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| 1 /////////////////////////////////////////////////////////////////////////////// |
| 2 // |
| 3 /// \file range_common.h |
| 4 /// \brief Common things for range encoder and decoder |
| 5 /// |
| 6 // Authors: Igor Pavlov |
| 7 // Lasse Collin |
| 8 // |
| 9 // This file has been put into the public domain. |
| 10 // You can do whatever you want with this file. |
| 11 // |
| 12 /////////////////////////////////////////////////////////////////////////////// |
| 13 |
| 14 #ifndef LZMA_RANGE_COMMON_H |
| 15 #define LZMA_RANGE_COMMON_H |
| 16 |
| 17 #ifdef HAVE_CONFIG_H |
| 18 # include "common.h" |
| 19 #endif |
| 20 |
| 21 |
| 22 /////////////// |
| 23 // Constants // |
| 24 /////////////// |
| 25 |
| 26 #define RC_SHIFT_BITS 8 |
| 27 #define RC_TOP_BITS 24 |
| 28 #define RC_TOP_VALUE (UINT32_C(1) << RC_TOP_BITS) |
| 29 #define RC_BIT_MODEL_TOTAL_BITS 11 |
| 30 #define RC_BIT_MODEL_TOTAL (UINT32_C(1) << RC_BIT_MODEL_TOTAL_BITS) |
| 31 #define RC_MOVE_BITS 5 |
| 32 |
| 33 |
| 34 //////////// |
| 35 // Macros // |
| 36 //////////// |
| 37 |
| 38 // Resets the probability so that both 0 and 1 have probability of 50 % |
| 39 #define bit_reset(prob) \ |
| 40 prob = RC_BIT_MODEL_TOTAL >> 1 |
| 41 |
| 42 // This does the same for a complete bit tree. |
| 43 // (A tree represented as an array.) |
| 44 #define bittree_reset(probs, bit_levels) \ |
| 45 for (uint32_t bt_i = 0; bt_i < (1 << (bit_levels)); ++bt_i) \ |
| 46 bit_reset((probs)[bt_i]) |
| 47 |
| 48 |
| 49 ////////////////////// |
| 50 // Type definitions // |
| 51 ////////////////////// |
| 52 |
| 53 /// \brief Type of probabilities used with range coder |
| 54 /// |
| 55 /// This needs to be at least 12-bit integer, so uint16_t is a logical choice. |
| 56 /// However, on some architecture and compiler combinations, a bigger type |
| 57 /// may give better speed, because the probability variables are accessed |
| 58 /// a lot. On the other hand, bigger probability type increases cache |
| 59 /// footprint, since there are 2 to 14 thousand probability variables in |
| 60 /// LZMA (assuming the limit of lc + lp <= 4; with lc + lp <= 12 there |
| 61 /// would be about 1.5 million variables). |
| 62 /// |
| 63 /// With malicious files, the initialization speed of the LZMA decoder can |
| 64 /// become important. In that case, smaller probability variables mean that |
| 65 /// there is less bytes to write to RAM, which makes initialization faster. |
| 66 /// With big probability type, the initialization can become so slow that it |
| 67 /// can be a problem e.g. for email servers doing virus scanning. |
| 68 /// |
| 69 /// I will be sticking to uint16_t unless some specific architectures |
| 70 /// are *much* faster (20-50 %) with uint32_t. |
| 71 typedef uint16_t probability; |
| 72 |
| 73 #endif |
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