1 | |
---|
2 | |
---|
3 | |
---|
4 | |
---|
5 | |
---|
6 | |
---|
7 | Network Working Group P. Deutsch |
---|
8 | Request for Comments: 1950 Aladdin Enterprises |
---|
9 | Category: Informational J-L. Gailly |
---|
10 | Info-ZIP |
---|
11 | May 1996 |
---|
12 | |
---|
13 | |
---|
14 | ZLIB Compressed Data Format Specification version 3.3 |
---|
15 | |
---|
16 | Status of This Memo |
---|
17 | |
---|
18 | This memo provides information for the Internet community. This memo |
---|
19 | does not specify an Internet standard of any kind. Distribution of |
---|
20 | this memo is unlimited. |
---|
21 | |
---|
22 | IESG Note: |
---|
23 | |
---|
24 | The IESG takes no position on the validity of any Intellectual |
---|
25 | Property Rights statements contained in this document. |
---|
26 | |
---|
27 | Notices |
---|
28 | |
---|
29 | Copyright (c) 1996 L. Peter Deutsch and Jean-Loup Gailly |
---|
30 | |
---|
31 | Permission is granted to copy and distribute this document for any |
---|
32 | purpose and without charge, including translations into other |
---|
33 | languages and incorporation into compilations, provided that the |
---|
34 | copyright notice and this notice are preserved, and that any |
---|
35 | substantive changes or deletions from the original are clearly |
---|
36 | marked. |
---|
37 | |
---|
38 | A pointer to the latest version of this and related documentation in |
---|
39 | HTML format can be found at the URL |
---|
40 | <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>. |
---|
41 | |
---|
42 | Abstract |
---|
43 | |
---|
44 | This specification defines a lossless compressed data format. The |
---|
45 | data can be produced or consumed, even for an arbitrarily long |
---|
46 | sequentially presented input data stream, using only an a priori |
---|
47 | bounded amount of intermediate storage. The format presently uses |
---|
48 | the DEFLATE compression method but can be easily extended to use |
---|
49 | other compression methods. It can be implemented readily in a manner |
---|
50 | not covered by patents. This specification also defines the ADLER-32 |
---|
51 | checksum (an extension and improvement of the Fletcher checksum), |
---|
52 | used for detection of data corruption, and provides an algorithm for |
---|
53 | computing it. |
---|
54 | |
---|
55 | |
---|
56 | |
---|
57 | |
---|
58 | Deutsch & Gailly Informational [Page 1] |
---|
59 | |
---|
60 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
---|
61 | |
---|
62 | |
---|
63 | Table of Contents |
---|
64 | |
---|
65 | 1. Introduction ................................................... 2 |
---|
66 | 1.1. Purpose ................................................... 2 |
---|
67 | 1.2. Intended audience ......................................... 3 |
---|
68 | 1.3. Scope ..................................................... 3 |
---|
69 | 1.4. Compliance ................................................ 3 |
---|
70 | 1.5. Definitions of terms and conventions used ................ 3 |
---|
71 | 1.6. Changes from previous versions ............................ 3 |
---|
72 | 2. Detailed specification ......................................... 3 |
---|
73 | 2.1. Overall conventions ....................................... 3 |
---|
74 | 2.2. Data format ............................................... 4 |
---|
75 | 2.3. Compliance ................................................ 7 |
---|
76 | 3. References ..................................................... 7 |
---|
77 | 4. Source code .................................................... 8 |
---|
78 | 5. Security Considerations ........................................ 8 |
---|
79 | 6. Acknowledgements ............................................... 8 |
---|
80 | 7. Authors' Addresses ............................................. 8 |
---|
81 | 8. Appendix: Rationale ............................................ 9 |
---|
82 | 9. Appendix: Sample code ..........................................10 |
---|
83 | |
---|
84 | 1. Introduction |
---|
85 | |
---|
86 | 1.1. Purpose |
---|
87 | |
---|
88 | The purpose of this specification is to define a lossless |
---|
89 | compressed data format that: |
---|
90 | |
---|
91 | * Is independent of CPU type, operating system, file system, |
---|
92 | and character set, and hence can be used for interchange; |
---|
93 | |
---|
94 | * Can be produced or consumed, even for an arbitrarily long |
---|
95 | sequentially presented input data stream, using only an a |
---|
96 | priori bounded amount of intermediate storage, and hence can |
---|
97 | be used in data communications or similar structures such as |
---|
98 | Unix filters; |
---|
99 | |
---|
100 | * Can use a number of different compression methods; |
---|
101 | |
---|
102 | * Can be implemented readily in a manner not covered by |
---|
103 | patents, and hence can be practiced freely. |
---|
104 | |
---|
105 | The data format defined by this specification does not attempt to |
---|
106 | allow random access to compressed data. |
---|
107 | |
---|
108 | |
---|
109 | |
---|
110 | |
---|
111 | |
---|
112 | |
---|
113 | |
---|
114 | Deutsch & Gailly Informational [Page 2] |
---|
115 | |
---|
116 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
---|
117 | |
---|
118 | |
---|
119 | 1.2. Intended audience |
---|
120 | |
---|
121 | This specification is intended for use by implementors of software |
---|
122 | to compress data into zlib format and/or decompress data from zlib |
---|
123 | format. |
---|
124 | |
---|
125 | The text of the specification assumes a basic background in |
---|
126 | programming at the level of bits and other primitive data |
---|
127 | representations. |
---|
128 | |
---|
129 | 1.3. Scope |
---|
130 | |
---|
131 | The specification specifies a compressed data format that can be |
---|
132 | used for in-memory compression of a sequence of arbitrary bytes. |
---|
133 | |
---|
134 | 1.4. Compliance |
---|
135 | |
---|
136 | Unless otherwise indicated below, a compliant decompressor must be |
---|
137 | able to accept and decompress any data set that conforms to all |
---|
138 | the specifications presented here; a compliant compressor must |
---|
139 | produce data sets that conform to all the specifications presented |
---|
140 | here. |
---|
141 | |
---|
142 | 1.5. Definitions of terms and conventions used |
---|
143 | |
---|
144 | byte: 8 bits stored or transmitted as a unit (same as an octet). |
---|
145 | (For this specification, a byte is exactly 8 bits, even on |
---|
146 | machines which store a character on a number of bits different |
---|
147 | from 8.) See below, for the numbering of bits within a byte. |
---|
148 | |
---|
149 | 1.6. Changes from previous versions |
---|
150 | |
---|
151 | Version 3.1 was the first public release of this specification. |
---|
152 | In version 3.2, some terminology was changed and the Adler-32 |
---|
153 | sample code was rewritten for clarity. In version 3.3, the |
---|
154 | support for a preset dictionary was introduced, and the |
---|
155 | specification was converted to RFC style. |
---|
156 | |
---|
157 | 2. Detailed specification |
---|
158 | |
---|
159 | 2.1. Overall conventions |
---|
160 | |
---|
161 | In the diagrams below, a box like this: |
---|
162 | |
---|
163 | +---+ |
---|
164 | | | <-- the vertical bars might be missing |
---|
165 | +---+ |
---|
166 | |
---|
167 | |
---|
168 | |
---|
169 | |
---|
170 | Deutsch & Gailly Informational [Page 3] |
---|
171 | |
---|
172 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
---|
173 | |
---|
174 | |
---|
175 | represents one byte; a box like this: |
---|
176 | |
---|
177 | +==============+ |
---|
178 | | | |
---|
179 | +==============+ |
---|
180 | |
---|
181 | represents a variable number of bytes. |
---|
182 | |
---|
183 | Bytes stored within a computer do not have a "bit order", since |
---|
184 | they are always treated as a unit. However, a byte considered as |
---|
185 | an integer between 0 and 255 does have a most- and least- |
---|
186 | significant bit, and since we write numbers with the most- |
---|
187 | significant digit on the left, we also write bytes with the most- |
---|
188 | significant bit on the left. In the diagrams below, we number the |
---|
189 | bits of a byte so that bit 0 is the least-significant bit, i.e., |
---|
190 | the bits are numbered: |
---|
191 | |
---|
192 | +--------+ |
---|
193 | |76543210| |
---|
194 | +--------+ |
---|
195 | |
---|
196 | Within a computer, a number may occupy multiple bytes. All |
---|
197 | multi-byte numbers in the format described here are stored with |
---|
198 | the MOST-significant byte first (at the lower memory address). |
---|
199 | For example, the decimal number 520 is stored as: |
---|
200 | |
---|
201 | 0 1 |
---|
202 | +--------+--------+ |
---|
203 | |00000010|00001000| |
---|
204 | +--------+--------+ |
---|
205 | ^ ^ |
---|
206 | | | |
---|
207 | | + less significant byte = 8 |
---|
208 | + more significant byte = 2 x 256 |
---|
209 | |
---|
210 | 2.2. Data format |
---|
211 | |
---|
212 | A zlib stream has the following structure: |
---|
213 | |
---|
214 | 0 1 |
---|
215 | +---+---+ |
---|
216 | |CMF|FLG| (more-->) |
---|
217 | +---+---+ |
---|
218 | |
---|
219 | |
---|
220 | |
---|
221 | |
---|
222 | |
---|
223 | |
---|
224 | |
---|
225 | |
---|
226 | Deutsch & Gailly Informational [Page 4] |
---|
227 | |
---|
228 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
---|
229 | |
---|
230 | |
---|
231 | (if FLG.FDICT set) |
---|
232 | |
---|
233 | 0 1 2 3 |
---|
234 | +---+---+---+---+ |
---|
235 | | DICTID | (more-->) |
---|
236 | +---+---+---+---+ |
---|
237 | |
---|
238 | +=====================+---+---+---+---+ |
---|
239 | |...compressed data...| ADLER32 | |
---|
240 | +=====================+---+---+---+---+ |
---|
241 | |
---|
242 | Any data which may appear after ADLER32 are not part of the zlib |
---|
243 | stream. |
---|
244 | |
---|
245 | CMF (Compression Method and flags) |
---|
246 | This byte is divided into a 4-bit compression method and a 4- |
---|
247 | bit information field depending on the compression method. |
---|
248 | |
---|
249 | bits 0 to 3 CM Compression method |
---|
250 | bits 4 to 7 CINFO Compression info |
---|
251 | |
---|
252 | CM (Compression method) |
---|
253 | This identifies the compression method used in the file. CM = 8 |
---|
254 | denotes the "deflate" compression method with a window size up |
---|
255 | to 32K. This is the method used by gzip and PNG (see |
---|
256 | references [1] and [2] in Chapter 3, below, for the reference |
---|
257 | documents). CM = 15 is reserved. It might be used in a future |
---|
258 | version of this specification to indicate the presence of an |
---|
259 | extra field before the compressed data. |
---|
260 | |
---|
261 | CINFO (Compression info) |
---|
262 | For CM = 8, CINFO is the base-2 logarithm of the LZ77 window |
---|
263 | size, minus eight (CINFO=7 indicates a 32K window size). Values |
---|
264 | of CINFO above 7 are not allowed in this version of the |
---|
265 | specification. CINFO is not defined in this specification for |
---|
266 | CM not equal to 8. |
---|
267 | |
---|
268 | FLG (FLaGs) |
---|
269 | This flag byte is divided as follows: |
---|
270 | |
---|
271 | bits 0 to 4 FCHECK (check bits for CMF and FLG) |
---|
272 | bit 5 FDICT (preset dictionary) |
---|
273 | bits 6 to 7 FLEVEL (compression level) |
---|
274 | |
---|
275 | The FCHECK value must be such that CMF and FLG, when viewed as |
---|
276 | a 16-bit unsigned integer stored in MSB order (CMF*256 + FLG), |
---|
277 | is a multiple of 31. |
---|
278 | |
---|
279 | |
---|
280 | |
---|
281 | |
---|
282 | Deutsch & Gailly Informational [Page 5] |
---|
283 | |
---|
284 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
---|
285 | |
---|
286 | |
---|
287 | FDICT (Preset dictionary) |
---|
288 | If FDICT is set, a DICT dictionary identifier is present |
---|
289 | immediately after the FLG byte. The dictionary is a sequence of |
---|
290 | bytes which are initially fed to the compressor without |
---|
291 | producing any compressed output. DICT is the Adler-32 checksum |
---|
292 | of this sequence of bytes (see the definition of ADLER32 |
---|
293 | below). The decompressor can use this identifier to determine |
---|
294 | which dictionary has been used by the compressor. |
---|
295 | |
---|
296 | FLEVEL (Compression level) |
---|
297 | These flags are available for use by specific compression |
---|
298 | methods. The "deflate" method (CM = 8) sets these flags as |
---|
299 | follows: |
---|
300 | |
---|
301 | 0 - compressor used fastest algorithm |
---|
302 | 1 - compressor used fast algorithm |
---|
303 | 2 - compressor used default algorithm |
---|
304 | 3 - compressor used maximum compression, slowest algorithm |
---|
305 | |
---|
306 | The information in FLEVEL is not needed for decompression; it |
---|
307 | is there to indicate if recompression might be worthwhile. |
---|
308 | |
---|
309 | compressed data |
---|
310 | For compression method 8, the compressed data is stored in the |
---|
311 | deflate compressed data format as described in the document |
---|
312 | "DEFLATE Compressed Data Format Specification" by L. Peter |
---|
313 | Deutsch. (See reference [3] in Chapter 3, below) |
---|
314 | |
---|
315 | Other compressed data formats are not specified in this version |
---|
316 | of the zlib specification. |
---|
317 | |
---|
318 | ADLER32 (Adler-32 checksum) |
---|
319 | This contains a checksum value of the uncompressed data |
---|
320 | (excluding any dictionary data) computed according to Adler-32 |
---|
321 | algorithm. This algorithm is a 32-bit extension and improvement |
---|
322 | of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073 |
---|
323 | standard. See references [4] and [5] in Chapter 3, below) |
---|
324 | |
---|
325 | Adler-32 is composed of two sums accumulated per byte: s1 is |
---|
326 | the sum of all bytes, s2 is the sum of all s1 values. Both sums |
---|
327 | are done modulo 65521. s1 is initialized to 1, s2 to zero. The |
---|
328 | Adler-32 checksum is stored as s2*65536 + s1 in most- |
---|
329 | significant-byte first (network) order. |
---|
330 | |
---|
331 | |
---|
332 | |
---|
333 | |
---|
334 | |
---|
335 | |
---|
336 | |
---|
337 | |
---|
338 | Deutsch & Gailly Informational [Page 6] |
---|
339 | |
---|
340 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
---|
341 | |
---|
342 | |
---|
343 | 2.3. Compliance |
---|
344 | |
---|
345 | A compliant compressor must produce streams with correct CMF, FLG |
---|
346 | and ADLER32, but need not support preset dictionaries. When the |
---|
347 | zlib data format is used as part of another standard data format, |
---|
348 | the compressor may use only preset dictionaries that are specified |
---|
349 | by this other data format. If this other format does not use the |
---|
350 | preset dictionary feature, the compressor must not set the FDICT |
---|
351 | flag. |
---|
352 | |
---|
353 | A compliant decompressor must check CMF, FLG, and ADLER32, and |
---|
354 | provide an error indication if any of these have incorrect values. |
---|
355 | A compliant decompressor must give an error indication if CM is |
---|
356 | not one of the values defined in this specification (only the |
---|
357 | value 8 is permitted in this version), since another value could |
---|
358 | indicate the presence of new features that would cause subsequent |
---|
359 | data to be interpreted incorrectly. A compliant decompressor must |
---|
360 | give an error indication if FDICT is set and DICTID is not the |
---|
361 | identifier of a known preset dictionary. A decompressor may |
---|
362 | ignore FLEVEL and still be compliant. When the zlib data format |
---|
363 | is being used as a part of another standard format, a compliant |
---|
364 | decompressor must support all the preset dictionaries specified by |
---|
365 | the other format. When the other format does not use the preset |
---|
366 | dictionary feature, a compliant decompressor must reject any |
---|
367 | stream in which the FDICT flag is set. |
---|
368 | |
---|
369 | 3. References |
---|
370 | |
---|
371 | [1] Deutsch, L.P.,"GZIP Compressed Data Format Specification", |
---|
372 | available in ftp://ftp.uu.net/pub/archiving/zip/doc/ |
---|
373 | |
---|
374 | [2] Thomas Boutell, "PNG (Portable Network Graphics) specification", |
---|
375 | available in ftp://ftp.uu.net/graphics/png/documents/ |
---|
376 | |
---|
377 | [3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification", |
---|
378 | available in ftp://ftp.uu.net/pub/archiving/zip/doc/ |
---|
379 | |
---|
380 | [4] Fletcher, J. G., "An Arithmetic Checksum for Serial |
---|
381 | Transmissions," IEEE Transactions on Communications, Vol. COM-30, |
---|
382 | No. 1, January 1982, pp. 247-252. |
---|
383 | |
---|
384 | [5] ITU-T Recommendation X.224, Annex D, "Checksum Algorithms," |
---|
385 | November, 1993, pp. 144, 145. (Available from |
---|
386 | gopher://info.itu.ch). ITU-T X.244 is also the same as ISO 8073. |
---|
387 | |
---|
388 | |
---|
389 | |
---|
390 | |
---|
391 | |
---|
392 | |
---|
393 | |
---|
394 | Deutsch & Gailly Informational [Page 7] |
---|
395 | |
---|
396 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
---|
397 | |
---|
398 | |
---|
399 | 4. Source code |
---|
400 | |
---|
401 | Source code for a C language implementation of a "zlib" compliant |
---|
402 | library is available at ftp://ftp.uu.net/pub/archiving/zip/zlib/. |
---|
403 | |
---|
404 | 5. Security Considerations |
---|
405 | |
---|
406 | A decoder that fails to check the ADLER32 checksum value may be |
---|
407 | subject to undetected data corruption. |
---|
408 | |
---|
409 | 6. Acknowledgements |
---|
410 | |
---|
411 | Trademarks cited in this document are the property of their |
---|
412 | respective owners. |
---|
413 | |
---|
414 | Jean-Loup Gailly and Mark Adler designed the zlib format and wrote |
---|
415 | the related software described in this specification. Glenn |
---|
416 | Randers-Pehrson converted this document to RFC and HTML format. |
---|
417 | |
---|
418 | 7. Authors' Addresses |
---|
419 | |
---|
420 | L. Peter Deutsch |
---|
421 | Aladdin Enterprises |
---|
422 | 203 Santa Margarita Ave. |
---|
423 | Menlo Park, CA 94025 |
---|
424 | |
---|
425 | Phone: (415) 322-0103 (AM only) |
---|
426 | FAX: (415) 322-1734 |
---|
427 | EMail: <ghost@aladdin.com> |
---|
428 | |
---|
429 | |
---|
430 | Jean-Loup Gailly |
---|
431 | |
---|
432 | EMail: <gzip@prep.ai.mit.edu> |
---|
433 | |
---|
434 | Questions about the technical content of this specification can be |
---|
435 | sent by email to |
---|
436 | |
---|
437 | Jean-Loup Gailly <gzip@prep.ai.mit.edu> and |
---|
438 | Mark Adler <madler@alumni.caltech.edu> |
---|
439 | |
---|
440 | Editorial comments on this specification can be sent by email to |
---|
441 | |
---|
442 | L. Peter Deutsch <ghost@aladdin.com> and |
---|
443 | Glenn Randers-Pehrson <randeg@alumni.rpi.edu> |
---|
444 | |
---|
445 | |
---|
446 | |
---|
447 | |
---|
448 | |
---|
449 | |
---|
450 | Deutsch & Gailly Informational [Page 8] |
---|
451 | |
---|
452 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
---|
453 | |
---|
454 | |
---|
455 | 8. Appendix: Rationale |
---|
456 | |
---|
457 | 8.1. Preset dictionaries |
---|
458 | |
---|
459 | A preset dictionary is specially useful to compress short input |
---|
460 | sequences. The compressor can take advantage of the dictionary |
---|
461 | context to encode the input in a more compact manner. The |
---|
462 | decompressor can be initialized with the appropriate context by |
---|
463 | virtually decompressing a compressed version of the dictionary |
---|
464 | without producing any output. However for certain compression |
---|
465 | algorithms such as the deflate algorithm this operation can be |
---|
466 | achieved without actually performing any decompression. |
---|
467 | |
---|
468 | The compressor and the decompressor must use exactly the same |
---|
469 | dictionary. The dictionary may be fixed or may be chosen among a |
---|
470 | certain number of predefined dictionaries, according to the kind |
---|
471 | of input data. The decompressor can determine which dictionary has |
---|
472 | been chosen by the compressor by checking the dictionary |
---|
473 | identifier. This document does not specify the contents of |
---|
474 | predefined dictionaries, since the optimal dictionaries are |
---|
475 | application specific. Standard data formats using this feature of |
---|
476 | the zlib specification must precisely define the allowed |
---|
477 | dictionaries. |
---|
478 | |
---|
479 | 8.2. The Adler-32 algorithm |
---|
480 | |
---|
481 | The Adler-32 algorithm is much faster than the CRC32 algorithm yet |
---|
482 | still provides an extremely low probability of undetected errors. |
---|
483 | |
---|
484 | The modulo on unsigned long accumulators can be delayed for 5552 |
---|
485 | bytes, so the modulo operation time is negligible. If the bytes |
---|
486 | are a, b, c, the second sum is 3a + 2b + c + 3, and so is position |
---|
487 | and order sensitive, unlike the first sum, which is just a |
---|
488 | checksum. That 65521 is prime is important to avoid a possible |
---|
489 | large class of two-byte errors that leave the check unchanged. |
---|
490 | (The Fletcher checksum uses 255, which is not prime and which also |
---|
491 | makes the Fletcher check insensitive to single byte changes 0 <-> |
---|
492 | 255.) |
---|
493 | |
---|
494 | The sum s1 is initialized to 1 instead of zero to make the length |
---|
495 | of the sequence part of s2, so that the length does not have to be |
---|
496 | checked separately. (Any sequence of zeroes has a Fletcher |
---|
497 | checksum of zero.) |
---|
498 | |
---|
499 | |
---|
500 | |
---|
501 | |
---|
502 | |
---|
503 | |
---|
504 | |
---|
505 | |
---|
506 | Deutsch & Gailly Informational [Page 9] |
---|
507 | |
---|
508 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
---|
509 | |
---|
510 | |
---|
511 | 9. Appendix: Sample code |
---|
512 | |
---|
513 | The following C code computes the Adler-32 checksum of a data buffer. |
---|
514 | It is written for clarity, not for speed. The sample code is in the |
---|
515 | ANSI C programming language. Non C users may find it easier to read |
---|
516 | with these hints: |
---|
517 | |
---|
518 | & Bitwise AND operator. |
---|
519 | >> Bitwise right shift operator. When applied to an |
---|
520 | unsigned quantity, as here, right shift inserts zero bit(s) |
---|
521 | at the left. |
---|
522 | << Bitwise left shift operator. Left shift inserts zero |
---|
523 | bit(s) at the right. |
---|
524 | ++ "n++" increments the variable n. |
---|
525 | % modulo operator: a % b is the remainder of a divided by b. |
---|
526 | |
---|
527 | #define BASE 65521 /* largest prime smaller than 65536 */ |
---|
528 | |
---|
529 | /* |
---|
530 | Update a running Adler-32 checksum with the bytes buf[0..len-1] |
---|
531 | and return the updated checksum. The Adler-32 checksum should be |
---|
532 | initialized to 1. |
---|
533 | |
---|
534 | Usage example: |
---|
535 | |
---|
536 | unsigned long adler = 1L; |
---|
537 | |
---|
538 | while (read_buffer(buffer, length) != EOF) { |
---|
539 | adler = update_adler32(adler, buffer, length); |
---|
540 | } |
---|
541 | if (adler != original_adler) error(); |
---|
542 | */ |
---|
543 | unsigned long update_adler32(unsigned long adler, |
---|
544 | unsigned char *buf, int len) |
---|
545 | { |
---|
546 | unsigned long s1 = adler & 0xffff; |
---|
547 | unsigned long s2 = (adler >> 16) & 0xffff; |
---|
548 | int n; |
---|
549 | |
---|
550 | for (n = 0; n < len; n++) { |
---|
551 | s1 = (s1 + buf[n]) % BASE; |
---|
552 | s2 = (s2 + s1) % BASE; |
---|
553 | } |
---|
554 | return (s2 << 16) + s1; |
---|
555 | } |
---|
556 | |
---|
557 | /* Return the adler32 of the bytes buf[0..len-1] */ |
---|
558 | |
---|
559 | |
---|
560 | |
---|
561 | |
---|
562 | Deutsch & Gailly Informational [Page 10] |
---|
563 | |
---|
564 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
---|
565 | |
---|
566 | |
---|
567 | unsigned long adler32(unsigned char *buf, int len) |
---|
568 | { |
---|
569 | return update_adler32(1L, buf, len); |
---|
570 | } |
---|
571 | |
---|
572 | |
---|
573 | |
---|
574 | |
---|
575 | |
---|
576 | |
---|
577 | |
---|
578 | |
---|
579 | |
---|
580 | |
---|
581 | |
---|
582 | |
---|
583 | |
---|
584 | |
---|
585 | |
---|
586 | |
---|
587 | |
---|
588 | |
---|
589 | |
---|
590 | |
---|
591 | |
---|
592 | |
---|
593 | |
---|
594 | |
---|
595 | |
---|
596 | |
---|
597 | |
---|
598 | |
---|
599 | |
---|
600 | |
---|
601 | |
---|
602 | |
---|
603 | |
---|
604 | |
---|
605 | |
---|
606 | |
---|
607 | |
---|
608 | |
---|
609 | |
---|
610 | |
---|
611 | |
---|
612 | |
---|
613 | |
---|
614 | |
---|
615 | |
---|
616 | |
---|
617 | |
---|
618 | Deutsch & Gailly Informational [Page 11] |
---|
619 | |
---|