range_common.h revision 213700
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. 71typedef uint16_t probability; 72 73#endif 74