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72 lines
2.3 KiB
C
72 lines
2.3 KiB
C
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///////////////////////////////////////////////////////////////////////////////
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//
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/// \file range_common.h
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/// \brief Common things for range encoder and decoder
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///
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// Authors: Igor Pavlov
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// Lasse Collin
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//
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// This file has been put into the public domain.
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// You can do whatever you want with this file.
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//
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///////////////////////////////////////////////////////////////////////////////
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#ifndef LZMA_RANGE_COMMON_H
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#define LZMA_RANGE_COMMON_H
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#include "common.h"
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///////////////
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// Constants //
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///////////////
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#define RC_SHIFT_BITS 8
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#define RC_TOP_BITS 24
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#define RC_TOP_VALUE (UINT32_C(1) << RC_TOP_BITS)
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#define RC_BIT_MODEL_TOTAL_BITS 11
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#define RC_BIT_MODEL_TOTAL (UINT32_C(1) << RC_BIT_MODEL_TOTAL_BITS)
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#define RC_MOVE_BITS 5
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////////////
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// Macros //
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////////////
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// Resets the probability so that both 0 and 1 have probability of 50 %
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#define bit_reset(prob) \
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prob = RC_BIT_MODEL_TOTAL >> 1
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// This does the same for a complete bit tree.
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// (A tree represented as an array.)
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#define bittree_reset(probs, bit_levels) \
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for (uint32_t bt_i = 0; bt_i < (1 << (bit_levels)); ++bt_i) \
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bit_reset((probs)[bt_i])
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//////////////////////
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// Type definitions //
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//////////////////////
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/// \brief Type of probabilities used with range coder
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///
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/// This needs to be at least 12-bit integer, so uint16_t is a logical choice.
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/// However, on some architecture and compiler combinations, a bigger type
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/// may give better speed, because the probability variables are accessed
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/// a lot. On the other hand, bigger probability type increases cache
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/// footprint, since there are 2 to 14 thousand probability variables in
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/// LZMA (assuming the limit of lc + lp <= 4; with lc + lp <= 12 there
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/// would be about 1.5 million variables).
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///
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/// With malicious files, the initialization speed of the LZMA decoder can
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/// become important. In that case, smaller probability variables mean that
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/// there is less bytes to write to RAM, which makes initialization faster.
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/// With big probability type, the initialization can become so slow that it
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/// can be a problem e.g. for email servers doing virus scanning.
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///
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/// I will be sticking to uint16_t unless some specific architectures
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/// are *much* faster (20-50 %) with uint32_t.
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typedef uint16_t probability;
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#endif
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