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