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