2011-12-04 15:14:43 +00:00
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File_Extractor library internals
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This describes the implementation and design.
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Contents
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--------
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* Framework
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* Archive abstraction
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* 7-ZIP
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* ZIP
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* RAR
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* GZIP
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* Binary
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Framework
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---------
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This library is essentially:
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* Several archive readers
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* Wrappers to provide common interface
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* File type detection
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The File_Extractor base class implements the common aspects of the
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interface and filters out invalid calls. It also provides default
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implementations for some operations. Each derived *_Extractor class then
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wraps the particular library.
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Then fex.h provides a stable, C-compatible wrapper over File_Extractor,
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and does file type identification.
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Archive abstraction
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-------------------
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An extractor is abstracted to these fundamental operations:
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* Open (either from path or custom reader)
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* Iterate over each file
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* Extract file's data (either gradually, or all at once into memory)
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* Seek to particular file in archive
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* Close
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The extractor can choose whether to open a file path immediately, or
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defer until the first stat() call. When extracting data, it can
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implement one or both of normal read and "give me a pointer to the data
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already in memory". If it only implements one, File_Extractor will
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implement the other in terms of it.
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7-ZIP
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-----
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A fairly simple wrapper over a slightly-modified LZMA library.
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Unfortunately you probably won't be able to drop a later version of the
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LZMA library sources in and recompile, as the interface and file
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arrangement tends to change with every release. The main change I've
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made is to declare functions extern "C" in headers, so C++ code can call
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them.
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ZIP
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---
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A complete implementation I wrote. Has nifty optimization that makes all
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disk reads begin and end on a multiple of 4K. Also minimizes number of
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accesses when initially reading catalog, and extracting file data.
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Correctly handles empty zip archives, unlike the popular unzip library
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which reports it as a bad archive.
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RAR
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---
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Wrapper over heavily-modified UnRAR extraction library based on the
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official UnRAR sources. The library is available separately with
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documentation and demos, in case you want to use it without the
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File_Extractor front-end (see unrar/changes.txt for more).
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GZIP
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----
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Uses zlib's built-in parsing. Also works with non-gzipped files, based
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on absence of two-byte Gzip header. Defers opening of file until
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fex_stat(), speeding mere scanning of filenames. Gzip_Reader and
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Zlib_Reader further divide the implementation into more manageable and
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reusable components.
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Binary
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------
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Just a minimal wrapper over C-style FILE.
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--
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Shay Green <gblargg@gmail.com>
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