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73 lines
2.3 KiB
73 lines
2.3 KiB
/////////////////////////////////////////////////////////////////////////////// |
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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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#ifdef HAVE_CONFIG_H |
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# include "common.h" |
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#endif |
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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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