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XZ Utils To-Do List
Known bugs
The test suite is too incomplete.
If the memory usage limit is less than about 13 MiB, xz is unable to
automatically scale down the compression settings enough even though
it would be possible by switching from BT2/BT3/BT4 match finder to
XZ Utils compress some files significantly worse than LZMA Utils.
This is due to faster compression presets used by XZ Utils, and
can often be worked around by using "xz --extreme". With some files
--extreme isn't enough though: it's most likely with files that
compress extremely well, so going from compression ratio of 0.003
to 0.004 means big relative increase in the compressed file size.
xz doesn't quote unprintable characters when it displays file names
given on the command line.
tuklib_exit() doesn't block signals => EINTR is possible.
SIGTSTP is not handled. If xz is stopped, the estimated remaining
time and calculated (de)compression speed won't make sense in the
progress indicator (xz --verbose).
If liblzma has created threads and fork() gets called, liblzma
code will break in the child process unless it calls exec() and
doesn't touch liblzma.
Missing features
Add support for storing metadata in .xz files. A preliminary
idea is to create a new Stream type for metadata. When both
metadata and data are wanted in the same .xz file, two or more
Streams would be concatenated.
The state stored in lzma_stream should be cloneable, which would
be mostly useful when using a preset dictionary in LZMA2, but
it may have other uses too. Compare to deflateCopy() in zlib.
Support LZMA_FINISH in raw decoder to indicate end of LZMA1 and
other streams that don't have an end of payload marker.
Adjust dictionary size when the input file size is known.
Maybe do this only if an option is given.
xz doesn't support copying extended attributes, access control
lists etc. from source to target file.
Multithreaded compression:
- Reduce memory usage of the current method.
- Implement threaded match finders.
- Implement pigz-style threading in LZMA2.
Multithreaded decompression
Buffer-to-buffer coding could use less RAM (especially when
decompressing LZMA1 or LZMA2).
I/O library is not implemented (similar to gzopen() in zlib).
It will be a separate library that supports uncompressed, .gz,
.bz2, .lzma, and .xz files.
Support changing lzma_options_lzma.mode with lzma_filters_update().
Support LZMA_FULL_FLUSH for lzma_stream_decoder() to stop at
Block and Stream boundaries.
lzma_strerror() to convert lzma_ret to human readable form?
This is tricky, because the same error codes are used with
slightly different meanings, and this cannot be fixed anymore.
Make it possible to adjust LZMA2 options in the middle of a Block
so that the encoding speed vs. compression ratio can be optimized
when the compressed data is streamed over network.
Improved BCJ filters. The current filters are small but they aren't
so great when compressing binary packages that contain various file
types. Specifically, they make things worse if there are static
libraries or Linux kernel modules. The filtering could also be
more effective (without getting overly complex), for example,
streamable variant BCJ2 from 7-Zip could be implemented.
Filter that autodetects specific data types in the input stream
and applies appropriate filters for the corrects parts of the input.
Perhaps combine this with the BCJ filter improvement point above.
Long-range LZ77 method as a separate filter or as a new LZMA2
match finder.
More tutorial programs are needed for liblzma.
Document the LZMA1 and LZMA2 algorithms.
Try to get the media type for .xz registered at IANA.