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xz_lzma2.h
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1 /*
2  * LZMA2 definitions
3  *
4  * Authors: Lasse Collin <[email protected]>
5  * Igor Pavlov <http://7-zip.org/>
6  *
7  * This file has been put into the public domain.
8  * You can do whatever you want with this file.
9  */
10 
11 #ifndef XZ_LZMA2_H
12 #define XZ_LZMA2_H
13 
14 /* Range coder constants */
15 #define RC_SHIFT_BITS 8
16 #define RC_TOP_BITS 24
17 #define RC_TOP_VALUE (1 << RC_TOP_BITS)
18 #define RC_BIT_MODEL_TOTAL_BITS 11
19 #define RC_BIT_MODEL_TOTAL (1 << RC_BIT_MODEL_TOTAL_BITS)
20 #define RC_MOVE_BITS 5
21 
22 /*
23  * Maximum number of position states. A position state is the lowest pb
24  * number of bits of the current uncompressed offset. In some places there
25  * are different sets of probabilities for different position states.
26  */
27 #define POS_STATES_MAX (1 << 4)
28 
29 /*
30  * This enum is used to track which LZMA symbols have occurred most recently
31  * and in which order. This information is used to predict the next symbol.
32  *
33  * Symbols:
34  * - Literal: One 8-bit byte
35  * - Match: Repeat a chunk of data at some distance
36  * - Long repeat: Multi-byte match at a recently seen distance
37  * - Short repeat: One-byte repeat at a recently seen distance
38  *
39  * The symbol names are in from STATE_oldest_older_previous. REP means
40  * either short or long repeated match, and NONLIT means any non-literal.
41  */
42 enum lzma_state {
55 };
56 
57 /* Total number of states */
58 #define STATES 12
59 
60 /* The lowest 7 states indicate that the previous state was a literal. */
61 #define LIT_STATES 7
62 
63 /* Indicate that the latest symbol was a literal. */
64 static inline void lzma_state_literal(enum lzma_state *state)
65 {
66  if (*state <= STATE_SHORTREP_LIT_LIT)
67  *state = STATE_LIT_LIT;
68  else if (*state <= STATE_LIT_SHORTREP)
69  *state -= 3;
70  else
71  *state -= 6;
72 }
73 
74 /* Indicate that the latest symbol was a match. */
75 static inline void lzma_state_match(enum lzma_state *state)
76 {
77  *state = *state < LIT_STATES ? STATE_LIT_MATCH : STATE_NONLIT_MATCH;
78 }
79 
80 /* Indicate that the latest state was a long repeated match. */
81 static inline void lzma_state_long_rep(enum lzma_state *state)
82 {
83  *state = *state < LIT_STATES ? STATE_LIT_LONGREP : STATE_NONLIT_REP;
84 }
85 
86 /* Indicate that the latest symbol was a short match. */
87 static inline void lzma_state_short_rep(enum lzma_state *state)
88 {
89  *state = *state < LIT_STATES ? STATE_LIT_SHORTREP : STATE_NONLIT_REP;
90 }
91 
92 /* Test if the previous symbol was a literal. */
93 static inline bool lzma_state_is_literal(enum lzma_state state)
94 {
95  return state < LIT_STATES;
96 }
97 
98 /* Each literal coder is divided in three sections:
99  * - 0x001-0x0FF: Without match byte
100  * - 0x101-0x1FF: With match byte; match bit is 0
101  * - 0x201-0x2FF: With match byte; match bit is 1
102  *
103  * Match byte is used when the previous LZMA symbol was something else than
104  * a literal (that is, it was some kind of match).
105  */
106 #define LITERAL_CODER_SIZE 0x300
107 
108 /* Maximum number of literal coders */
109 #define LITERAL_CODERS_MAX (1 << 4)
110 
111 /* Minimum length of a match is two bytes. */
112 #define MATCH_LEN_MIN 2
113 
114 /* Match length is encoded with 4, 5, or 10 bits.
115  *
116  * Length Bits
117  * 2-9 4 = Choice=0 + 3 bits
118  * 10-17 5 = Choice=1 + Choice2=0 + 3 bits
119  * 18-273 10 = Choice=1 + Choice2=1 + 8 bits
120  */
121 #define LEN_LOW_BITS 3
122 #define LEN_LOW_SYMBOLS (1 << LEN_LOW_BITS)
123 #define LEN_MID_BITS 3
124 #define LEN_MID_SYMBOLS (1 << LEN_MID_BITS)
125 #define LEN_HIGH_BITS 8
126 #define LEN_HIGH_SYMBOLS (1 << LEN_HIGH_BITS)
127 #define LEN_SYMBOLS (LEN_LOW_SYMBOLS + LEN_MID_SYMBOLS + LEN_HIGH_SYMBOLS)
128 
129 /*
130  * Maximum length of a match is 273 which is a result of the encoding
131  * described above.
132  */
133 #define MATCH_LEN_MAX (MATCH_LEN_MIN + LEN_SYMBOLS - 1)
134 
135 /*
136  * Different sets of probabilities are used for match distances that have
137  * very short match length: Lengths of 2, 3, and 4 bytes have a separate
138  * set of probabilities for each length. The matches with longer length
139  * use a shared set of probabilities.
140  */
141 #define DIST_STATES 4
142 
143 /*
144  * Get the index of the appropriate probability array for decoding
145  * the distance slot.
146  */
147 static inline uint32_t lzma_get_dist_state(uint32_t len)
148 {
149  return len < DIST_STATES + MATCH_LEN_MIN
150  ? len - MATCH_LEN_MIN : DIST_STATES - 1;
151 }
152 
153 /*
154  * The highest two bits of a 32-bit match distance are encoded using six bits.
155  * This six-bit value is called a distance slot. This way encoding a 32-bit
156  * value takes 6-36 bits, larger values taking more bits.
157  */
158 #define DIST_SLOT_BITS 6
159 #define DIST_SLOTS (1 << DIST_SLOT_BITS)
160 
161 /* Match distances up to 127 are fully encoded using probabilities. Since
162  * the highest two bits (distance slot) are always encoded using six bits,
163  * the distances 0-3 don't need any additional bits to encode, since the
164  * distance slot itself is the same as the actual distance. DIST_MODEL_START
165  * indicates the first distance slot where at least one additional bit is
166  * needed.
167  */
168 #define DIST_MODEL_START 4
169 
170 /*
171  * Match distances greater than 127 are encoded in three pieces:
172  * - distance slot: the highest two bits
173  * - direct bits: 2-26 bits below the highest two bits
174  * - alignment bits: four lowest bits
175  *
176  * Direct bits don't use any probabilities.
177  *
178  * The distance slot value of 14 is for distances 128-191.
179  */
180 #define DIST_MODEL_END 14
181 
182 /* Distance slots that indicate a distance <= 127. */
183 #define FULL_DISTANCES_BITS (DIST_MODEL_END / 2)
184 #define FULL_DISTANCES (1 << FULL_DISTANCES_BITS)
185 
186 /*
187  * For match distances greater than 127, only the highest two bits and the
188  * lowest four bits (alignment) is encoded using probabilities.
189  */
190 #define ALIGN_BITS 4
191 #define ALIGN_SIZE (1 << ALIGN_BITS)
192 #define ALIGN_MASK (ALIGN_SIZE - 1)
193 
194 /* Total number of all probability variables */
195 #define PROBS_TOTAL (1846 + LITERAL_CODERS_MAX * LITERAL_CODER_SIZE)
196 
197 /*
198  * LZMA remembers the four most recent match distances. Reusing these
199  * distances tends to take less space than re-encoding the actual
200  * distance value.
201  */
202 #define REPS 4
203 
204 #endif