IDLE; Return value = @code{BZ_STREAM_END}
      else
         Next state = FINISHING; Return value = @code{BZ_FINISHING}

FINISHING/other
      Illegal.
      Return value = @code{BZ_SEQUENCE_ERROR}
@end display

That still looks complicated?  Well, fair enough.  The usual sequence
of calls for compressing a load of data is:
@itemize @bullet
@item Get started with @code{BZ2_bzCompressInit}.
@item Shovel data in and shlurp out its compressed form using zero or more
calls of @code{BZ2_bzCompress} with action = @code{BZ_RUN}.
@item Finish up.  
Repeatedly call @code{BZ2_bzCompress} with action = @code{BZ_FINISH}, 
copying out the compressed output, until @code{BZ_STREAM_END} is returned.
@item Close up and go home.  Call @code{BZ2_bzCompressEnd}.
@end itemize
If the data you want to compress fits into your input buffer all
at once, you can skip the calls of @code{BZ2_bzCompress ( ..., BZ_RUN )} and 
just do the @code{BZ2_bzCompress ( ..., BZ_FINISH )} calls.

All required memory is allocated by @code{BZ2_bzCompressInit}.  The
compression library can accept any data at all (obviously).  So you
shouldn't get any error return values from the @code{BZ2_bzCompress} calls.
If you do, they will be @code{BZ_SEQUENCE_ERROR}, and indicate a bug in
your programming.

Trivial other possible return values:
@display
      @code{BZ_PARAM_ERROR}   
         if @code{strm} is @code{NULL}, or @code{strm->s} is @code{NULL}
@end display

@subsection @code{BZ2_bzCompressEnd}
@example
int BZ2_bzCompressEnd ( bz_stream *strm );
@end example
Releases all memory associated with a compression stream.

Possible return values:
@display
   @code{BZ_PARAM_ERROR}    if @code{strm} is @code{NULL} or @code{strm->s} is @code{NULL}
   @code{BZ_OK}    otherwise
@end display


@subsection @code{BZ2_bzDecompressInit}
@example
int BZ2_bzDecompressInit ( bz_stream *strm, int verbosity, int small );
@end example
Prepares for decompression.  As with @code{BZ2_bzCompressInit}, a
@code{bz_stream} record should be allocated and initialised before the
call.  Fields @code{bzalloc}, @code{bzfree} and @code{opaque} should be
set if a custom memory allocator is required, or made @code{NULL} for
the normal @code{malloc}/@code{free} routines.  Upon return, the internal
state will have been initialised, and @code{total_in} and
@code{total_out} will be zero.

For the meaning of parameter @code{verbosity}, see @code{BZ2_bzCompressInit}.

If @code{small} is nonzero, the library will use an alternative
decompression algorithm which uses less memory but at the cost of
decompressing more slowly (roughly speaking, half the speed, but the
maximum memory requirement drops to around 2300k).  See Chapter 2 for
more information on memory management.

Note that the amount of memory needed to decompress
a stream cannot be determined until the stream's header has been read,
so even if @code{BZ2_bzDecompressInit} succeeds, a subsequent
@code{BZ2_bzDecompress} could fail with @code{BZ_MEM_ERROR}.

Possible return values:
@display
      @code{BZ_CONFIG_ERROR}
         if the library has been mis-compiled
      @code{BZ_PARAM_ERROR}
         if @code{(small != 0 && small != 1)}
         or @code{(verbosity < 0 || verbosity > 4)}
      @code{BZ_MEM_ERROR}
         if insufficient memory is available
@end display

Allowable next actions:
@display
      @code{BZ2_bzDecompress}
         if @code{BZ_OK} was returned
      no specific action required in case of error
@end display

 

@subsection @code{BZ2_bzDecompress}
@example
int BZ2_bzDecompress ( bz_stream *strm );
@end example
Provides more input and/out output buffer space for the library.  The
caller maintains input and output buffers, and uses @code{BZ2_bzDecompress}
to transfer data between them.

Before each call to @code{BZ2_bzDecompress}, @code{next_in} 
should point at the compressed data,
and @code{avail_in} should indicate how many bytes the library
may read.  @code{BZ2_bzDecompress} updates @code{next_in}, @code{avail_in} 
and @code{total_in}
to reflect the number of bytes it has read.

Similarly, @code{next_out} should point to a buffer in which the uncompressed
output is to be placed, with @code{avail_out} indicating how much output space
is available.  @code{BZ2_bzCompress} updates @code{next_out},
@code{avail_out} and @code{total_out} to reflect
the number of bytes output.

You may provide and remove as little or as much data as you like on
each call of @code{BZ2_bzDecompress}.  
In the limit, it is acceptable to
supply and remove data one byte at a time, although this would be
terribly inefficient.  You should always ensure that at least one
byte of output space is available at each call.

Use of @code{BZ2_bzDecompress} is simpler than @code{BZ2_bzCompress}.

You should provide input and remove output as described above, and
repeatedly call @code{BZ2_bzDecompress} until @code{BZ_STREAM_END} is
returned.  Appearance of @code{BZ_STREAM_END} denotes that
@code{BZ2_bzDecompress} has detected the logical end of the compressed
stream.  @code{BZ2_bzDecompress} will not produce @code{BZ_STREAM_END} until
all output data has been placed into the output buffer, so once
@code{BZ_STREAM_END} appears, you are guaranteed to have available all
the decompressed output, and @code{BZ2_bzDecompressEnd} can safely be
called.

If case of an error return value, you should call @code{BZ2_bzDecompressEnd}
to clean up and release memory.

Possible return values:
@display
      @code{BZ_PARAM_ERROR}
         if @code{strm} is @code{NULL} or @code{strm->s} is @code{NULL}
         or @code{strm->avail_out < 1}
      @code{BZ_DATA_ERROR}
         if a data integrity error is detected in the compressed stream
      @code{BZ_DATA_ERROR_MAGIC}
         if the compressed stream doesn't begin with the right magic bytes
      @code{BZ_MEM_ERROR}
         if there wasn't enough memory available
      @code{BZ_STREAM_END}
         if the logical end of the data stream was detected and all
         output in has been consumed, eg @code{s->avail_out > 0}
      @code{BZ_OK}
         otherwise
@end display
Allowable next actions:
@display
      @code{BZ2_bzDecompress}
         if @code{BZ_OK} was returned
      @code{BZ2_bzDecompressEnd}
         otherwise
@end display


@subsection @code{BZ2_bzDecompressEnd}
@example
int BZ2_bzDecompressEnd ( bz_stream *strm );
@end example
Releases all memory associated with a decompression stream.

Possible return values:
@display
      @code{BZ_PARAM_ERROR}
         if @code{strm} is @code{NULL} or @code{strm->s} is @code{NULL}
      @code{BZ_OK}
         otherwise
@end display

Allowable next actions:
@display
      None.
@end display


@section High-level interface

This interface provides functions for reading and writing 
@code{bzip2} format files.  First, some general points.

@itemize @bullet
@item All of the functions take an @code{int*} first argument,
  @code{bzerror}.
  After each call, @code{bzerror} should be consulted first to determine
  the outcome of the call.  If @code{bzerror} is @code{BZ_OK}, 
  the call completed
  successfully, and only then should the return value of the function
  (if any) be consulted.  If @code{bzerror} is @code{BZ_IO_ERROR}, 
  there was an error
  reading/writing the underlying compressed file, and you should
  then consult @code{errno}/@code{perror} to determine the 
  cause of the difficulty.
  @code{bzerror} may also be set to various other values; precise details are
  given on a per-function basis below.
@item If @code{bzerror} indicates an error 
  (ie, anything except @code{BZ_OK} and @code{BZ_STREAM_END}),
  you should immediately call @code{BZ2_bzReadClose} (or @code{BZ2_bzWriteClose},
  depending on whether you are attempting to read or to write)
  to free up all resources associated
  with the stream.  Once an error has been indicated, behaviour of all calls
  except @code{BZ2_bzReadClose} (@code{BZ2_bzWriteClose}) is undefined.  
  The implication is that (1) @code{bzerror} should
  be checked after each call, and (2) if @code{bzerror} indicates an error, 
  @code{BZ2_bzReadClose} (@code{BZ2_bzWriteClose}) should then be called to clean up.
@item The @code{FILE*} arguments passed to
   @code{BZ2_bzReadOpen}/@code{BZ2_bzWriteOpen}  
  should be set to binary mode.
  Most Unix systems will do this by default, but other platforms,
  including Windows and Mac, will not.  If you omit this, you may
  encounter problems when moving code to new platforms.
@item Memory allocation requests are handled by
  @code{malloc}/@code{free}.  
  At present
  there is no facility for user-defined memory allocators in the file I/O
  functions (could easily be added, though).
@end itemize



@subsection @code{BZ2_bzReadOpen}
@example
   typedef void BZFILE;

   BZFILE *BZ2_bzReadOpen ( int *bzerror, FILE *f, 
                            int small, int verbosity,
                            void *unused, int nUnused );
@end example
Prepare to read compressed data from file handle @code{f}.  @code{f}
should refer to a file which has been opened for reading, and for which
the error indicator (@code{ferror(f)})is not set.  If @code{small} is 1,
the library will try to decompress using less memory, at the expense of
speed.

For reasons explained below, @code{BZ2_bzRead} will decompress the
@code{nUnused} bytes starting at @code{unused}, before starting to read
from the file @code{f}.  At most @code{BZ_MAX_UNUSED} bytes may be
supplied like this.  If this facility is not required, you should pass
@code{NULL} and @code{0} for @code{unused} and n@code{Unused}
respectively.

For the meaning of parameters @code{small} and @code{verbosity},
see @code{BZ2_bzDecompressInit}.

The amount of memory needed to decompress a file cannot be determined
until the file's header has been read.  So it is possible that
@code{BZ2_bzReadOpen} returns @code{BZ_OK} but a subsequent call of
@code{BZ2_bzRead} will return @code{BZ_MEM_ERROR}.

Possible assignments to @code{bzerror}:
@display
      @code{BZ_CONFIG_ERROR}
         if the library has been mis-compiled
      @code{BZ_PARAM_ERROR}
         if @code{f} is @code{NULL} 
         or @code{small} is neither @code{0} nor @code{1}                 
         or @code{(unused == NULL && nUnused != 0)}
         or @code{(unused != NULL && !(0 <= nUnused <= BZ_MAX_UNUSED))}
      @code{BZ_IO_ERROR}    
         if @code{ferror(f)} is nonzero
      @code{BZ_MEM_ERROR}   
         if insufficient memory is available
      @code{BZ_OK}
         otherwise.
@end display

Possible return values:
@display
      Pointer to an abstract @code{BZFILE}        
         if @code{bzerror} is @code{BZ_OK}   
      @code{NULL}
         otherwise
@end display

Allowable next actions:
@display
      @code{BZ2_bzRead}
         if @code{bzerror} is @code{BZ_OK}   
      @code{BZ2_bzClose} 
         otherwise
@end display


@subsection @code{BZ2_bzRead}
@example
   int BZ2_bzRead ( int *bzerror, BZFILE *b, void *buf, int len );
@end example
Reads up to @code{len} (uncompressed) bytes from the compressed file 
@code{b} into
the buffer @code{buf}.  If the read was successful, 
@code{bzerror} is set to @code{BZ_OK}
and the number of bytes read is returned.  If the logical end-of-stream
was detected, @code{bzerror} will be set to @code{BZ_STREAM_END}, 
and the number
of bytes read is returned.  All other @code{bzerror} values denote an error.

@code{BZ2_bzRead} will supply @code{len} bytes,
unless the logical stream end is detected
or an error occurs.  Because of this, it is possible to detect the 
stream end by observing when the number of bytes returned is 
less than the number
requested.  Nevertheless, this is regarded as inadvisable; you should
instead check @code{bzerror} after every call and watch out for
@code{BZ_STREAM_END}.

Internally, @code{BZ2_bzRead} copies data from the compressed file in chunks
of size @code{BZ_MAX_UNUSED} bytes
before decompressing it.  If the file contains more bytes than strictly
needed to reach the logical end-of-stream, @code{BZ2_bzRead} will almost certainly
read some of the trailing data before signalling @code{BZ_SEQUENCE_END}.
To collect the read but unused data once @code{BZ_SEQUENCE_END} has 
appeared, call @code{BZ2_bzReadGetUnused} immediately before @code{BZ2_bzReadClose}.

Possible assignments to @code{bzerror}:
@display
      @code{BZ_PARAM_ERROR}
         if @code{b} is @code{NULL} or @code{buf} is @code{NULL} or @code{len < 0}
      @code{BZ_SEQUENCE_ERROR} 
         if @code{b} was opened with @code{BZ2_bzWriteOpen}
      @code{BZ_IO_ERROR} 
         if there is an error reading from the compressed file
      @code{BZ_UNEXPECTED_EOF} 
         if the compressed file ended before the logical end-of-stream was detected
      @code{BZ_DATA_ERROR} 
         if a data integrity error was detected in the compressed stream
      @code{BZ_DATA_ERROR_MAGIC}
         if the stream does not begin with the requisite header bytes (ie, is not 
         a @code{bzip2} data file).  This is really a special case of @code{BZ_DATA_ERROR}.
      @code{BZ_MEM_ERROR} 
         if insufficient memory was available
      @code{BZ_STREAM_END} 
         if the logical end of stream was detected.
      @code{BZ_OK}
         otherwise.
@end display

Possible return values:
@display
      number of bytes read
         if @code{bzerror} is @code{BZ_OK} or @code{BZ_STREAM_END}
      undefined
         otherwise
@end display

Allowable next actions:
@display
      collect data from @code{buf}, then @code{BZ2_bzRead} or @code{BZ2_bzReadClose}
         if @code{bzerror} is @code{BZ_OK} 
      collect data from @code{buf}, then @code{BZ2_bzReadClose} or @code{BZ2_bzReadGetUnused} 
         if @code{bzerror} is @code{BZ_SEQUENCE_END}   
      @code{BZ2_bzReadClose} 
         otherwise
@end display



@subsection @code{BZ2_bzReadGetUnused}
@example
   void BZ2_bzReadGetUnused ( int* bzerror, BZFILE *b, 
                              void** unused, int* nUnused );
@end example
Returns data which was read from the compressed file but was not needed
to get to the logical end-of-stream.  @code{*unused} is set to the address
of the data, and @code{*nUnused} to the number of bytes.  @code{*nUnused} will
be set to a value between @code{0} and @code{BZ_MAX_UNUSED} inclusive.

This function may only be called once @code{BZ2_bzRead} has signalled 
@code{BZ_STREAM_END} but before @code{BZ2_bzReadClose}.

Possible assignments to @code{bzerror}:
@display
      @code{BZ_PARAM_ERROR} 
         if @code{b} is @code{NULL} 
         or @code{unused} is @code{NULL} or @code{nUnused} is @code{NULL}
      @code{BZ_SEQUENCE_ERROR} 
         if @code{BZ_STREAM_END} has not been signalled
         or if @code{b} was opened with @code{BZ2_bzWriteOpen}
     @code{BZ_OK}
         otherwise
@end display

Allowable next actions:
@display 
      @code{BZ2_bzReadClose}
@end display


@subsection @code{BZ2_bzReadClose}
@example
   void BZ2_bzReadClose ( int *bzerror, BZFILE *b );
@end example
Releases all memory pertaining to the compressed file @code{b}.  
@code{BZ2_bzReadClose} does not call @code{fclose} on the underlying file
handle, so you should do that yourself if appropriate.
@code{BZ2_bzReadClose} should be called to clean up after all error
situations.

Possible assignments to @code{bzerror}:
@display
      @code{BZ_SEQUENCE_ERROR} 
         if @code{b} was opened with @code{BZ2_bzOpenWrite} 
      @code{BZ_OK} 
         otherwise
@end display

Allowable next actions:
@display
      none
@end display



@subsection @code{BZ2_bzWriteOpen}
@example
   BZFILE *BZ2_bzWriteOpen ( int *bzerror, FILE *f, 
                             int blockSize100k, int verbosity,
                             int workFactor );
@end example
Prepare to write compressed data to file handle @code{f}.  
@code{f} should refer to
a file which has been opened for writing, and for which the error
indicator (@code{ferror(f)})is not set.  

For the meaning of parameters @code{blockSize100k},
@code{verbosity} and @code{workFactor}, see
@* @code{BZ2_bzCompressInit}.

All required memory is allocated at this stage, so if the call
completes successfully, @code{BZ_MEM_ERROR} cannot be signalled by a
subsequent call to @code{BZ2_bzWrite}.

Possible assignments to @code{bzerror}:
@display 
      @code{BZ_CONFIG_ERROR}
         if the library has been mis-compiled
      @code{BZ_PARAM_ERROR} 
         if @code{f} is @code{NULL} 
         or @code{blockSize100k < 1} or @code{blockSize100k > 9}
      @code{BZ_IO_ERROR} 
         if @code{ferror(f)} is nonzero
      @code{BZ_MEM_ERROR} 
         if insufficient memory is available
      @code{BZ_OK} 
         otherwise
@end display

Possible return values:
@display
      Pointer to an abstract @code{BZFILE}  
         if @code{bzerror} is @code{BZ_OK}   
      @code{NULL} 
         otherwise
@end display

Allowable next actions:
@display
      @code{BZ2_bzWrite} 
         if @code{bzerror} is @code{BZ_OK} 
         (you could go directly to @code{BZ2_bzWriteClose}, but this would be pretty pointless)
      @code{BZ2_bzWriteClose} 
         otherwise
@end display



@subsection @code{BZ2_bzWrite}
@example
   void BZ2_bzWrite ( int *bzerror, BZFILE *b, void *buf, int len );
@end example
Absorbs @code{len} bytes from the buffer @code{buf}, eventually to be
compressed and written to the file.

Possible assignments to @code{bzerror}:
@display
      @code{BZ_PARAM_ERROR} 
         if @code{b} is @code{NULL} or @code{buf} is @code{NULL} or @code{len < 0}
      @code{BZ_SEQUENCE_ERROR} 
         if b was opened with @code{BZ2_bzReadOpen}
      @code{BZ_IO_ERROR} 
         if there is an error writing the compressed file.
      @code{BZ_OK} 
         otherwise
@end display




@subsection @code{BZ2_bzWriteClose}
@example
   void BZ2_bzWriteClose ( int *bzerror, BZFILE* f,
                           int abandon,
                           unsigned int* nbytes_in,
                           unsigned int* nbytes_out );

   void BZ2_bzWriteClose64 ( int *bzerror, BZFILE* f,
                             int abandon,
                             unsigned int* nbytes_in_lo32,
                             unsigned int* nbytes_in_hi32,
                             unsigned int* nbytes_out_lo32,
                             unsigned int* nbytes_out_hi32 );
@end example

Compresses and flushes to the compressed file all data so far supplied
by @code{BZ2_bzWrite}.  The logical end-of-stream markers are also written, so
subsequent calls to @code{BZ2_bzWrite} are illegal.  All memory associated 
with the compressed file @code{b} is released.  
@code{fflush} is called on the
compressed file, but it is not @code{fclose}'d.

If @code{BZ2_bzWriteClose} is called to clean up after an error, the only
action is to release the memory.  The library records the error codes
issued by previous calls, so this situation will be detected
automatically.  There is no attempt to complete the compression
operation, nor to @code{fflush} the compressed file.  You can force this
behaviour to happen even in the case of no error, by passing a nonzero
value to @code{abandon}.

If @code{nbytes_in} is non-null, @code{*nbytes_in} will be set to be the
total volume of uncompressed data handled.  Similarly, @code{nbytes_out}
will be set to the total volume of compressed data written.  For 
compatibility with older versions of the library, @code{BZ2_bzWriteClose}
only yields the lower 32 bits of these counts.  Use
@code{BZ2_bzWriteClose64} if you want the full 64 bit counts.  These
two functions are otherwise absolutely identical.


Possible assignments to @code{bzerror}:
@display
      @code{BZ_SEQUENCE_ERROR} 
         if @code{b} was opened with @code{BZ2_bzReadOpen}
      @code{BZ_IO_ERROR} 
         if there is an error writing the compressed file
      @code{BZ_OK} 
         otherwise
@end display

@subsection Handling embedded compressed data streams

The high-level library facilitates use of
@code{bzip2} data streams which form some part of a surrounding, larger
data stream.
@itemize @bullet
@item For writing, the library takes an open file handle, writes
compressed data to it, @code{fflush}es it but does not @code{fclose} it.
The calling application can write its own data before and after the
compressed data stream, using that same file handle.
@item Reading is more complex, and the facilities are not as general
as they could be since generality is hard to reconcile with efficiency.
@code{BZ2_bzRead} reads from the compressed file in blocks of size
@code{BZ_MAX_UNUSED} bytes, and in doing so probably will overshoot
the logical end of compressed stream.
To recover this data once decompression has
ended, call @code{BZ2_bzReadGetUnused} after the last call of @code{BZ2_bzRead}
(the one returning @code{BZ_STREAM_END}) but before calling
@code{BZ2_bzReadClose}.
@end itemize

This mechanism makes it easy to decompress multiple @code{bzip2}
streams placed end-to-end.  As the end of one stream, when @code{BZ2_bzRead}
returns @code{BZ_STREAM_END}, call @code{BZ2_bzReadGetUnused} to collect the
unused data (copy it into your own buffer somewhere).  
That data forms the start of the next compressed stream.
To start uncompressing that next stream, call @code{BZ2_bzReadOpen} again,
feeding in the unused data via the @code{unused}/@code{nUnused}
parameters.
Keep doing this until @code{BZ_STREAM_END} return coincides with the
physical end of file (@code{feof(f)}).  In this situation
@code{BZ2_bzReadGetUnused}
will of course return no data.

This should give some feel for how the high-level interface can be used.
If you require extra flexibility, you'll have to bite the bullet and get
to grips with the low-level interface.

@subsection Standard file-reading/writing code
Here's how you'd write data to a compressed file:
@example @code
FILE*   f;
BZFILE* b;
int     nBuf;
char    buf[ /* whatever size you like */ ];
int     bzerror;
int     nWritten;

f = fopen ( "myfile.bz2", "w" );
if (!f) @{
   /* handle error */
@}
b = BZ2_bzWriteOpen ( &bzerror, f, 9 );
if (bzerror != BZ_OK) @{
   BZ2_bzWriteClose ( b );
   /* handle error */
@}

while ( /* condition */ ) @{
   /* get data to write into buf, and set nBuf appropriately */
   nWritten = BZ2_bzWrite ( &bzerror, b, buf, nBuf );
   if (bzerror == BZ_IO_ERROR) @{ 
      BZ2_bzWriteClose ( &bzerror, b );
      /* handle error */
   @}
@}

BZ2_bzWriteClose ( &bzerror, b );
if (bzerror == BZ_IO_ERROR) @{
   /* handle error */
@}
@end example
And to read from a compressed file:
@example
FILE*   f;
BZFILE* b;
int     nBuf;
char    buf[ /* whatever size you like */ ];
int     bzerror;
int     nWritten;

f = fopen ( "myfile.bz2", "r" );
if (!f) @{
   /* handle error */
@}
b = BZ2_bzReadOpen ( &bzerror, f, 0, NULL, 0 );
if (bzerror != BZ_OK) @{
   BZ2_bzReadClose ( &bzerror, b );
   /* handle error */
@}

bzerror = BZ_OK;
while (bzerror == BZ_OK && /* arbitrary other conditions */) @{
   nBuf = BZ2_bzRead ( &bzerror, b, buf, /* size of buf */ );
   if (bzerror == BZ_OK) @{
      /* do something with buf[0 .. nBuf-1] */
   @}
@}
if (bzerror != BZ_STREAM_END) @{
   BZ2_bzReadClose ( &bzerror, b );
   /* handle error */
@} else @{
   BZ2_bzReadClose ( &bzerror );
@}
@end example



@section Utility functions
@subsection @code{BZ2_bzBuffToBuffCompress}
@example
   int BZ2_bzBuffToBuffCompress( char*         dest,
                                 unsigned int* destLen,
                                 char*         source,
                                 unsigned int  sourceLen,
                                 int           blockSize100k,
                                 int           verbosity,
                                 int           workFactor );
@end example
Attempts to compress the data in @code{source[0 .. sourceLen-1]}
into the destination buffer, @code{dest[0 .. *destLen-1]}.
If the destination buffer is big enough, @code{*destLen} is
set to the size of the compressed data, and @code{BZ_OK} is
returned.  If the compressed data won't fit, @code{*destLen}
is unchanged, and @code{BZ_OUTBUFF_FULL} is returned.

Compression in this manner is a one-shot event, done with a single call
to this function.  The resulting compressed data is a complete
@code{bzip2} format data stream.  There is no mechanism for making
additional calls to provide extra input data.  If you want that kind of
mechanism, use the low-level interface.

For the meaning of parameters @code{blockSize100k}, @code{verbosity}
and @code{workFactor}, @* see @code{BZ2_bzCompressInit}.

To guarantee that the compressed data will fit in its buffer, allocate
an output buffer of size 1% larger than the uncompressed data, plus
six hundred extra bytes.

@code{BZ2_bzBuffToBuffDecompress} will not write data at or
beyond @code{dest[*destLen]}, even in case of buffer overflow.

Possible return values:
@display
      @code{BZ_CONFIG_ERROR}
         if the library has been mis-compiled
      @code{BZ_PARAM_ERROR} 
         if @code{dest} is @code{NULL} or @code{destLen} is @code{NULL}
         or @code{blockSize100k < 1} or @code{blockSize100k > 9}
         or @code{verbosity < 0} or @code{verbosity > 4} 
         or @code{workFactor < 0} or @code{workFactor > 250}
      @code{BZ_MEM_ERROR}
         if insufficient memory is available 
      @code{BZ_OUTBUFF_FULL}
         if the size of the compressed data exceeds @code{*destLen}
      @code{BZ_OK} 
         otherwise
@end display



@subsection @code{BZ2_bzBuffToBuffDecompress}
@example
   int BZ2_bzBuffToBuffDecompress ( char*         dest,
                                    unsigned int* destLen,
                                    char*         source,
                                    unsigned int  sourceLen,
                                    int           small,
                                    int           verbosity );
@end example
Attempts to decompress the data in @code{source[0 .. sourceLen-1]}
into the destination buffer, @code{dest[0 .. *destLen-1]}.
If the destination buffer is big enough, @code{*destLen} is
set to the size of the uncompressed data, and @code{BZ_OK} is
returned.  If the compressed data won't fit, @code{*destLen}
is unchanged, and @code{BZ_OUTBUFF_FULL} is returned.

@code{source} is assumed to hold a complete @code{bzip2} format
data stream.  @* @code{BZ2_bzBuffToBuffDecompress} tries to decompress
the entirety of the stream into the output buffer.

For the meaning of parameters @code{small} and @code{verbosity},
see @code{BZ2_bzDecompressInit}.

Because the compression ratio of the compressed data cannot be known in
advance, there is no easy way to guarantee that the output buffer will
be big enough.  You may of course make arrangements in your code to
record the size of the uncompressed data, but such a mechanism is beyond
the scope of this library.

@code{BZ2_bzBuffToBuffDecompress} will not write data at or
beyond @code{dest[*destLen]}, even in case of buffer overflow.

Possible return values:
@display
      @code{BZ_CONFIG_ERROR}
         if the library has been mis-compiled
      @code{BZ_PARAM_ERROR} 
         if @code{dest} is @code{NULL} or @code{destLen} is @code{NULL}
         or @code{small != 0 && small != 1}
         or @code{verbosity < 0} or @code{verbosity > 4} 
      @code{BZ_MEM_ERROR}
         if insufficient memory is available 
      @code{BZ_OUTBUFF_FULL}
         if the size of the compressed data exceeds @code{*destLen}
      @code{BZ_DATA_ERROR}
         if a data integrity error was detected in the compressed data
      @code{BZ_DATA_ERROR_MAGIC}
         if the compressed data doesn't begin with the right magic bytes
      @code{BZ_UNEXPECTED_EOF}
         if the compressed data ends unexpectedly
      @code{BZ_OK} 
         otherwise
@end display



@section @code{zlib} compatibility functions
Yoshioka Tsuneo has contributed some functions to
give better @code{zlib} compatibility.  These functions are
@code{BZ2_bzopen}, @code{BZ2_bzread}, @code{BZ2_bzwrite}, @code{BZ2_bzflush},
@code{BZ2_bzclose},
@code{BZ2_bzerror} and @code{BZ2_bzlibVersion}.
These functions are not (yet) officially part of
the library.  If they break, you get to keep all the pieces.
Nevertheless, I think they work ok.
@example
typedef void BZFILE;

const char * BZ2_bzlibVersion ( void );
@end example
Returns a string indicating the library version.
@example
BZFILE * BZ2_bzopen  ( const char *path, const char *mode );
BZFILE * BZ2_bzdopen ( int        fd,    const char *mode );
@end example
Opens a @code{.bz2} file for reading or writing, using either its name
or a pre-existing file descriptor. 
Analogous to @code{fopen} and @code{fdopen}.
@example         
int BZ2_bzread  ( BZFILE* b, void* buf, int len );
int BZ2_bzwrite ( BZFILE* b, void* buf, int len );
@end example
Reads/writes data from/to a previously opened @code{BZFILE}.
Analogous to @code{fread} and @code{fwrite}.
@example
int  BZ2_bzflush ( BZFILE* b );
void BZ2_bzclose ( BZFILE* b );
@end example
Flushes/closes a @code{BZFILE}.  @code{BZ2_bzflush} doesn't actually do
anything.  Analogous to @code{fflush} and @code{fclose}.

@example 
const char * BZ2_bzerror ( BZFILE *b, int *errnum )
@end example
Returns a string describing the more recent error status of
@code{b}, and also sets @code{*errnum} to its numerical value.


@section Using the library in a @code{stdio}-free environment

@subsection Getting rid of @code{stdio}

In a deeply embedded application, you might want to use just
the memory-to-memory functions.  You can do this conveniently
by compiling the library with preprocessor symbol @code{BZ_NO_STDIO}
defined.  Doing this gives you a library containing only the following
eight functions:

@code{BZ2_bzCompressInit}, @code{BZ2_bzCompress}, @code{BZ2_bzCompressEnd} @*
@code{BZ2_bzDecompressInit}, @code{BZ2_bzDecompress}, @code{BZ2_bzDecompressEnd} @*
@code{BZ2_bzBuffToBuffCompress}, @code{BZ2_bzBuffToBuffDecompress}

When compiled like this, all functions will ignore @code{verbosity}
settings.

@subsection Critical error handling
@code{libbzip2} contains a number of internal assertion checks which
should, needless to say, never be activated.  Nevertheless, if an
assertion should fail, behaviour depends on whether or not the library
was compiled with @code{BZ_NO_STDIO} set.

For a normal compile, an assertion failure yields the message
@example
   bzip2/libbzip2: internal error number N.
   This is a bug in bzip2/libbzip2, 1.0.2, 30-Dec-2001.
   Please report it to me at: jseward@@acm.org.  If this happened
   when you were using some program which uses libbzip2 as a
   component, you should also report this bug to the author(s)
   of that program.  Please make an effort to report this bug;
   timely and accurate bug reports eventually lead to higher
   quality software.  Thanks.  Julian Seward, 30 December 2001.
@end example
where @code{N} is some error code number.  If @code{N == 1007}, it also
prints some extra text advising the reader that unreliable memory is
often associated with internal error 1007.  (This is a
frequently-observed-phenomenon with versions 1.0.0/1.0.1).

@code{exit(3)} is then called.

For a @code{stdio}-free library, assertion failures result
in a call to a function declared as:
@example
   extern void bz_internal_error ( int errcode );
@end example
The relevant code is passed as a parameter.  You should supply
such a function.

In either case, once an assertion failure has occurred, any 
@code{bz_stream} records involved can be regarded as invalid.
You should not attempt to resume normal operation with them.

You may, of course, change critical error handling to suit
your needs.  As I said above, critical errors indicate bugs
in the library and should not occur.  All "normal" error
situations are indicated via error return codes from functions,
and can be recovered from.


@section Making a Windows DLL
Everything related to Windows has been contributed by Yoshioka Tsuneo
@* (@code{QWF00133@@niftyserve.or.jp} /
@code{tsuneo-y@@is.aist-nara.ac.jp}), so you should send your queries to
him (but perhaps Cc: me, @code{jseward@@acm.org}).

My vague understanding of what to do is: using Visual C++ 5.0,
open the project file @code{libbz2.dsp}, and build.  That's all.

If you can't
open the project file for some reason, make a new one, naming these files:
@code{blocksort.c}, @code{bzlib.c}, @code{compress.c}, 
@code{crctable.c}, @code{decompress.c}, @code{huffman.c}, @*
@code{randtable.c} and @code{libbz2.def}.  You will also need
to name the header files @code{bzlib.h} and @code{bzlib_private.h}.

If you don't use VC++, you may need to define the proprocessor symbol
@code{_WIN32}. 

Finally, @code{dlltest.c} is a sample program using the DLL.  It has a
project file, @code{dlltest.dsp}.

If you just want a makefile for Visual C, have a look at
@code{makefile.msc}.

Be aware that if you compile @code{bzip2} itself on Win32, you must set
@code{BZ_UNIX} to 0 and @code{BZ_LCCWIN32} to 1, in the file
@code{bzip2.c}, before compiling.  Otherwise the resulting binary won't
work correctly.

I haven't tried any of this stuff myself, but it all looks plausible.



@chapter Miscellanea

These are just some random thoughts of mine.  Your mileage may
vary.

@section Limitations of the compressed file format
@code{bzip2-1.0}, @code{0.9.5} and @code{0.9.0}
use exactly the same file format as the previous
version, @code{bzip2-0.1}.  This decision was made in the interests of
stability.  Creating yet another incompatible compressed file format
would create further confusion and disruption for users.

Nevertheless, this is not a painless decision.  Development
work since the release of @code{bzip2-0.1} in August 1997
has shown complexities in the file format which slow down
decompression and, in retrospect, are unnecessary.  These are:
@itemize @bullet
@item The run-length encoder, which is the first of the 
      compression transformations, is entirely irrelevant.
      The original purpose was to protect the sorting algorithm
      from the very worst case input: a string of repeated
      symbols.  But algorithm steps Q6a and Q6b in the original
      Burrows-Wheeler technical report (SRC-124) show how
      repeats can be handled without difficulty in block
      sorting.
@item The randomisation mechanism doesn't really need to be
      there.  Udi Manber and Gene Myers published a suffix
      array construction algorithm a few years back, which
      can be employed to sort any block, no matter how 
      repetitive, in O(N log N) time.  Subsequent work by
      Kunihiko Sadakane has produced a derivative O(N (log N)^2) 
      algorithm which usually outperforms the Manber-Myers
      algorithm.

      I could have changed to Sadakane's algorithm, but I find
      it to be slower than @code{bzip2}'s existing algorithm for
      most inputs, and the randomisation mechanism protects
      adequately against bad cases.  I didn't think it was
      a good tradeoff to make.  Partly this is due to the fact
      that I was not flooded with email complaints about
      @code{bzip2-0.1}'s performance on repetitive data, so
      perhaps it isn't a problem for real inputs.

      Probably the best long-term solution,
      and the one I have incorporated into 0.9.5 and above,
      is to use the existing sorting
      algorithm initially, and fall back to a O(N (log N)^2)
      algorithm if the standard algorithm gets into difficulties.
@item The compressed file format was never designed to be
      handled by a library, and I have had to jump though
      some hoops to produce an efficient implementation of
      decompression.  It's a bit hairy.  Try passing
      @code{decompress.c} through the C preprocessor 
      and you'll see what I mean.  Much of this complexity
      could have been avoided if the compressed size of
      each block of data was recorded in the data stream.
@item An Adler-32 checksum, rather than a CRC32 checksum,
      would be faster to compute.
@end itemize
It would be fair to say that the @code{bzip2} format was frozen
before I properly and fully understood the performance
consequences of doing so.

Improvements which I was able to incorporate into
0.9.0, despite using the same file format, are:
@itemize @bullet
@item Single array implementation of the inverse BWT.  This
      significantly speeds up decompression, presumably
      because it reduces the number of cache misses.
@item Faster inverse MTF transform for large MTF values.  The
      new implementation is based on the notion of sliding blocks
      of values.
@item @code{bzip2-0.9.0} now reads and writes files with @code{fread}
      and @code{fwrite}; version 0.1 used @code{putc} and @code{getc}.
      Duh!  Well, you live and learn.

@end itemize
Further ahead, it would be nice 
to be able to do random access into files.  This will 
require some careful design of compressed file formats.



@section Portability issues
After some consideration, I have decided not to use
GNU @code{autoconf} to configure 0.9.5 or 1.0.

@code{autoconf}, admirable and wonderful though it is, 
mainly assists with portability problems between Unix-like
platforms.  But @code{bzip2} doesn't have much in the way
of portability problems on Unix; most of the difficulties appear
when porting to the Mac, or to Microsoft's operating systems.
@code{autoconf} doesn't help in those cases, and brings in a 
whole load of new complexity.

Most people should be able to compile the library and program
under Unix straight out-of-the-box, so to speak, especially 
if you have a version of GNU C available.

There are a couple of @code{__inline__} directives in the code.  GNU C
(@code{gcc}) should be able to handle them.  If you're not using
GNU C, your C compiler shouldn't see them at all.
If your compiler does, for some reason, see them and doesn't
like them, just @code{#define} @code{__inline__} to be @code{/* */}.  One
easy way to do this is to compile with the flag @code{-D__inline__=}, 
which should be understood by most Unix compilers.

If you still have difficulties, try compiling with the macro
@code{BZ_STRICT_ANSI} defined.  This should enable you to build the
library in a strictly ANSI compliant environment.  Building the program
itself like this is dangerous and not supported, since you remove
@code{bzip2}'s checks against compressing directories, symbolic links,
devices, and other not-really-a-file entities.  This could cause
filesystem corruption!

One other thing: if you create a @code{bzip2} binary for public
distribution, please try and link it statically (@code{gcc -s}).  This
avoids all sorts of library-version issues that others may encounter
later on.

If you build @code{bzip2} on Win32, you must set @code{BZ_UNIX} to 0 and
@code{BZ_LCCWIN32} to 1, in the file @code{bzip2.c}, before compiling.
Otherwise the resulting binary won't work correctly.



@section Reporting bugs
I tried pretty hard to make sure @code{bzip2} is
bug free, both by design and by testing.  Hopefully
you'll never need to read this section for real.

Nevertheless, if @code{bzip2} dies with a segmentation
fault, a bus error or an internal assertion failure, it
will ask you to email me a bug report.  Experience with
version 0.1 shows that almost all these problems can
be traced to either compiler bugs or hardware problems.
@itemize @bullet
@item
Recompile the program with no optimisation, and see if it
works.  And/or try a different compiler.
I heard all sorts of stories about various flavours
of GNU C (and other compilers) generating bad code for
@code{bzip2}, and I've run across two such examples myself.

2.7.X versions of GNU C are known to generate bad code from
time to time, at high optimisation levels.  
If you get problems, try using the flags
@code{-O2} @code{-fomit-frame-pointer} @code{-fno-strength-reduce}.
You should specifically @emph{not} use @code{-funroll-loops}.

You may notice that the Makefile runs six tests as part of
the build process.  If the program passes all of these, it's
a pretty good (but not 100%) indication that the compiler has
done its job correctly.
@item
If @code{bzip2} crashes randomly, and the crashes are not
repeatable, you may have a flaky memory subsystem.  @code{bzip2}
really hammers your memory hierarchy, and if it's a bit marginal,
you may get these problems.  Ditto if your disk or I/O subsystem
is slowly failing.  Yup, this really does happen.

Try using a different machine of the same type, and see if
you can repeat the problem.
@item This isn't really a bug, but ... If @code{bzip2} tells
you your file is corrupted on decompression, and you
obtained the file via FTP, there is a possibility that you
forgot to tell FTP to do a binary mode transfer.  That absolutely
will cause the file to be non-decompressible.  You'll have to transfer
it again.
@end itemize

If you've incorporated @code{libbzip2} into your own program
and are getting problems, please, please, please, check that the 
parameters you are passing in calls to the library, are
correct, and in accordance with what the documentation says
is allowable.  I have tried to make the library robust against
such problems, but I'm sure I haven't succeeded.

Finally, if the above comments don't help, you'll have to send
me a bug report.  Now, it's just amazing how many people will 
send me a bug report saying something like
@display
   bzip2 crashed with segmentation fault on my machine
@end display
and absolutely nothing else.  Needless to say, a such a report
is @emph{totally, utterly, completely and comprehensively 100% useless; 
a waste of your time, my time, and net bandwidth}.
With no details at all, there's no way I can possibly begin
to figure out what the problem is.

The rules of the game are: facts, facts, facts.  Don't omit
them because "oh, they won't be relevant".  At the bare 
minimum:
@display
   Machine type.  Operating system version.  
   Exact version of @code{bzip2} (do @code{bzip2 -V}).  
   Exact version of the compiler used.  
   Flags passed to the compiler.
@end display
However, the most important single thing that will help me is
the file that you were trying to compress or decompress at the
time the problem happened.  Without that, my ability to do anything
more than speculate about the cause, is limited.

Please remember that I connect to the Internet with a modem, so
you should contact me before mailing me huge files.


@section Did you get the right package?

@code{bzip2} is a resource hog.  It soaks up large amounts of CPU cycles
and memory.  Also, it gives very large latencies.  In the worst case, you
can feed many megabytes of uncompressed data into the library before
getting any compressed output, so this probably rules out applications
requiring interactive behaviour.

These aren't faults of my implementation, I hope, but more
an intrinsic property of the Burrows-Wheeler transform (unfortunately).  
Maybe this isn't what you want.

If you want a compressor and/or library which is faster, uses less
memory but gets pretty good compression, and has minimal latency,
consider Jean-loup
Gailly's and Mark Adler's work, @code{zlib-1.1.3} and
@code{gzip-1.2.4}.  Look for them at

@code{http://www.zlib.org} and
@code{http://www.gzip.org} respectively.

For something faster and lighter still, you might try Markus F X J
Oberhumer's @code{LZO} real-time compression/decompression library, at
@* @code{http://wildsau.idv.uni-linz.ac.at/mfx/lzo.html}.

If you want to use the @code{bzip2} algorithms to compress small blocks
of data, 64k bytes or smaller, for example on an on-the-fly disk
compressor, you'd be well advised not to use this library.  Instead,
I've made a special library tuned for that kind of use.  It's part of
@code{e2compr-0.40}, an on-the-fly disk compressor for the Linux
@code{ext2} filesystem.  Look at
@code{http://www.netspace.net.au/~reiter/e2compr}.



@section Testing

A record of the tests I've done.

First, some data sets:
@itemize @bullet
@item B: a directory containing 6001 files, one for every length in the
      range 0 to 6000 bytes.  The files contain random lowercase
      letters.  18.7 megabytes.
@item H: my home directory tree.  Documents, source code, mail files,
      compressed data.  H contains B, and also a directory of 
      files designed as boundary cases for the sorting; mostly very
      repetitive, nasty files.  565 megabytes.
@item A: directory tree holding various applications built from source:
      @code{egcs}, @code{gcc-2.8.1}, KDE, GTK, Octave, etc.
      2200 megabytes.
@end itemize
The tests conducted are as follows.  Each test means compressing 
(a copy of) each file in the data set, decompressing it and
comparing it against the original.

First, a bunch of tests with block sizes and internal buffer
sizes set very small, 
to detect any problems with the
blocking and buffering mechanisms.  
This required modifying the source code so as to try to 
break it.
@enumerate
@item Data set H, with
      buffer size of 1 byte, and block size of 23 bytes.
@item Data set B, buffer sizes 1 byte, block size 1 byte.
@item As (2) but small-mode decompression.
@item As (2) with block size 2 bytes.
@item As (2) with block size 3 bytes.
@item As (2) with block size 4 bytes.
@item As (2) with block size 5 bytes.
@item As (2) with block size 6 bytes and small-mode decompression.
@item H with buffer size of 1 byte, but normal block
      size (up to 900000 bytes).
@end enumerate
Then some tests with unmodified source code.
@enumerate
@item H, all settings normal.
@item As (1), with small-mode decompress.
@item H, compress with flag @code{-1}.
@item H, compress with flag @code{-s}, decompress with flag @code{-s}.
@item Forwards compatibility: H, @code{bzip2-0.1pl2} compressing,
      @code{bzip2-0.9.5} decompressing, all settings normal.
@item Backwards compatibility:  H, @code{bzip2-0.9.5} compressing,
      @code{bzip2-0.1pl2} decompressing, all settings normal.
@item Bigger tests: A, all settings normal.
@item As (7), using the fallback (Sadakane-like) sorting algorithm.
@item As (8), compress with flag @code{-1}, decompress with flag
      @code{-s}.
@item H, using the fallback sorting algorithm.
@item Forwards compatibility: A, @code{bzip2-0.1pl2} compressing,
      @code{bzip2-0.9.5} decompressing, all settings normal.
@item Backwards compatibility:  A, @code{bzip2-0.9.5} compressing,
      @code{bzip2-0.1pl2} decompressing, all settings normal.
@item Misc test: about 400 megabytes of @code{.tar} files with
      @code{bzip2} compiled with Checker (a memory access error
       detector, like Purify).
@item Misc tests to make sure it builds and runs ok on non-Linux/x86
      platforms.
@end enumerate
These tests were conducted on a 225 MHz IDT WinChip machine, running
Linux 2.0.36.  They represent nearly a week of continuous computation.
All tests completed successfully.


@section Further reading
@code{bzip2} is not research work, in the sense that it doesn't present
any new ideas.  Rather, it's an engineering exercise based on existing
ideas.

Four documents describe essentially all the ideas behind @code{bzip2}:
@example
Michael Burrows and D. J. Wheeler:
  "A block-sorting lossless data compression algorithm"
   10th May 1994. 
   Digital SRC Research Report 124.
   ftp://ftp.digital.com/pub/DEC/SRC/research-reports/SRC-124.ps.gz
   If you have trouble finding it, try searching at the
   New Zealand Digital Library, http://www.nzdl.org.

Daniel S. Hirschberg and Debra A. LeLewer
  "Efficient Decoding of Prefix Codes"
   Communications of the ACM, April 1990, Vol 33, Number 4.
   You might be able to get an electronic copy of this
      from the ACM Digital Library.

David J. Wheeler
   Program bred3.c and accompanying document bred3.ps.
   This contains the idea behind the multi-table Huffman
   coding scheme.
   ftp://ftp.cl.cam.ac.uk/users/djw3/

Jon L. Bentley and Robert Sedgewick
  "Fast Algorithms for Sorting and Searching Strings"
   Available from Sedgewick's web page,
   www.cs.princeton.edu/~rs
@end example
The following paper gives valuable additional insights into the
algorithm, but is not immediately the basis of any code
used in bzip2.
@example
Peter Fenwick:
   Block Sorting Text Compression
   Proceedings of the 19th Australasian Computer Science Conference,
     Melbourne, Australia.  Jan 31 - Feb 2, 1996.
   ftp://ftp.cs.auckland.ac.nz/pub/peter-f/ACSC96paper.ps
@end example
Kunihiko Sadakane's sorting algorithm, mentioned above,
is available from:
@example
http://naomi.is.s.u-tokyo.ac.jp/~sada/papers/Sada98b.ps.gz
@end example
The Manber-Myers suffix array construction
algorithm is described in a paper
available from:
@example
http://www.cs.arizona.edu/people/gene/PAPERS/suffix.ps
@end example
Finally, the following paper documents some recent investigations
I made into the performance of sorting algorithms:
@example
Julian Seward:
   On the Performance of BWT Sorting Algorithms
   Proceedings of the IEEE Data Compression Conference 2000
     Snowbird, Utah.  28-30 March 2000.
@end example


@contents

@bye