2018-11-30 15:36:49 -05:00
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/*
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* Copyright (c) uzlib authors
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*
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* This software is provided 'as-is', without any express
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* or implied warranty. In no event will the authors be
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* held liable for any damages arising from the use of
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* this software.
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*
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* Permission is granted to anyone to use this software
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* for any purpose, including commercial applications,
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* and to alter it and redistribute it freely, subject to
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* the following restrictions:
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*
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* 1. The origin of this software must not be
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* misrepresented; you must not claim that you
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* wrote the original software. If you use this
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* software in a product, an acknowledgment in
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* the product documentation would be appreciated
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* but is not required.
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*
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* 2. Altered source versions must be plainly marked
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* as such, and must not be misrepresented as
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* being the original software.
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*
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* 3. This notice may not be removed or altered from
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* any source distribution.
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*/
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/* This files contains type declaration and prototypes for defl_static.c.
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They may be altered/distinct from the originals used in PuTTY source
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code. */
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struct Outbuf {
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lib/uzlib: Add memory-efficient, streaming LZ77 compression support.
The compression algorithm implemented in this commit uses much less memory
compared to the standard way of implementing it using a hash table and
large look-back window. In particular the algorithm here doesn't allocate
hash table to store indices into the history of the previously seen text.
Instead it simply does a brute-force-search of the history text to find a
match for the compressor. This is slower (linear search vs hash table
lookup) but with a small enough history (eg 512 bytes) it's not that slow.
And a small history does not impact the compression too much.
To give some more concrete numbers comparing memory use between the
approaches:
- Standard approach: inplace compression, all text to compress must be in
RAM (or at least memory addressable), and then an additional 16k bytes
RAM of hash table pointers, pointing into the text
- The approach in this commit: streaming compression, only a limited amount
of previous text must be in RAM (user selectable, defaults to 512 bytes).
To compress, say, 1k of data, the standard approach requires all that data
to be in RAM, plus an additional 16k of RAM for the hash table pointers.
With this commit, you only need the 1k of data in RAM. Or if it's
streaming from a file (or elsewhere), you could get away with only 256
bytes of RAM for the sliding history and still get very decent compression.
In summary: because compression takes such a large amount of RAM (in the
standard algorithm) and it's not really suitable for microcontrollers, the
approach taken in this commit is to minimise RAM usage as much as possible,
and still have acceptable performance (speed and compression ratio).
Signed-off-by: Damien George <damien@micropython.org>
2023-01-17 23:46:23 -05:00
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void *dest_write_data;
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void (*dest_write_cb)(struct Outbuf *outbuf, uint8_t byte);
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2018-11-30 15:36:49 -05:00
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unsigned long outbits;
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int noutbits;
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int comp_disabled;
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};
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void zlib_start_block(struct Outbuf *ctx);
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void zlib_finish_block(struct Outbuf *ctx);
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void zlib_literal(struct Outbuf *ectx, unsigned char c);
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void zlib_match(struct Outbuf *ectx, int distance, int len);
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