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. 69typedef uint16_t probability; 70 71#endif 72