xz/xz_lzma2.h

   1 /*
   2  * LZMA2 definitions
   3  *
   4  * Authors: Lasse Collin <lasse.collin@tukaani.org>
   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 {
  43         STATE_LIT_LIT,
  44         STATE_MATCH_LIT_LIT,
  45         STATE_REP_LIT_LIT,
  46         STATE_SHORTREP_LIT_LIT,
  47         STATE_MATCH_LIT,
  48         STATE_REP_LIT,
  49         STATE_SHORTREP_LIT,
  50         STATE_LIT_MATCH,
  51         STATE_LIT_LONGREP,
  52         STATE_LIT_SHORTREP,
  53         STATE_NONLIT_MATCH,
  54         STATE_NONLIT_REP
  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