ce188d4d |
1 | LZMA specification (DRAFT version)\r |
2 | ----------------------------------\r |
3 | \r |
4 | Author: Igor Pavlov\r |
5 | Date: 2015-06-14\r |
6 | \r |
7 | This specification defines the format of LZMA compressed data and lzma file format.\r |
8 | \r |
9 | Notation \r |
10 | --------\r |
11 | \r |
12 | We use the syntax of C++ programming language.\r |
13 | We use the following types in C++ code:\r |
14 | unsigned - unsigned integer, at least 16 bits in size\r |
15 | int - signed integer, at least 16 bits in size\r |
16 | UInt64 - 64-bit unsigned integer\r |
17 | UInt32 - 32-bit unsigned integer\r |
18 | UInt16 - 16-bit unsigned integer\r |
19 | Byte - 8-bit unsigned integer\r |
20 | bool - boolean type with two possible values: false, true\r |
21 | \r |
22 | \r |
23 | lzma file format\r |
24 | ================\r |
25 | \r |
26 | The lzma file contains the raw LZMA stream and the header with related properties.\r |
27 | \r |
28 | The files in that format use ".lzma" extension.\r |
29 | \r |
30 | The lzma file format layout:\r |
31 | \r |
32 | Offset Size Description\r |
33 | \r |
34 | 0 1 LZMA model properties (lc, lp, pb) in encoded form\r |
35 | 1 4 Dictionary size (32-bit unsigned integer, little-endian)\r |
36 | 5 8 Uncompressed size (64-bit unsigned integer, little-endian)\r |
37 | 13 Compressed data (LZMA stream)\r |
38 | \r |
39 | LZMA properties:\r |
40 | \r |
41 | name Range Description\r |
42 | \r |
43 | lc [0, 8] the number of "literal context" bits\r |
44 | lp [0, 4] the number of "literal pos" bits\r |
45 | pb [0, 4] the number of "pos" bits\r |
46 | dictSize [0, 2^32 - 1] the dictionary size \r |
47 | \r |
48 | The following code encodes LZMA properties:\r |
49 | \r |
50 | void EncodeProperties(Byte *properties)\r |
51 | {\r |
52 | properties[0] = (Byte)((pb * 5 + lp) * 9 + lc);\r |
53 | Set_UInt32_LittleEndian(properties + 1, dictSize);\r |
54 | }\r |
55 | \r |
56 | If the value of dictionary size in properties is smaller than (1 << 12),\r |
57 | the LZMA decoder must set the dictionary size variable to (1 << 12).\r |
58 | \r |
59 | #define LZMA_DIC_MIN (1 << 12)\r |
60 | \r |
61 | unsigned lc, pb, lp;\r |
62 | UInt32 dictSize;\r |
63 | UInt32 dictSizeInProperties;\r |
64 | \r |
65 | void DecodeProperties(const Byte *properties)\r |
66 | {\r |
67 | unsigned d = properties[0];\r |
68 | if (d >= (9 * 5 * 5))\r |
69 | throw "Incorrect LZMA properties";\r |
70 | lc = d % 9;\r |
71 | d /= 9;\r |
72 | pb = d / 5;\r |
73 | lp = d % 5;\r |
74 | dictSizeInProperties = 0;\r |
75 | for (int i = 0; i < 4; i++)\r |
76 | dictSizeInProperties |= (UInt32)properties[i + 1] << (8 * i);\r |
77 | dictSize = dictSizeInProperties;\r |
78 | if (dictSize < LZMA_DIC_MIN)\r |
79 | dictSize = LZMA_DIC_MIN;\r |
80 | }\r |
81 | \r |
82 | If "Uncompressed size" field contains ones in all 64 bits, it means that\r |
83 | uncompressed size is unknown and there is the "end marker" in stream,\r |
84 | that indicates the end of decoding point.\r |
85 | In opposite case, if the value from "Uncompressed size" field is not\r |
86 | equal to ((2^64) - 1), the LZMA stream decoding must be finished after\r |
87 | specified number of bytes (Uncompressed size) is decoded. And if there \r |
88 | is the "end marker", the LZMA decoder must read that marker also.\r |
89 | \r |
90 | \r |
91 | The new scheme to encode LZMA properties\r |
92 | ----------------------------------------\r |
93 | \r |
94 | If LZMA compression is used for some another format, it's recommended to\r |
95 | use a new improved scheme to encode LZMA properties. That new scheme was\r |
96 | used in xz format that uses the LZMA2 compression algorithm.\r |
97 | The LZMA2 is a new compression algorithm that is based on the LZMA algorithm.\r |
98 | \r |
99 | The dictionary size in LZMA2 is encoded with just one byte and LZMA2 supports\r |
100 | only reduced set of dictionary sizes:\r |
101 | (2 << 11), (3 << 11),\r |
102 | (2 << 12), (3 << 12),\r |
103 | ...\r |
104 | (2 << 30), (3 << 30),\r |
105 | (2 << 31) - 1\r |
106 | \r |
107 | The dictionary size can be extracted from encoded value with the following code:\r |
108 | \r |
109 | dictSize = (p == 40) ? 0xFFFFFFFF : (((UInt32)2 | ((p) & 1)) << ((p) / 2 + 11));\r |
110 | \r |
111 | Also there is additional limitation (lc + lp <= 4) in LZMA2 for values of \r |
112 | "lc" and "lp" properties:\r |
113 | \r |
114 | if (lc + lp > 4)\r |
115 | throw "Unsupported properties: (lc + lp) > 4";\r |
116 | \r |
117 | There are some advantages for LZMA decoder with such (lc + lp) value\r |
118 | limitation. It reduces the maximum size of tables allocated by decoder.\r |
119 | And it reduces the complexity of initialization procedure, that can be \r |
120 | important to keep high speed of decoding of big number of small LZMA streams.\r |
121 | \r |
122 | It's recommended to use that limitation (lc + lp <= 4) for any new format\r |
123 | that uses LZMA compression. Note that the combinations of "lc" and "lp" \r |
124 | parameters, where (lc + lp > 4), can provide significant improvement in \r |
125 | compression ratio only in some rare cases.\r |
126 | \r |
127 | The LZMA properties can be encoded into two bytes in new scheme:\r |
128 | \r |
129 | Offset Size Description\r |
130 | \r |
131 | 0 1 The dictionary size encoded with LZMA2 scheme\r |
132 | 1 1 LZMA model properties (lc, lp, pb) in encoded form\r |
133 | \r |
134 | \r |
135 | The RAM usage \r |
136 | =============\r |
137 | \r |
138 | The RAM usage for LZMA decoder is determined by the following parts:\r |
139 | \r |
140 | 1) The Sliding Window (from 4 KiB to 4 GiB).\r |
141 | 2) The probability model counter arrays (arrays of 16-bit variables).\r |
142 | 3) Some additional state variables (about 10 variables of 32-bit integers).\r |
143 | \r |
144 | \r |
145 | The RAM usage for Sliding Window\r |
146 | --------------------------------\r |
147 | \r |
148 | There are two main scenarios of decoding:\r |
149 | \r |
150 | 1) The decoding of full stream to one RAM buffer.\r |
151 | \r |
152 | If we decode full LZMA stream to one output buffer in RAM, the decoder \r |
153 | can use that output buffer as sliding window. So the decoder doesn't \r |
154 | need additional buffer allocated for sliding window.\r |
155 | \r |
156 | 2) The decoding to some external storage.\r |
157 | \r |
158 | If we decode LZMA stream to external storage, the decoder must allocate\r |
159 | the buffer for sliding window. The size of that buffer must be equal \r |
160 | or larger than the value of dictionary size from properties of LZMA stream.\r |
161 | \r |
162 | In this specification we describe the code for decoding to some external\r |
163 | storage. The optimized version of code for decoding of full stream to one\r |
164 | output RAM buffer can require some minor changes in code.\r |
165 | \r |
166 | \r |
167 | The RAM usage for the probability model counters\r |
168 | ------------------------------------------------\r |
169 | \r |
170 | The size of the probability model counter arrays is calculated with the \r |
171 | following formula:\r |
172 | \r |
173 | size_of_prob_arrays = 1846 + 768 * (1 << (lp + lc))\r |
174 | \r |
175 | Each probability model counter is 11-bit unsigned integer.\r |
176 | If we use 16-bit integer variables (2-byte integers) for these probability \r |
177 | model counters, the RAM usage required by probability model counter arrays \r |
178 | can be estimated with the following formula:\r |
179 | \r |
180 | RAM = 4 KiB + 1.5 KiB * (1 << (lp + lc))\r |
181 | \r |
182 | For example, for default LZMA parameters (lp = 0 and lc = 3), the RAM usage is\r |
183 | \r |
184 | RAM_lc3_lp0 = 4 KiB + 1.5 KiB * 8 = 16 KiB\r |
185 | \r |
186 | The maximum RAM state usage is required for decoding the stream with lp = 4 \r |
187 | and lc = 8:\r |
188 | \r |
189 | RAM_lc8_lp4 = 4 KiB + 1.5 KiB * 4096 = 6148 KiB\r |
190 | \r |
191 | If the decoder uses LZMA2's limited property condition \r |
192 | (lc + lp <= 4), the RAM usage will be not larger than\r |
193 | \r |
194 | RAM_lc_lp_4 = 4 KiB + 1.5 KiB * 16 = 28 KiB\r |
195 | \r |
196 | \r |
197 | The RAM usage for encoder\r |
198 | -------------------------\r |
199 | \r |
200 | There are many variants for LZMA encoding code.\r |
201 | These variants have different values for memory consumption.\r |
202 | Note that memory consumption for LZMA Encoder can not be \r |
203 | smaller than memory consumption of LZMA Decoder for same stream.\r |
204 | \r |
205 | The RAM usage required by modern effective implementation of \r |
206 | LZMA Encoder can be estimated with the following formula:\r |
207 | \r |
208 | Encoder_RAM_Usage = 4 MiB + 11 * dictionarySize.\r |
209 | \r |
210 | But there are some modes of the encoder that require less memory.\r |
211 | \r |
212 | \r |
213 | LZMA Decoding\r |
214 | =============\r |
215 | \r |
216 | The LZMA compression algorithm uses LZ-based compression with Sliding Window\r |
217 | and Range Encoding as entropy coding method.\r |
218 | \r |
219 | \r |
220 | Sliding Window\r |
221 | --------------\r |
222 | \r |
223 | LZMA uses Sliding Window compression similar to LZ77 algorithm.\r |
224 | \r |
225 | LZMA stream must be decoded to the sequence that consists\r |
226 | of MATCHES and LITERALS:\r |
227 | \r |
228 | - a LITERAL is a 8-bit character (one byte).\r |
229 | The decoder just puts that LITERAL to the uncompressed stream.\r |
230 | \r |
231 | - a MATCH is a pair of two numbers (DISTANCE-LENGTH pair).\r |
232 | The decoder takes one byte exactly "DISTANCE" characters behind\r |
233 | current position in the uncompressed stream and puts it to \r |
234 | uncompressed stream. The decoder must repeat it "LENGTH" times.\r |
235 | \r |
236 | The "DISTANCE" can not be larger than dictionary size.\r |
237 | And the "DISTANCE" can not be larger than the number of bytes in\r |
238 | the uncompressed stream that were decoded before that match.\r |
239 | \r |
240 | In this specification we use cyclic buffer to implement Sliding Window\r |
241 | for LZMA decoder:\r |
242 | \r |
243 | class COutWindow\r |
244 | {\r |
245 | Byte *Buf;\r |
246 | UInt32 Pos;\r |
247 | UInt32 Size;\r |
248 | bool IsFull;\r |
249 | \r |
250 | public:\r |
251 | unsigned TotalPos;\r |
252 | COutStream OutStream;\r |
253 | \r |
254 | COutWindow(): Buf(NULL) {}\r |
255 | ~COutWindow() { delete []Buf; }\r |
256 | \r |
257 | void Create(UInt32 dictSize)\r |
258 | {\r |
259 | Buf = new Byte[dictSize];\r |
260 | Pos = 0;\r |
261 | Size = dictSize;\r |
262 | IsFull = false;\r |
263 | TotalPos = 0;\r |
264 | }\r |
265 | \r |
266 | void PutByte(Byte b)\r |
267 | {\r |
268 | TotalPos++;\r |
269 | Buf[Pos++] = b;\r |
270 | if (Pos == Size)\r |
271 | {\r |
272 | Pos = 0;\r |
273 | IsFull = true;\r |
274 | }\r |
275 | OutStream.WriteByte(b);\r |
276 | }\r |
277 | \r |
278 | Byte GetByte(UInt32 dist) const\r |
279 | {\r |
280 | return Buf[dist <= Pos ? Pos - dist : Size - dist + Pos];\r |
281 | }\r |
282 | \r |
283 | void CopyMatch(UInt32 dist, unsigned len)\r |
284 | {\r |
285 | for (; len > 0; len--)\r |
286 | PutByte(GetByte(dist));\r |
287 | }\r |
288 | \r |
289 | bool CheckDistance(UInt32 dist) const\r |
290 | {\r |
291 | return dist <= Pos || IsFull;\r |
292 | }\r |
293 | \r |
294 | bool IsEmpty() const\r |
295 | {\r |
296 | return Pos == 0 && !IsFull;\r |
297 | }\r |
298 | };\r |
299 | \r |
300 | \r |
301 | In another implementation it's possible to use one buffer that contains \r |
302 | Sliding Window and the whole data stream after uncompressing.\r |
303 | \r |
304 | \r |
305 | Range Decoder\r |
306 | -------------\r |
307 | \r |
308 | LZMA algorithm uses Range Encoding (1) as entropy coding method.\r |
309 | \r |
310 | LZMA stream contains just one very big number in big-endian encoding.\r |
311 | LZMA decoder uses the Range Decoder to extract a sequence of binary\r |
312 | symbols from that big number.\r |
313 | \r |
314 | The state of the Range Decoder:\r |
315 | \r |
316 | struct CRangeDecoder\r |
317 | {\r |
318 | UInt32 Range; \r |
319 | UInt32 Code;\r |
320 | InputStream *InStream;\r |
321 | \r |
322 | bool Corrupted;\r |
323 | }\r |
324 | \r |
325 | The notes about UInt32 type for the "Range" and "Code" variables:\r |
326 | \r |
327 | It's possible to use 64-bit (unsigned or signed) integer type\r |
328 | for the "Range" and the "Code" variables instead of 32-bit unsigned,\r |
329 | but some additional code must be used to truncate the values to \r |
330 | low 32-bits after some operations.\r |
331 | \r |
332 | If the programming language does not support 32-bit unsigned integer type \r |
333 | (like in case of JAVA language), it's possible to use 32-bit signed integer, \r |
334 | but some code must be changed. For example, it's required to change the code\r |
335 | that uses comparison operations for UInt32 variables in this specification.\r |
336 | \r |
337 | The Range Decoder can be in some states that can be treated as \r |
338 | "Corruption" in LZMA stream. The Range Decoder uses the variable "Corrupted":\r |
339 | \r |
340 | (Corrupted == false), if the Range Decoder has not detected any corruption.\r |
341 | (Corrupted == true), if the Range Decoder has detected some corruption.\r |
342 | \r |
343 | The reference LZMA Decoder ignores the value of the "Corrupted" variable.\r |
344 | So it continues to decode the stream, even if the corruption can be detected\r |
345 | in the Range Decoder. To provide the full compatibility with output of the \r |
346 | reference LZMA Decoder, another LZMA Decoder implementations must also \r |
347 | ignore the value of the "Corrupted" variable.\r |
348 | \r |
349 | The LZMA Encoder is required to create only such LZMA streams, that will not \r |
350 | lead the Range Decoder to states, where the "Corrupted" variable is set to true.\r |
351 | \r |
352 | The Range Decoder reads first 5 bytes from input stream to initialize\r |
353 | the state:\r |
354 | \r |
355 | bool CRangeDecoder::Init()\r |
356 | {\r |
357 | Corrupted = false;\r |
358 | Range = 0xFFFFFFFF;\r |
359 | Code = 0;\r |
360 | \r |
361 | Byte b = InStream->ReadByte();\r |
362 | \r |
363 | for (int i = 0; i < 4; i++)\r |
364 | Code = (Code << 8) | InStream->ReadByte();\r |
365 | \r |
366 | if (b != 0 || Code == Range)\r |
367 | Corrupted = true;\r |
368 | return b == 0;\r |
369 | }\r |
370 | \r |
371 | The LZMA Encoder always writes ZERO in initial byte of compressed stream.\r |
372 | That scheme allows to simplify the code of the Range Encoder in the \r |
373 | LZMA Encoder. If initial byte is not equal to ZERO, the LZMA Decoder must\r |
374 | stop decoding and report error.\r |
375 | \r |
376 | After the last bit of data was decoded by Range Decoder, the value of the\r |
377 | "Code" variable must be equal to 0. The LZMA Decoder must check it by \r |
378 | calling the IsFinishedOK() function:\r |
379 | \r |
380 | bool IsFinishedOK() const { return Code == 0; }\r |
381 | \r |
382 | If there is corruption in data stream, there is big probability that\r |
383 | the "Code" value will be not equal to 0 in the Finish() function. So that\r |
384 | check in the IsFinishedOK() function provides very good feature for \r |
385 | corruption detection.\r |
386 | \r |
387 | The value of the "Range" variable before each bit decoding can not be smaller \r |
388 | than ((UInt32)1 << 24). The Normalize() function keeps the "Range" value in \r |
389 | described range.\r |
390 | \r |
391 | #define kTopValue ((UInt32)1 << 24)\r |
392 | \r |
393 | void CRangeDecoder::Normalize()\r |
394 | {\r |
395 | if (Range < kTopValue)\r |
396 | {\r |
397 | Range <<= 8;\r |
398 | Code = (Code << 8) | InStream->ReadByte();\r |
399 | }\r |
400 | }\r |
401 | \r |
402 | Notes: if the size of the "Code" variable is larger than 32 bits, it's\r |
403 | required to keep only low 32 bits of the "Code" variable after the change\r |
404 | in Normalize() function.\r |
405 | \r |
406 | If the LZMA Stream is not corrupted, the value of the "Code" variable is\r |
407 | always smaller than value of the "Range" variable.\r |
408 | But the Range Decoder ignores some types of corruptions, so the value of\r |
409 | the "Code" variable can be equal or larger than value of the "Range" variable\r |
410 | for some "Corrupted" archives.\r |
411 | \r |
412 | \r |
413 | LZMA uses Range Encoding only with binary symbols of two types:\r |
414 | 1) binary symbols with fixed and equal probabilities (direct bits)\r |
415 | 2) binary symbols with predicted probabilities\r |
416 | \r |
417 | The DecodeDirectBits() function decodes the sequence of direct bits:\r |
418 | \r |
419 | UInt32 CRangeDecoder::DecodeDirectBits(unsigned numBits)\r |
420 | {\r |
421 | UInt32 res = 0;\r |
422 | do\r |
423 | {\r |
424 | Range >>= 1;\r |
425 | Code -= Range;\r |
426 | UInt32 t = 0 - ((UInt32)Code >> 31);\r |
427 | Code += Range & t;\r |
428 | \r |
429 | if (Code == Range)\r |
430 | Corrupted = true;\r |
431 | \r |
432 | Normalize();\r |
433 | res <<= 1;\r |
434 | res += t + 1;\r |
435 | }\r |
436 | while (--numBits);\r |
437 | return res;\r |
438 | }\r |
439 | \r |
440 | \r |
441 | The Bit Decoding with Probability Model\r |
442 | ---------------------------------------\r |
443 | \r |
444 | The task of Bit Probability Model is to estimate probabilities of binary\r |
445 | symbols. And then it provides the Range Decoder with that information.\r |
446 | The better prediction provides better compression ratio.\r |
447 | The Bit Probability Model uses statistical data of previous decoded\r |
448 | symbols.\r |
449 | \r |
450 | That estimated probability is presented as 11-bit unsigned integer value\r |
451 | that represents the probability of symbol "0".\r |
452 | \r |
453 | #define kNumBitModelTotalBits 11\r |
454 | \r |
455 | Mathematical probabilities can be presented with the following formulas:\r |
456 | probability(symbol_0) = prob / 2048.\r |
457 | probability(symbol_1) = 1 - Probability(symbol_0) = \r |
458 | = 1 - prob / 2048 = \r |
459 | = (2048 - prob) / 2048\r |
460 | where the "prob" variable contains 11-bit integer probability counter.\r |
461 | \r |
462 | It's recommended to use 16-bit unsigned integer type, to store these 11-bit\r |
463 | probability values:\r |
464 | \r |
465 | typedef UInt16 CProb;\r |
466 | \r |
467 | Each probability value must be initialized with value ((1 << 11) / 2),\r |
468 | that represents the state, where probabilities of symbols 0 and 1 \r |
469 | are equal to 0.5:\r |
470 | \r |
471 | #define PROB_INIT_VAL ((1 << kNumBitModelTotalBits) / 2)\r |
472 | \r |
473 | The INIT_PROBS macro is used to initialize the array of CProb variables:\r |
474 | \r |
475 | #define INIT_PROBS(p) \\r |
476 | { for (unsigned i = 0; i < sizeof(p) / sizeof(p[0]); i++) p[i] = PROB_INIT_VAL; }\r |
477 | \r |
478 | \r |
479 | The DecodeBit() function decodes one bit.\r |
480 | The LZMA decoder provides the pointer to CProb variable that contains \r |
481 | information about estimated probability for symbol 0 and the Range Decoder \r |
482 | updates that CProb variable after decoding. The Range Decoder increases \r |
483 | estimated probability of the symbol that was decoded:\r |
484 | \r |
485 | #define kNumMoveBits 5\r |
486 | \r |
487 | unsigned CRangeDecoder::DecodeBit(CProb *prob)\r |
488 | {\r |
489 | unsigned v = *prob;\r |
490 | UInt32 bound = (Range >> kNumBitModelTotalBits) * v;\r |
491 | unsigned symbol;\r |
492 | if (Code < bound)\r |
493 | {\r |
494 | v += ((1 << kNumBitModelTotalBits) - v) >> kNumMoveBits;\r |
495 | Range = bound;\r |
496 | symbol = 0;\r |
497 | }\r |
498 | else\r |
499 | {\r |
500 | v -= v >> kNumMoveBits;\r |
501 | Code -= bound;\r |
502 | Range -= bound;\r |
503 | symbol = 1;\r |
504 | }\r |
505 | *prob = (CProb)v;\r |
506 | Normalize();\r |
507 | return symbol;\r |
508 | }\r |
509 | \r |
510 | \r |
511 | The Binary Tree of bit model counters\r |
512 | -------------------------------------\r |
513 | \r |
514 | LZMA uses a tree of Bit model variables to decode symbol that needs\r |
515 | several bits for storing. There are two versions of such trees in LZMA:\r |
516 | 1) the tree that decodes bits from high bit to low bit (the normal scheme).\r |
517 | 2) the tree that decodes bits from low bit to high bit (the reverse scheme).\r |
518 | \r |
519 | Each binary tree structure supports different size of decoded symbol\r |
520 | (the size of binary sequence that contains value of symbol).\r |
521 | If that size of decoded symbol is "NumBits" bits, the tree structure \r |
522 | uses the array of (2 << NumBits) counters of CProb type. \r |
523 | But only ((2 << NumBits) - 1) items are used by encoder and decoder.\r |
524 | The first item (the item with index equal to 0) in array is unused.\r |
525 | That scheme with unused array's item allows to simplify the code.\r |
526 | \r |
527 | unsigned BitTreeReverseDecode(CProb *probs, unsigned numBits, CRangeDecoder *rc)\r |
528 | {\r |
529 | unsigned m = 1;\r |
530 | unsigned symbol = 0;\r |
531 | for (unsigned i = 0; i < numBits; i++)\r |
532 | {\r |
533 | unsigned bit = rc->DecodeBit(&probs[m]);\r |
534 | m <<= 1;\r |
535 | m += bit;\r |
536 | symbol |= (bit << i);\r |
537 | }\r |
538 | return symbol;\r |
539 | }\r |
540 | \r |
541 | template <unsigned NumBits>\r |
542 | class CBitTreeDecoder\r |
543 | {\r |
544 | CProb Probs[(unsigned)1 << NumBits];\r |
545 | \r |
546 | public:\r |
547 | \r |
548 | void Init()\r |
549 | {\r |
550 | INIT_PROBS(Probs);\r |
551 | }\r |
552 | \r |
553 | unsigned Decode(CRangeDecoder *rc)\r |
554 | {\r |
555 | unsigned m = 1;\r |
556 | for (unsigned i = 0; i < NumBits; i++)\r |
557 | m = (m << 1) + rc->DecodeBit(&Probs[m]);\r |
558 | return m - ((unsigned)1 << NumBits);\r |
559 | }\r |
560 | \r |
561 | unsigned ReverseDecode(CRangeDecoder *rc)\r |
562 | {\r |
563 | return BitTreeReverseDecode(Probs, NumBits, rc);\r |
564 | }\r |
565 | };\r |
566 | \r |
567 | \r |
568 | LZ part of LZMA \r |
569 | ---------------\r |
570 | \r |
571 | LZ part of LZMA describes details about the decoding of MATCHES and LITERALS.\r |
572 | \r |
573 | \r |
574 | The Literal Decoding\r |
575 | --------------------\r |
576 | \r |
577 | The LZMA Decoder uses (1 << (lc + lp)) tables with CProb values, where \r |
578 | each table contains 0x300 CProb values:\r |
579 | \r |
580 | CProb *LitProbs;\r |
581 | \r |
582 | void CreateLiterals()\r |
583 | {\r |
584 | LitProbs = new CProb[(UInt32)0x300 << (lc + lp)];\r |
585 | }\r |
586 | \r |
587 | void InitLiterals()\r |
588 | {\r |
589 | UInt32 num = (UInt32)0x300 << (lc + lp);\r |
590 | for (UInt32 i = 0; i < num; i++)\r |
591 | LitProbs[i] = PROB_INIT_VAL;\r |
592 | }\r |
593 | \r |
594 | To select the table for decoding it uses the context that consists of\r |
595 | (lc) high bits from previous literal and (lp) low bits from value that\r |
596 | represents current position in outputStream.\r |
597 | \r |
598 | If (State > 7), the Literal Decoder also uses "matchByte" that represents \r |
599 | the byte in OutputStream at position the is the DISTANCE bytes before \r |
600 | current position, where the DISTANCE is the distance in DISTANCE-LENGTH pair\r |
601 | of latest decoded match.\r |
602 | \r |
603 | The following code decodes one literal and puts it to Sliding Window buffer:\r |
604 | \r |
605 | void DecodeLiteral(unsigned state, UInt32 rep0)\r |
606 | {\r |
607 | unsigned prevByte = 0;\r |
608 | if (!OutWindow.IsEmpty())\r |
609 | prevByte = OutWindow.GetByte(1);\r |
610 | \r |
611 | unsigned symbol = 1;\r |
612 | unsigned litState = ((OutWindow.TotalPos & ((1 << lp) - 1)) << lc) + (prevByte >> (8 - lc));\r |
613 | CProb *probs = &LitProbs[(UInt32)0x300 * litState];\r |
614 | \r |
615 | if (state >= 7)\r |
616 | {\r |
617 | unsigned matchByte = OutWindow.GetByte(rep0 + 1);\r |
618 | do\r |
619 | {\r |
620 | unsigned matchBit = (matchByte >> 7) & 1;\r |
621 | matchByte <<= 1;\r |
622 | unsigned bit = RangeDec.DecodeBit(&probs[((1 + matchBit) << 8) + symbol]);\r |
623 | symbol = (symbol << 1) | bit;\r |
624 | if (matchBit != bit)\r |
625 | break;\r |
626 | }\r |
627 | while (symbol < 0x100);\r |
628 | }\r |
629 | while (symbol < 0x100)\r |
630 | symbol = (symbol << 1) | RangeDec.DecodeBit(&probs[symbol]);\r |
631 | OutWindow.PutByte((Byte)(symbol - 0x100));\r |
632 | }\r |
633 | \r |
634 | \r |
635 | The match length decoding\r |
636 | -------------------------\r |
637 | \r |
638 | The match length decoder returns normalized (zero-based value) \r |
639 | length of match. That value can be converted to real length of the match \r |
640 | with the following code:\r |
641 | \r |
642 | #define kMatchMinLen 2\r |
643 | \r |
644 | matchLen = len + kMatchMinLen;\r |
645 | \r |
646 | The match length decoder can return the values from 0 to 271.\r |
647 | And the corresponded real match length values can be in the range \r |
648 | from 2 to 273.\r |
649 | \r |
650 | The following scheme is used for the match length encoding:\r |
651 | \r |
652 | Binary encoding Binary Tree structure Zero-based match length \r |
653 | sequence (binary + decimal):\r |
654 | \r |
655 | 0 xxx LowCoder[posState] xxx\r |
656 | 1 0 yyy MidCoder[posState] yyy + 8\r |
657 | 1 1 zzzzzzzz HighCoder zzzzzzzz + 16\r |
658 | \r |
659 | LZMA uses bit model variable "Choice" to decode the first selection bit.\r |
660 | \r |
661 | If the first selection bit is equal to 0, the decoder uses binary tree \r |
662 | LowCoder[posState] to decode 3-bit zero-based match length (xxx).\r |
663 | \r |
664 | If the first selection bit is equal to 1, the decoder uses bit model \r |
665 | variable "Choice2" to decode the second selection bit.\r |
666 | \r |
667 | If the second selection bit is equal to 0, the decoder uses binary tree \r |
668 | MidCoder[posState] to decode 3-bit "yyy" value, and zero-based match\r |
669 | length is equal to (yyy + 8).\r |
670 | \r |
671 | If the second selection bit is equal to 1, the decoder uses binary tree \r |
672 | HighCoder to decode 8-bit "zzzzzzzz" value, and zero-based \r |
673 | match length is equal to (zzzzzzzz + 16).\r |
674 | \r |
675 | LZMA uses "posState" value as context to select the binary tree \r |
676 | from LowCoder and MidCoder binary tree arrays:\r |
677 | \r |
678 | unsigned posState = OutWindow.TotalPos & ((1 << pb) - 1);\r |
679 | \r |
680 | The full code of the length decoder:\r |
681 | \r |
682 | class CLenDecoder\r |
683 | {\r |
684 | CProb Choice;\r |
685 | CProb Choice2;\r |
686 | CBitTreeDecoder<3> LowCoder[1 << kNumPosBitsMax];\r |
687 | CBitTreeDecoder<3> MidCoder[1 << kNumPosBitsMax];\r |
688 | CBitTreeDecoder<8> HighCoder;\r |
689 | \r |
690 | public:\r |
691 | \r |
692 | void Init()\r |
693 | {\r |
694 | Choice = PROB_INIT_VAL;\r |
695 | Choice2 = PROB_INIT_VAL;\r |
696 | HighCoder.Init();\r |
697 | for (unsigned i = 0; i < (1 << kNumPosBitsMax); i++)\r |
698 | {\r |
699 | LowCoder[i].Init();\r |
700 | MidCoder[i].Init();\r |
701 | }\r |
702 | }\r |
703 | \r |
704 | unsigned Decode(CRangeDecoder *rc, unsigned posState)\r |
705 | {\r |
706 | if (rc->DecodeBit(&Choice) == 0)\r |
707 | return LowCoder[posState].Decode(rc);\r |
708 | if (rc->DecodeBit(&Choice2) == 0)\r |
709 | return 8 + MidCoder[posState].Decode(rc);\r |
710 | return 16 + HighCoder.Decode(rc);\r |
711 | }\r |
712 | };\r |
713 | \r |
714 | The LZMA decoder uses two instances of CLenDecoder class.\r |
715 | The first instance is for the matches of "Simple Match" type,\r |
716 | and the second instance is for the matches of "Rep Match" type:\r |
717 | \r |
718 | CLenDecoder LenDecoder;\r |
719 | CLenDecoder RepLenDecoder;\r |
720 | \r |
721 | \r |
722 | The match distance decoding\r |
723 | ---------------------------\r |
724 | \r |
725 | LZMA supports dictionary sizes up to 4 GiB minus 1.\r |
726 | The value of match distance (decoded by distance decoder) can be \r |
727 | from 1 to 2^32. But the distance value that is equal to 2^32 is used to\r |
728 | indicate the "End of stream" marker. So real largest match distance \r |
729 | that is used for LZ-window match is (2^32 - 1).\r |
730 | \r |
731 | LZMA uses normalized match length (zero-based length) \r |
732 | to calculate the context state "lenState" do decode the distance value:\r |
733 | \r |
734 | #define kNumLenToPosStates 4\r |
735 | \r |
736 | unsigned lenState = len;\r |
737 | if (lenState > kNumLenToPosStates - 1)\r |
738 | lenState = kNumLenToPosStates - 1;\r |
739 | \r |
740 | The distance decoder returns the "dist" value that is zero-based value \r |
741 | of match distance. The real match distance can be calculated with the\r |
742 | following code:\r |
743 | \r |
744 | matchDistance = dist + 1; \r |
745 | \r |
746 | The state of the distance decoder and the initialization code: \r |
747 | \r |
748 | #define kEndPosModelIndex 14\r |
749 | #define kNumFullDistances (1 << (kEndPosModelIndex >> 1))\r |
750 | #define kNumAlignBits 4\r |
751 | \r |
752 | CBitTreeDecoder<6> PosSlotDecoder[kNumLenToPosStates];\r |
753 | CProb PosDecoders[1 + kNumFullDistances - kEndPosModelIndex];\r |
754 | CBitTreeDecoder<kNumAlignBits> AlignDecoder;\r |
755 | \r |
756 | void InitDist()\r |
757 | {\r |
758 | for (unsigned i = 0; i < kNumLenToPosStates; i++)\r |
759 | PosSlotDecoder[i].Init();\r |
760 | AlignDecoder.Init();\r |
761 | INIT_PROBS(PosDecoders);\r |
762 | }\r |
763 | \r |
764 | At first stage the distance decoder decodes 6-bit "posSlot" value with bit\r |
765 | tree decoder from PosSlotDecoder array. It's possible to get 2^6=64 different \r |
766 | "posSlot" values.\r |
767 | \r |
768 | unsigned posSlot = PosSlotDecoder[lenState].Decode(&RangeDec);\r |
769 | \r |
770 | The encoding scheme for distance value is shown in the following table:\r |
771 | \r |
772 | posSlot (decimal) /\r |
773 | zero-based distance (binary)\r |
774 | 0 0\r |
775 | 1 1\r |
776 | 2 10\r |
777 | 3 11\r |
778 | \r |
779 | 4 10 x\r |
780 | 5 11 x\r |
781 | 6 10 xx\r |
782 | 7 11 xx\r |
783 | 8 10 xxx\r |
784 | 9 11 xxx\r |
785 | 10 10 xxxx\r |
786 | 11 11 xxxx\r |
787 | 12 10 xxxxx\r |
788 | 13 11 xxxxx\r |
789 | \r |
790 | 14 10 yy zzzz\r |
791 | 15 11 yy zzzz\r |
792 | 16 10 yyy zzzz\r |
793 | 17 11 yyy zzzz\r |
794 | ...\r |
795 | 62 10 yyyyyyyyyyyyyyyyyyyyyyyyyy zzzz\r |
796 | 63 11 yyyyyyyyyyyyyyyyyyyyyyyyyy zzzz\r |
797 | \r |
798 | where \r |
799 | "x ... x" means the sequence of binary symbols encoded with binary tree and \r |
800 | "Reverse" scheme. It uses separated binary tree for each posSlot from 4 to 13.\r |
801 | "y" means direct bit encoded with range coder.\r |
802 | "zzzz" means the sequence of four binary symbols encoded with binary\r |
803 | tree with "Reverse" scheme, where one common binary tree "AlignDecoder"\r |
804 | is used for all posSlot values.\r |
805 | \r |
806 | If (posSlot < 4), the "dist" value is equal to posSlot value.\r |
807 | \r |
808 | If (posSlot >= 4), the decoder uses "posSlot" value to calculate the value of\r |
809 | the high bits of "dist" value and the number of the low bits.\r |
810 | \r |
811 | If (4 <= posSlot < kEndPosModelIndex), the decoder uses bit tree decoders.\r |
812 | (one separated bit tree decoder per one posSlot value) and "Reverse" scheme.\r |
813 | In this implementation we use one CProb array "PosDecoders" that contains \r |
814 | all CProb variables for all these bit decoders.\r |
815 | \r |
816 | if (posSlot >= kEndPosModelIndex), the middle bits are decoded as direct \r |
817 | bits from RangeDecoder and the low 4 bits are decoded with a bit tree \r |
818 | decoder "AlignDecoder" with "Reverse" scheme.\r |
819 | \r |
820 | The code to decode zero-based match distance:\r |
821 | \r |
822 | unsigned DecodeDistance(unsigned len)\r |
823 | {\r |
824 | unsigned lenState = len;\r |
825 | if (lenState > kNumLenToPosStates - 1)\r |
826 | lenState = kNumLenToPosStates - 1;\r |
827 | \r |
828 | unsigned posSlot = PosSlotDecoder[lenState].Decode(&RangeDec);\r |
829 | if (posSlot < 4)\r |
830 | return posSlot;\r |
831 | \r |
832 | unsigned numDirectBits = (unsigned)((posSlot >> 1) - 1);\r |
833 | UInt32 dist = ((2 | (posSlot & 1)) << numDirectBits);\r |
834 | if (posSlot < kEndPosModelIndex)\r |
835 | dist += BitTreeReverseDecode(PosDecoders + dist - posSlot, numDirectBits, &RangeDec);\r |
836 | else\r |
837 | {\r |
838 | dist += RangeDec.DecodeDirectBits(numDirectBits - kNumAlignBits) << kNumAlignBits;\r |
839 | dist += AlignDecoder.ReverseDecode(&RangeDec);\r |
840 | }\r |
841 | return dist;\r |
842 | }\r |
843 | \r |
844 | \r |
845 | \r |
846 | LZMA Decoding modes\r |
847 | -------------------\r |
848 | \r |
849 | There are 2 types of LZMA streams:\r |
850 | \r |
851 | 1) The stream with "End of stream" marker.\r |
852 | 2) The stream without "End of stream" marker.\r |
853 | \r |
854 | And the LZMA Decoder supports 3 modes of decoding:\r |
855 | \r |
856 | 1) The unpack size is undefined. The LZMA decoder stops decoding after \r |
857 | getting "End of stream" marker. \r |
858 | The input variables for that case:\r |
859 | \r |
860 | markerIsMandatory = true\r |
861 | unpackSizeDefined = false\r |
862 | unpackSize contains any value\r |
863 | \r |
864 | 2) The unpack size is defined and LZMA decoder supports both variants, \r |
865 | where the stream can contain "End of stream" marker or the stream is\r |
866 | finished without "End of stream" marker. The LZMA decoder must detect \r |
867 | any of these situations.\r |
868 | The input variables for that case:\r |
869 | \r |
870 | markerIsMandatory = false\r |
871 | unpackSizeDefined = true\r |
872 | unpackSize contains unpack size\r |
873 | \r |
874 | 3) The unpack size is defined and the LZMA stream must contain \r |
875 | "End of stream" marker\r |
876 | The input variables for that case:\r |
877 | \r |
878 | markerIsMandatory = true\r |
879 | unpackSizeDefined = true\r |
880 | unpackSize contains unpack size\r |
881 | \r |
882 | \r |
883 | The main loop of decoder\r |
884 | ------------------------\r |
885 | \r |
886 | The main loop of LZMA decoder:\r |
887 | \r |
888 | Initialize the LZMA state.\r |
889 | loop\r |
890 | {\r |
891 | // begin of loop\r |
892 | Check "end of stream" conditions.\r |
893 | Decode Type of MATCH / LITERAL. \r |
894 | If it's LITERAL, decode LITERAL value and put the LITERAL to Window.\r |
895 | If it's MATCH, decode the length of match and the match distance. \r |
896 | Check error conditions, check end of stream conditions and copy\r |
897 | the sequence of match bytes from sliding window to current position\r |
898 | in window.\r |
899 | Go to begin of loop\r |
900 | }\r |
901 | \r |
902 | The reference implementation of LZMA decoder uses "unpackSize" variable\r |
903 | to keep the number of remaining bytes in output stream. So it reduces \r |
904 | "unpackSize" value after each decoded LITERAL or MATCH.\r |
905 | \r |
906 | The following code contains the "end of stream" condition check at the start\r |
907 | of the loop:\r |
908 | \r |
909 | if (unpackSizeDefined && unpackSize == 0 && !markerIsMandatory)\r |
910 | if (RangeDec.IsFinishedOK())\r |
911 | return LZMA_RES_FINISHED_WITHOUT_MARKER;\r |
912 | \r |
913 | LZMA uses three types of matches:\r |
914 | \r |
915 | 1) "Simple Match" - the match with distance value encoded with bit models.\r |
916 | \r |
917 | 2) "Rep Match" - the match that uses the distance from distance\r |
918 | history table.\r |
919 | \r |
920 | 3) "Short Rep Match" - the match of single byte length, that uses the latest \r |
921 | distance from distance history table.\r |
922 | \r |
923 | The LZMA decoder keeps the history of latest 4 match distances that were used \r |
924 | by decoder. That set of 4 variables contains zero-based match distances and \r |
925 | these variables are initialized with zero values:\r |
926 | \r |
927 | UInt32 rep0 = 0, rep1 = 0, rep2 = 0, rep3 = 0;\r |
928 | \r |
929 | The LZMA decoder uses binary model variables to select type of MATCH or LITERAL:\r |
930 | \r |
931 | #define kNumStates 12\r |
932 | #define kNumPosBitsMax 4\r |
933 | \r |
934 | CProb IsMatch[kNumStates << kNumPosBitsMax];\r |
935 | CProb IsRep[kNumStates];\r |
936 | CProb IsRepG0[kNumStates];\r |
937 | CProb IsRepG1[kNumStates];\r |
938 | CProb IsRepG2[kNumStates];\r |
939 | CProb IsRep0Long[kNumStates << kNumPosBitsMax];\r |
940 | \r |
941 | The decoder uses "state" variable value to select exact variable \r |
942 | from "IsRep", "IsRepG0", "IsRepG1" and "IsRepG2" arrays.\r |
943 | The "state" variable can get the value from 0 to 11.\r |
944 | Initial value for "state" variable is zero:\r |
945 | \r |
946 | unsigned state = 0;\r |
947 | \r |
948 | The "state" variable is updated after each LITERAL or MATCH with one of the\r |
949 | following functions:\r |
950 | \r |
951 | unsigned UpdateState_Literal(unsigned state)\r |
952 | {\r |
953 | if (state < 4) return 0;\r |
954 | else if (state < 10) return state - 3;\r |
955 | else return state - 6;\r |
956 | }\r |
957 | unsigned UpdateState_Match (unsigned state) { return state < 7 ? 7 : 10; }\r |
958 | unsigned UpdateState_Rep (unsigned state) { return state < 7 ? 8 : 11; }\r |
959 | unsigned UpdateState_ShortRep(unsigned state) { return state < 7 ? 9 : 11; }\r |
960 | \r |
961 | The decoder calculates "state2" variable value to select exact variable from \r |
962 | "IsMatch" and "IsRep0Long" arrays:\r |
963 | \r |
964 | unsigned posState = OutWindow.TotalPos & ((1 << pb) - 1);\r |
965 | unsigned state2 = (state << kNumPosBitsMax) + posState;\r |
966 | \r |
967 | The decoder uses the following code flow scheme to select exact \r |
968 | type of LITERAL or MATCH:\r |
969 | \r |
970 | IsMatch[state2] decode\r |
971 | 0 - the Literal\r |
972 | 1 - the Match\r |
973 | IsRep[state] decode\r |
974 | 0 - Simple Match\r |
975 | 1 - Rep Match\r |
976 | IsRepG0[state] decode\r |
977 | 0 - the distance is rep0\r |
978 | IsRep0Long[state2] decode\r |
979 | 0 - Short Rep Match\r |
980 | 1 - Rep Match 0\r |
981 | 1 - \r |
982 | IsRepG1[state] decode\r |
983 | 0 - Rep Match 1\r |
984 | 1 - \r |
985 | IsRepG2[state] decode\r |
986 | 0 - Rep Match 2\r |
987 | 1 - Rep Match 3\r |
988 | \r |
989 | \r |
990 | LITERAL symbol\r |
991 | --------------\r |
992 | If the value "0" was decoded with IsMatch[state2] decoding, we have "LITERAL" type.\r |
993 | \r |
994 | At first the LZMA decoder must check that it doesn't exceed \r |
995 | specified uncompressed size:\r |
996 | \r |
997 | if (unpackSizeDefined && unpackSize == 0)\r |
998 | return LZMA_RES_ERROR;\r |
999 | \r |
1000 | Then it decodes literal value and puts it to sliding window:\r |
1001 | \r |
1002 | DecodeLiteral(state, rep0);\r |
1003 | \r |
1004 | Then the decoder must update the "state" value and "unpackSize" value;\r |
1005 | \r |
1006 | state = UpdateState_Literal(state);\r |
1007 | unpackSize--;\r |
1008 | \r |
1009 | Then the decoder must go to the begin of main loop to decode next Match or Literal.\r |
1010 | \r |
1011 | \r |
1012 | Simple Match\r |
1013 | ------------\r |
1014 | \r |
1015 | If the value "1" was decoded with IsMatch[state2] decoding,\r |
1016 | we have the "Simple Match" type.\r |
1017 | \r |
1018 | The distance history table is updated with the following scheme:\r |
1019 | \r |
1020 | rep3 = rep2;\r |
1021 | rep2 = rep1;\r |
1022 | rep1 = rep0;\r |
1023 | \r |
1024 | The zero-based length is decoded with "LenDecoder":\r |
1025 | \r |
1026 | len = LenDecoder.Decode(&RangeDec, posState);\r |
1027 | \r |
1028 | The state is update with UpdateState_Match function:\r |
1029 | \r |
1030 | state = UpdateState_Match(state);\r |
1031 | \r |
1032 | and the new "rep0" value is decoded with DecodeDistance:\r |
1033 | \r |
1034 | rep0 = DecodeDistance(len);\r |
1035 | \r |
1036 | That "rep0" will be used as zero-based distance for current match.\r |
1037 | \r |
1038 | If the value of "rep0" is equal to 0xFFFFFFFF, it means that we have \r |
1039 | "End of stream" marker, so we can stop decoding and check finishing \r |
1040 | condition in Range Decoder:\r |
1041 | \r |
1042 | if (rep0 == 0xFFFFFFFF)\r |
1043 | return RangeDec.IsFinishedOK() ?\r |
1044 | LZMA_RES_FINISHED_WITH_MARKER :\r |
1045 | LZMA_RES_ERROR;\r |
1046 | \r |
1047 | If uncompressed size is defined, LZMA decoder must check that it doesn't \r |
1048 | exceed that specified uncompressed size:\r |
1049 | \r |
1050 | if (unpackSizeDefined && unpackSize == 0)\r |
1051 | return LZMA_RES_ERROR;\r |
1052 | \r |
1053 | Also the decoder must check that "rep0" value is not larger than dictionary size\r |
1054 | and is not larger than the number of already decoded bytes:\r |
1055 | \r |
1056 | if (rep0 >= dictSize || !OutWindow.CheckDistance(rep0))\r |
1057 | return LZMA_RES_ERROR;\r |
1058 | \r |
1059 | Then the decoder must copy match bytes as described in \r |
1060 | "The match symbols copying" section.\r |
1061 | \r |
1062 | \r |
1063 | Rep Match\r |
1064 | ---------\r |
1065 | \r |
1066 | If the LZMA decoder has decoded the value "1" with IsRep[state] variable,\r |
1067 | we have "Rep Match" type.\r |
1068 | \r |
1069 | At first the LZMA decoder must check that it doesn't exceed \r |
1070 | specified uncompressed size:\r |
1071 | \r |
1072 | if (unpackSizeDefined && unpackSize == 0)\r |
1073 | return LZMA_RES_ERROR;\r |
1074 | \r |
1075 | Also the decoder must return error, if the LZ window is empty:\r |
1076 | \r |
1077 | if (OutWindow.IsEmpty())\r |
1078 | return LZMA_RES_ERROR;\r |
1079 | \r |
1080 | If the match type is "Rep Match", the decoder uses one of the 4 variables of\r |
1081 | distance history table to get the value of distance for current match.\r |
1082 | And there are 4 corresponding ways of decoding flow. \r |
1083 | \r |
1084 | The decoder updates the distance history with the following scheme \r |
1085 | depending from type of match:\r |
1086 | \r |
1087 | - "Rep Match 0" or "Short Rep Match":\r |
1088 | ; LZMA doesn't update the distance history \r |
1089 | \r |
1090 | - "Rep Match 1":\r |
1091 | UInt32 dist = rep1;\r |
1092 | rep1 = rep0;\r |
1093 | rep0 = dist;\r |
1094 | \r |
1095 | - "Rep Match 2":\r |
1096 | UInt32 dist = rep2;\r |
1097 | rep2 = rep1;\r |
1098 | rep1 = rep0;\r |
1099 | rep0 = dist;\r |
1100 | \r |
1101 | - "Rep Match 3":\r |
1102 | UInt32 dist = rep3;\r |
1103 | rep3 = rep2;\r |
1104 | rep2 = rep1;\r |
1105 | rep1 = rep0;\r |
1106 | rep0 = dist;\r |
1107 | \r |
1108 | Then the decoder decodes exact subtype of "Rep Match" using "IsRepG0", "IsRep0Long",\r |
1109 | "IsRepG1", "IsRepG2".\r |
1110 | \r |
1111 | If the subtype is "Short Rep Match", the decoder updates the state, puts \r |
1112 | the one byte from window to current position in window and goes to next \r |
1113 | MATCH/LITERAL symbol (the begin of main loop):\r |
1114 | \r |
1115 | state = UpdateState_ShortRep(state);\r |
1116 | OutWindow.PutByte(OutWindow.GetByte(rep0 + 1));\r |
1117 | unpackSize--;\r |
1118 | continue;\r |
1119 | \r |
1120 | In other cases (Rep Match 0/1/2/3), it decodes the zero-based \r |
1121 | length of match with "RepLenDecoder" decoder:\r |
1122 | \r |
1123 | len = RepLenDecoder.Decode(&RangeDec, posState);\r |
1124 | \r |
1125 | Then it updates the state:\r |
1126 | \r |
1127 | state = UpdateState_Rep(state);\r |
1128 | \r |
1129 | Then the decoder must copy match bytes as described in \r |
1130 | "The Match symbols copying" section.\r |
1131 | \r |
1132 | \r |
1133 | The match symbols copying\r |
1134 | -------------------------\r |
1135 | \r |
1136 | If we have the match (Simple Match or Rep Match 0/1/2/3), the decoder must\r |
1137 | copy the sequence of bytes with calculated match distance and match length.\r |
1138 | If uncompressed size is defined, LZMA decoder must check that it doesn't \r |
1139 | exceed that specified uncompressed size:\r |
1140 | \r |
1141 | len += kMatchMinLen;\r |
1142 | bool isError = false;\r |
1143 | if (unpackSizeDefined && unpackSize < len)\r |
1144 | {\r |
1145 | len = (unsigned)unpackSize;\r |
1146 | isError = true;\r |
1147 | }\r |
1148 | OutWindow.CopyMatch(rep0 + 1, len);\r |
1149 | unpackSize -= len;\r |
1150 | if (isError)\r |
1151 | return LZMA_RES_ERROR;\r |
1152 | \r |
1153 | Then the decoder must go to the begin of main loop to decode next MATCH or LITERAL.\r |
1154 | \r |
1155 | \r |
1156 | \r |
1157 | NOTES\r |
1158 | -----\r |
1159 | \r |
1160 | This specification doesn't describe the variant of decoder implementation \r |
1161 | that supports partial decoding. Such partial decoding case can require some \r |
1162 | changes in "end of stream" condition checks code. Also such code \r |
1163 | can use additional status codes, returned by decoder.\r |
1164 | \r |
1165 | This specification uses C++ code with templates to simplify describing.\r |
1166 | The optimized version of LZMA decoder doesn't need templates.\r |
1167 | Such optimized version can use just two arrays of CProb variables:\r |
1168 | 1) The dynamic array of CProb variables allocated for the Literal Decoder.\r |
1169 | 2) The one common array that contains all other CProb variables.\r |
1170 | \r |
1171 | \r |
1172 | References: \r |
1173 | \r |
1174 | 1. G. N. N. Martin, Range encoding: an algorithm for removing redundancy \r |
1175 | from a digitized message, Video & Data Recording Conference, \r |
1176 | Southampton, UK, July 24-27, 1979.\r |