142 lines
5.7 KiB
Plaintext
142 lines
5.7 KiB
Plaintext
unrar_core source code changes
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------------------------------
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Unrar_core is based on UnRAR (unrarsrc-3.8.5.tar.gz) by Alexander L.
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Roshal. The original sources have been HEAVILY modified, trimmed down,
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and purged of all OS-specific calls for file access and other
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unnecessary operations. Support for encryption, recovery records, and
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segmentation has been REMOVED. See license.txt for licensing. In
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particular, this code cannot be used to re-create the RAR compression
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algorithm, which is proprietary.
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If you obtained this code as a part of my File_Extractor library and
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want to use it on its own, get my unrar_core library, which includes
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examples and documentation.
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The source is as close as possible to the original, to make it simple to
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update when a new version of UnRAR comes out. In many places the
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original names and object nesting are kept, even though it's a bit
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harder to follow. See rar.hpp for the main "glue".
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Website: http://www.slack.net/~ant/
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E-mail : Shay Green <gblargg@gmail.com>
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Contents
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--------
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* Diff-friendly changes
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* Removal of features
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* Error reporting changes
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* Minor tweaks
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* Unrar findings
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Diff-friendly changes
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---------------------
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To make my source code changes more easily visible with a line-based
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file diff, I've tried to make changes by inserting or deleting lines,
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rather than modifying them. So if the original declared a static array
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static int array [4] = { 1, 2, 3, 4 };
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and I want to make it const, I add the const on a line before
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const // added
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static int array [4] = { 1, 2, 3, 4 };
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rather than on the same line
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static const int array [4] = { 1, 2, 3, 4 };
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This way a diff will simply show an added line, making it clear what was
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added. If I simply inserted const on the same line, it wouldn't be as
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clear what all I had changed.
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I've also made use of several macros rather than changing the source
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text. For example, since a class name like Unpack might easily conflict,
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I've renamed it to Rar_Unpack by using #define Unpack Rar_Unpack rather
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than changing the source text. These macros are only defined when
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compiling the library sources; the user-visible unrar.h is very clean.
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Removal of features
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-------------------
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This library is meant for simple access to common archives without
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having to extract them first. Encryption, segmentation, huge files, and
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self-extracting archives aren't common for things that need to be
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accessed in this manner, so I've removed support for them. Also,
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encryption adds complexity to the code that must be maintained.
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Segmentation would require a way to specify the other segments.
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Error reporting changes
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-----------------------
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The original used C++ exceptions to report errors. I've eliminated use
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of these through a combination of error codes and longjmp. This allows
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use of the library from C or some other language which doesn't easily
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support exceptions.
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I tried to make as few changes as possible in the conversion. Due to the
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number of places file reads occur, propagating an error code via return
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statements would have required too many code changes. Instead, I perform
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the read, save the error code, and return 0 bytes read in case of an
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error. I also ensure that the calling code interprets this zero in an
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acceptable way. I then check this saved error code after the operation
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completes, and have it take priority over the error the RAR code
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returned. I do a similar thing for write errors.
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Minor tweaks
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------------
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- Eliminated as many GCC warnings as reasonably possible.
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- Non-class array allocations now use malloc(), allowing the code to be
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linked without the standard C++ library (particularly, operator new).
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Class object allocations use a class-specific allocator that just calls
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malloc(), also avoiding calls to operator new.
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- Made all unchanging static data const. Several pieces of static data
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in the original code weren't marked const where they could be.
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- Initialization of some static tables was done on an as-needed basis,
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creating a problem when extracting from archives in multiple threads.
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This initialization can now be done by the user before any archives are
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opened.
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- Tweaked CopyString, the major bottleneck during compression. I inlined
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it, cached some variables in locals in case the compiler couldn't easily
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see that the memory accesses don't modify them, and made them use
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memcpy() where possible. This improved performance by at least 20% on
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x86. Perhaps it won't work as well on files with lots of smaller string
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matches.
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- Some .cpp files are #included by others. I've added guards to these so
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that you can simply compile all .cpp files and not get any redefinition
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errors.
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- The current solid extraction position is kept track of, allowing the
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user to randomly extract files without regard to proper extraction
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order. The library optimizes solid extraction and only restarts it if
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the user is extracting a file earlier in the archive than the last
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solid-extracted one.
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- Most of the time a solid file's data is already contiguously in the
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internal Unpack::Window, which unrar_extract_mem() takes advantage of.
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This avoids extra allocation in many cases.
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- Allocation of Unpack is delayed until the first extraction, rather
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than being allocated immediately on opening the archive. This allows
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scanning with minimal memory usage.
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Unrar findings
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--------------
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- Apparently the LHD_SOLID flag indicates that file depends on previous
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files, rather than that later files depend on the current file's
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contents. Thus this flag can't be used to intelligently decide which
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files need to be internally extracted when skipping them, making it
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necessary to internally extract every file before the one to be
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extracted, if the archive is solid.
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--
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Shay Green <gblargg@gmail.com>
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