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