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authorParménides GV <parmegv@sdf.org>2015-06-04 19:20:15 +0200
committerParménides GV <parmegv@sdf.org>2015-06-04 19:20:15 +0200
commit27594eeae6f40a402bc3110f06d57975168e74e3 (patch)
treecdabf6571e6f4ff07205fd6921d8095539a1fcdc /app/lzo/doc/LZO.FAQ
parent8dc4f58d96892fbfd83094fb85b1d17656035290 (diff)
ics-openvpn as a submodule! beautiful
ics-openvpn is now officially on GitHub, and they track openssl and openvpn as submodules, so it's easier to update everything. Just a git submodule update --recursive. I've also set up soft links to native modules from ics-openvpn in app, so that we don't copy files in Gradle (which was causing problems with the submodules .git* files, not being copied). That makes the repo cleaner.
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-============================================================================
-LZO Frequently Asked Questions
-============================================================================
-
-
-I hate reading docs - just tell me how to add compression to my program
-=======================================================================
-
-This is for the impatient: take a look at examples/simple.c and
-examples/lzopack.c and see how easy this is.
-
-But you will come back to read the documentation later, won't you ?
-
-
-Can you explain the naming conventions of the algorithms ?
-==========================================================
-
-Let's take a look at LZO1X:
-
- The algorithm name is LZO1X.
- The algorithm category is LZO1.
- Various compression levels are implemented.
-
- LZO1X-999
- !---------- algorithm category
- !--------- algorithm type
- !!!----- compression level (1-9, 99, 999)
-
- LZO1X-1(11)
- !---------- algorithm category
- !--------- algorithm type
- !------- compression level (1-9, 99, 999)
- !!---- memory level (memory requirements for compression)
-
-All compression/memory levels generate the same compressed data format,
-so e.g. the LZO1X decompressor handles all LZO1X-* compression levels
-(for more information about the decompressors see below).
-
-Category LZO1 algorithms: compressed data format is strictly byte aligned
-Category LZO2 algorithms: uses bit-shifting, slower decompression
-
-
-Why are there so many algorithms ?
-==================================
-
-Because of historical reasons - I want to support unlimited
-backward compatibility.
-
-Don't get misled by the size of the library - using one algorithm
-increases the size of your application by only a few KiB.
-
-If you just want to add a little bit of data compression to your
-application you may be looking for miniLZO.
-See minilzo/README.LZO for more information.
-
-
-Which algorithm should I use ?
-==============================
-
-LZO1X seems to be best choice in many cases, so:
-- when going for speed use LZO1X-1
-- when generating pre-compressed data use LZO1X-999
-- if you have little memory available for compression use LZO1X-1(11)
- or LZO1X-1(12)
-
-Of course, your mileage may vary, and you are encouraged to run your
-own experiments. Try LZO1Y and LZO1F next.
-
-
-What's the difference between the decompressors per algorithm ?
-===============================================================
-
-Once again let's use LZO1X for explanation:
-
-- lzo1x_decompress
- The 'standard' decompressor. Pretty fast - use this whenever possible.
-
- This decompressor expects valid compressed data.
- If the compressed data gets corrupted somehow (e.g. transmission
- via an erroneous channel, disk errors, ...) it will probably crash
- your application because absolutely no additional checks are done.
-
-- lzo1x_decompress_safe
- The 'safe' decompressor. Somewhat slower.
-
- This decompressor will catch all compressed data violations and
- return an error code in this case - it will never crash.
-
-- lzo1x_decompress_asm
- Same as lzo1x_decompress - written in assembler.
-
-- lzo1x_decompress_asm_safe
- Same as lzo1x_decompress_safe - written in assembler.
-
-- lzo1x_decompress_asm_fast
- Similar to lzo1x_decompress_asm - but even faster.
-
- For reasons of speed this decompressor can write up to 3 bytes
- past the end of the decompressed (output) block.
- [ technical note: because data is transferred in 32-bit units ]
-
- Use this when you are decompressing from one memory block to
- another memory block - just provide output space for 3 extra bytes.
- You shouldn't use it if e.g. you are directly decompressing to video
- memory (because the extra bytes will be show up on the screen).
-
-- lzo1x_decompress_asm_fast_safe
- This is the safe version of lzo1x_decompress_asm_fast.
-
-
-Notes:
-------
-- When using a safe decompressor you must pass the number of
- bytes available in 'dst' via the parameter 'dst_len'.
-
-- If you want to be sure that your data is not corrupted you must
- use a checksum - just using the safe decompressor is not enough,
- because many data errors will not result in a compressed data violation.
-
-- Assembler versions are only available for the i386 family yet.
- Please see also asm/i386/00README.TXT
-
-- You should test if the assembler versions are actually faster
- than the C version on your machine - some compilers can do a very
- good optimization job and they also can optimize the code
- for a specific processor.
-
-
-What is this optimization thing ?
-=================================
-
-The compressors use a heuristic approach - they sometimes code
-information that doesn't improve compression ratio.
-
-Optimization removes this superfluos information in order to
-increase decompression speed.
-
-Optimization works similar to decompression except that the
-compressed data is modified as well. The length of the compressed
-data block will not change - only the compressed data-bytes will
-get rearranged a little bit.
-
-Don't expect too much, though - my tests have shown that the
-optimization step improves decompression speed by about 1-3%.
-
-
-I need even more decompression speed...
-=======================================
-
-Many RISC processors (like MIPS) can transfer 32-bit words much
-faster than bytes - this can significantly speed up decompression.
-So after verifying that everything works fine you can try if activating
-the LZO_ALIGNED_OK_4 macro improves LZO1X and LZO1Y decompression
-performance. Change the file config.h accordingly and recompile everything.
-
-On an i386 architecture you should evaluate the assembler versions.
-
-
-How can I reduce memory requirements when (de)compressing ?
-===========================================================
-
-If you cleverly arrange your data, you can do an overlapping (in-place)
-decompression which means that you can decompress to the *same*
-block where the compressed data resides. This effectively removes
-the space requirements for holding the compressed data block.
-
-This technique is essential e.g. for usage in an executable packer.
-
-You can also partly overlay the buffers when doing compression.
-
-See examples/overlap.c for a working example.
-
-
-Can you give a cookbook for using pre-compressed data ?
-=======================================================
-
-Let's assume you use LZO1X-999.
-
-1) pre-compression step
- - call lzo_init()
- - call lzo1x_999_compress()
- - call lzo1x_optimize()
- - compute an adler32 checksum of the *compressed* data
- - store the compressed data and the checksum in a file
- - if you are paranoid you should verify decompression now
-
-2) decompression step within your application
- - call lzo_init()
- - load your compressed data and the checksum
- - optionally verify the checksum of the compressed data
- (so that you can use the standard decompressor)
- - decompress
-
-See examples/precomp.c and examples/precomp2.c for a working example.
-
-
-How much can my data expand during compression ?
-================================================
-
-LZO will expand incompressible data by a little amount.
-I still haven't computed the exact values, but I suggest using
-these formulas for a worst-case expansion calculation:
-
- Algorithm LZO1, LZO1A, LZO1B, LZO1C, LZO1F, LZO1X, LZO1Y, LZO1Z:
- ----------------------------------------------------------------
- output_block_size = input_block_size + (input_block_size / 16) + 64 + 3
-
- [This is about 106% for a large block size.]
-
- Algorithm LZO2A:
- ----------------
- output_block_size = input_block_size + (input_block_size / 8) + 128 + 3
-