xref: /freebsd/contrib/xz/src/liblzma/rangecoder/range_common.h (revision 5e53a4f90f82c4345f277dd87cc9292f26e04a29)
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.
71 typedef uint16_t probability;
72 
73 #endif
74