authorgravatar for andrew@ziglang.orgAndrew Kelley <andrew@ziglang.org> 2022-12-18 23:12:45-05:00
committergravatar for noreply@github.comGitHub <noreply@github.com> 2022-12-18 23:12:45-05:00
logd1f61f2d6801240c593bcf5a3219020ecae0e736
tree15875ab5fae54f0be59401f30897b1228bcfe93e
parentaca9c74e80e106309b9783ff251ab0cdd3fb9626
parent8f98a2b90fe0834af34a943886c67855ba6c4f7b
signaturebadge-question-mark Signed by PGP key 4AEE18F83AFDEB23

Merge pull request #13999 from ziglang/gitattributes

Update gitattributes and re-enable std lib compression test on Windows

18 files changed, 1644 insertions(+), 1644 deletions(-)

.gitattributes+2-2
......@@ -1,9 +1,9 @@
11*.zig text eol=lf
22*.txt text eol=lf
33langref.html.in text eol=lf
4deps/SoftFloat-3e/*.txt text eol=crlf
4lib/std/compress/testdata/** binary
5lib/std/compress/deflate/testdata/** binary
56
6deps/** linguist-vendored
77lib/include/** linguist-vendored
88lib/libc/** linguist-vendored
99lib/libcxx/** linguist-vendored
build.zig+1-1
......@@ -106,7 +106,7 @@ pub fn build(b: *Builder) !void {
106106 .install_dir = .lib,
107107 .install_subdir = "zig",
108108 .exclude_extensions = &[_][]const u8{
109 // exclude files from lib/std/compress/
109 // exclude files from lib/std/compress/testdata
110110 ".gz",
111111 ".z.0",
112112 ".z.9",
ci/aarch64-windows.ps1+5
......@@ -31,6 +31,11 @@ if ((git rev-parse --is-shallow-repository) -eq "true") {
3131 git fetch --unshallow # `git describe` won't work on a shallow repo
3232}
3333
34# Fix files that have their gitattributes changed
35git rm -rf lib
36git restore --staged lib
37git checkout lib
38
3439Write-Output "Building from source..."
3540Remove-Item -Path 'build-release' -Recurse -Force -ErrorAction Ignore
3641New-Item -Path 'build-release' -ItemType Directory
lib/std/compress/deflate/huffman_bit_writer.zig-5
......@@ -848,11 +848,6 @@ test "writeBlockHuff" {
848848 // Tests huffman encoding against reference files to detect possible regressions.
849849 // If encoding/bit allocation changes you can regenerate these files
850850
851 if (builtin.os.tag == .windows) {
852 // https://github.com/ziglang/zig/issues/13892
853 return error.SkipZigTest;
854 }
855
856851 try testBlockHuff(
857852 "huffman-null-max.input",
858853 "huffman-null-max.golden",
lib/std/compress/gzip.zig+2-2
......@@ -172,8 +172,8 @@ fn testReader(data: []const u8, comptime expected: []const u8) !void {
172172// SHA256=164ef0897b4cbec63abf1b57f069f3599bd0fb7c72c2a4dee21bd7e03ec9af67
173173test "compressed data" {
174174 try testReader(
175 @embedFile("rfc1952.txt.gz"),
176 @embedFile("rfc1952.txt"),
175 @embedFile("testdata/rfc1952.txt.gz"),
176 @embedFile("testdata/rfc1952.txt"),
177177 );
178178}
179179
lib/std/compress/rfc1951.txt deleted-955
......@@ -1,955 +0,0 @@
1
2
3
4
5
6
7Network Working Group P. Deutsch
8Request for Comments: 1951 Aladdin Enterprises
9Category: Informational May 1996
10
11
12 DEFLATE Compressed Data Format Specification version 1.3
13
14Status of This Memo
15
16 This memo provides information for the Internet community. This memo
17 does not specify an Internet standard of any kind. Distribution of
18 this memo is unlimited.
19
20IESG Note:
21
22 The IESG takes no position on the validity of any Intellectual
23 Property Rights statements contained in this document.
24
25Notices
26
27 Copyright (c) 1996 L. Peter Deutsch
28
29 Permission is granted to copy and distribute this document for any
30 purpose and without charge, including translations into other
31 languages and incorporation into compilations, provided that the
32 copyright notice and this notice are preserved, and that any
33 substantive changes or deletions from the original are clearly
34 marked.
35
36 A pointer to the latest version of this and related documentation in
37 HTML format can be found at the URL
38 <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>.
39
40Abstract
41
42 This specification defines a lossless compressed data format that
43 compresses data using a combination of the LZ77 algorithm and Huffman
44 coding, with efficiency comparable to the best currently available
45 general-purpose compression methods. The data can be produced or
46 consumed, even for an arbitrarily long sequentially presented input
47 data stream, using only an a priori bounded amount of intermediate
48 storage. The format can be implemented readily in a manner not
49 covered by patents.
50
51
52
53
54
55
56
57
58Deutsch Informational [Page 1]
59
60RFC 1951 DEFLATE Compressed Data Format Specification May 1996
61
62
63Table 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 ............................ 4
72 2. Compressed representation overview ............................. 4
73 3. Detailed specification ......................................... 5
74 3.1. Overall conventions ....................................... 5
75 3.1.1. Packing into bytes .................................. 5
76 3.2. Compressed block format ................................... 6
77 3.2.1. Synopsis of prefix and Huffman coding ............... 6
78 3.2.2. Use of Huffman coding in the "deflate" format ....... 7
79 3.2.3. Details of block format ............................. 9
80 3.2.4. Non-compressed blocks (BTYPE=00) ................... 11
81 3.2.5. Compressed blocks (length and distance codes) ...... 11
82 3.2.6. Compression with fixed Huffman codes (BTYPE=01) .... 12
83 3.2.7. Compression with dynamic Huffman codes (BTYPE=10) .. 13
84 3.3. Compliance ............................................... 14
85 4. Compression algorithm details ................................. 14
86 5. References .................................................... 16
87 6. Security Considerations ....................................... 16
88 7. Source code ................................................... 16
89 8. Acknowledgements .............................................. 16
90 9. Author's Address .............................................. 17
91
921. Introduction
93
94 1.1. Purpose
95
96 The purpose of this specification is to define a lossless
97 compressed data format that:
98 * Is independent of CPU type, operating system, file system,
99 and character set, and hence can be used for interchange;
100 * Can be produced or consumed, even for an arbitrarily long
101 sequentially presented input data stream, using only an a
102 priori bounded amount of intermediate storage, and hence
103 can be used in data communications or similar structures
104 such as Unix filters;
105 * Compresses data with efficiency comparable to the best
106 currently available general-purpose compression methods,
107 and in particular considerably better than the "compress"
108 program;
109 * Can be implemented readily in a manner not covered by
110 patents, and hence can be practiced freely;
111
112
113
114Deutsch Informational [Page 2]
115
116RFC 1951 DEFLATE Compressed Data Format Specification May 1996
117
118
119 * Is compatible with the file format produced by the current
120 widely used gzip utility, in that conforming decompressors
121 will be able to read data produced by the existing gzip
122 compressor.
123
124 The data format defined by this specification does not attempt to:
125
126 * Allow random access to compressed data;
127 * Compress specialized data (e.g., raster graphics) as well
128 as the best currently available specialized algorithms.
129
130 A simple counting argument shows that no lossless compression
131 algorithm can compress every possible input data set. For the
132 format defined here, the worst case expansion is 5 bytes per 32K-
133 byte block, i.e., a size increase of 0.015% for large data sets.
134 English text usually compresses by a factor of 2.5 to 3;
135 executable files usually compress somewhat less; graphical data
136 such as raster images may compress much more.
137
138 1.2. Intended audience
139
140 This specification is intended for use by implementors of software
141 to compress data into "deflate" format and/or decompress data from
142 "deflate" format.
143
144 The text of the specification assumes a basic background in
145 programming at the level of bits and other primitive data
146 representations. Familiarity with the technique of Huffman coding
147 is helpful but not required.
148
149 1.3. Scope
150
151 The specification specifies a method for representing a sequence
152 of bytes as a (usually shorter) sequence of bits, and a method for
153 packing the latter bit sequence into bytes.
154
155 1.4. Compliance
156
157 Unless otherwise indicated below, a compliant decompressor must be
158 able to accept and decompress any data set that conforms to all
159 the specifications presented here; a compliant compressor must
160 produce data sets that conform to all the specifications presented
161 here.
162
163 1.5. Definitions of terms and conventions used
164
165 Byte: 8 bits stored or transmitted as a unit (same as an octet).
166 For this specification, a byte is exactly 8 bits, even on machines
167
168
169
170Deutsch Informational [Page 3]
171
172RFC 1951 DEFLATE Compressed Data Format Specification May 1996
173
174
175 which store a character on a number of bits different from eight.
176 See below, for the numbering of bits within a byte.
177
178 String: a sequence of arbitrary bytes.
179
180 1.6. Changes from previous versions
181
182 There have been no technical changes to the deflate format since
183 version 1.1 of this specification. In version 1.2, some
184 terminology was changed. Version 1.3 is a conversion of the
185 specification to RFC style.
186
1872. Compressed representation overview
188
189 A compressed data set consists of a series of blocks, corresponding
190 to successive blocks of input data. The block sizes are arbitrary,
191 except that non-compressible blocks are limited to 65,535 bytes.
192
193 Each block is compressed using a combination of the LZ77 algorithm
194 and Huffman coding. The Huffman trees for each block are independent
195 of those for previous or subsequent blocks; the LZ77 algorithm may
196 use a reference to a duplicated string occurring in a previous block,
197 up to 32K input bytes before.
198
199 Each block consists of two parts: a pair of Huffman code trees that
200 describe the representation of the compressed data part, and a
201 compressed data part. (The Huffman trees themselves are compressed
202 using Huffman encoding.) The compressed data consists of a series of
203 elements of two types: literal bytes (of strings that have not been
204 detected as duplicated within the previous 32K input bytes), and
205 pointers to duplicated strings, where a pointer is represented as a
206 pair <length, backward distance>. The representation used in the
207 "deflate" format limits distances to 32K bytes and lengths to 258
208 bytes, but does not limit the size of a block, except for
209 uncompressible blocks, which are limited as noted above.
210
211 Each type of value (literals, distances, and lengths) in the
212 compressed data is represented using a Huffman code, using one code
213 tree for literals and lengths and a separate code tree for distances.
214 The code trees for each block appear in a compact form just before
215 the compressed data for that block.
216
217
218
219
220
221
222
223
224
225
226Deutsch Informational [Page 4]
227
228RFC 1951 DEFLATE Compressed Data Format Specification May 1996
229
230
2313. Detailed specification
232
233 3.1. Overall conventions In the diagrams below, a box like this:
234
235 +---+
236 | | <-- the vertical bars might be missing
237 +---+
238
239 represents one byte; a box like this:
240
241 +==============+
242 | |
243 +==============+
244
245 represents a variable number of bytes.
246
247 Bytes stored within a computer do not have a "bit order", since
248 they are always treated as a unit. However, a byte considered as
249 an integer between 0 and 255 does have a most- and least-
250 significant bit, and since we write numbers with the most-
251 significant digit on the left, we also write bytes with the most-
252 significant bit on the left. In the diagrams below, we number the
253 bits of a byte so that bit 0 is the least-significant bit, i.e.,
254 the bits are numbered:
255
256 +--------+
257 |76543210|
258 +--------+
259
260 Within a computer, a number may occupy multiple bytes. All
261 multi-byte numbers in the format described here are stored with
262 the least-significant byte first (at the lower memory address).
263 For example, the decimal number 520 is stored as:
264
265 0 1
266 +--------+--------+
267 |00001000|00000010|
268 +--------+--------+
269 ^ ^
270 | |
271 | + more significant byte = 2 x 256
272 + less significant byte = 8
273
274 3.1.1. Packing into bytes
275
276 This document does not address the issue of the order in which
277 bits of a byte are transmitted on a bit-sequential medium,
278 since the final data format described here is byte- rather than
279
280
281
282Deutsch Informational [Page 5]
283
284RFC 1951 DEFLATE Compressed Data Format Specification May 1996
285
286
287 bit-oriented. However, we describe the compressed block format
288 in below, as a sequence of data elements of various bit
289 lengths, not a sequence of bytes. We must therefore specify
290 how to pack these data elements into bytes to form the final
291 compressed byte sequence:
292
293 * Data elements are packed into bytes in order of
294 increasing bit number within the byte, i.e., starting
295 with the least-significant bit of the byte.
296 * Data elements other than Huffman codes are packed
297 starting with the least-significant bit of the data
298 element.
299 * Huffman codes are packed starting with the most-
300 significant bit of the code.
301
302 In other words, if one were to print out the compressed data as
303 a sequence of bytes, starting with the first byte at the
304 *right* margin and proceeding to the *left*, with the most-
305 significant bit of each byte on the left as usual, one would be
306 able to parse the result from right to left, with fixed-width
307 elements in the correct MSB-to-LSB order and Huffman codes in
308 bit-reversed order (i.e., with the first bit of the code in the
309 relative LSB position).
310
311 3.2. Compressed block format
312
313 3.2.1. Synopsis of prefix and Huffman coding
314
315 Prefix coding represents symbols from an a priori known
316 alphabet by bit sequences (codes), one code for each symbol, in
317 a manner such that different symbols may be represented by bit
318 sequences of different lengths, but a parser can always parse
319 an encoded string unambiguously symbol-by-symbol.
320
321 We define a prefix code in terms of a binary tree in which the
322 two edges descending from each non-leaf node are labeled 0 and
323 1 and in which the leaf nodes correspond one-for-one with (are
324 labeled with) the symbols of the alphabet; then the code for a
325 symbol is the sequence of 0's and 1's on the edges leading from
326 the root to the leaf labeled with that symbol. For example:
327
328
329
330
331
332
333
334
335
336
337
338Deutsch Informational [Page 6]
339
340RFC 1951 DEFLATE Compressed Data Format Specification May 1996
341
342
343 /\ Symbol Code
344 0 1 ------ ----
345 / \ A 00
346 /\ B B 1
347 0 1 C 011
348 / \ D 010
349 A /\
350 0 1
351 / \
352 D C
353
354 A parser can decode the next symbol from an encoded input
355 stream by walking down the tree from the root, at each step
356 choosing the edge corresponding to the next input bit.
357
358 Given an alphabet with known symbol frequencies, the Huffman
359 algorithm allows the construction of an optimal prefix code
360 (one which represents strings with those symbol frequencies
361 using the fewest bits of any possible prefix codes for that
362 alphabet). Such a code is called a Huffman code. (See
363 reference [1] in Chapter 5, references for additional
364 information on Huffman codes.)
365
366 Note that in the "deflate" format, the Huffman codes for the
367 various alphabets must not exceed certain maximum code lengths.
368 This constraint complicates the algorithm for computing code
369 lengths from symbol frequencies. Again, see Chapter 5,
370 references for details.
371
372 3.2.2. Use of Huffman coding in the "deflate" format
373
374 The Huffman codes used for each alphabet in the "deflate"
375 format have two additional rules:
376
377 * All codes of a given bit length have lexicographically
378 consecutive values, in the same order as the symbols
379 they represent;
380
381 * Shorter codes lexicographically precede longer codes.
382
383
384
385
386
387
388
389
390
391
392
393
394Deutsch Informational [Page 7]
395
396RFC 1951 DEFLATE Compressed Data Format Specification May 1996
397
398
399 We could recode the example above to follow this rule as
400 follows, assuming that the order of the alphabet is ABCD:
401
402 Symbol Code
403 ------ ----
404 A 10
405 B 0
406 C 110
407 D 111
408
409 I.e., 0 precedes 10 which precedes 11x, and 110 and 111 are
410 lexicographically consecutive.
411
412 Given this rule, we can define the Huffman code for an alphabet
413 just by giving the bit lengths of the codes for each symbol of
414 the alphabet in order; this is sufficient to determine the
415 actual codes. In our example, the code is completely defined
416 by the sequence of bit lengths (2, 1, 3, 3). The following
417 algorithm generates the codes as integers, intended to be read
418 from most- to least-significant bit. The code lengths are
419 initially in tree[I].Len; the codes are produced in
420 tree[I].Code.
421
422 1) Count the number of codes for each code length. Let
423 bl_count[N] be the number of codes of length N, N >= 1.
424
425 2) Find the numerical value of the smallest code for each
426 code length:
427
428 code = 0;
429 bl_count[0] = 0;
430 for (bits = 1; bits <= MAX_BITS; bits++) {
431 code = (code + bl_count[bits-1]) << 1;
432 next_code[bits] = code;
433 }
434
435 3) Assign numerical values to all codes, using consecutive
436 values for all codes of the same length with the base
437 values determined at step 2. Codes that are never used
438 (which have a bit length of zero) must not be assigned a
439 value.
440
441 for (n = 0; n <= max_code; n++) {
442 len = tree[n].Len;
443 if (len != 0) {
444 tree[n].Code = next_code[len];
445 next_code[len]++;
446 }
447
448
449
450Deutsch Informational [Page 8]
451
452RFC 1951 DEFLATE Compressed Data Format Specification May 1996
453
454
455 }
456
457 Example:
458
459 Consider the alphabet ABCDEFGH, with bit lengths (3, 3, 3, 3,
460 3, 2, 4, 4). After step 1, we have:
461
462 N bl_count[N]
463 - -----------
464 2 1
465 3 5
466 4 2
467
468 Step 2 computes the following next_code values:
469
470 N next_code[N]
471 - ------------
472 1 0
473 2 0
474 3 2
475 4 14
476
477 Step 3 produces the following code values:
478
479 Symbol Length Code
480 ------ ------ ----
481 A 3 010
482 B 3 011
483 C 3 100
484 D 3 101
485 E 3 110
486 F 2 00
487 G 4 1110
488 H 4 1111
489
490 3.2.3. Details of block format
491
492 Each block of compressed data begins with 3 header bits
493 containing the following data:
494
495 first bit BFINAL
496 next 2 bits BTYPE
497
498 Note that the header bits do not necessarily begin on a byte
499 boundary, since a block does not necessarily occupy an integral
500 number of bytes.
501
502
503
504
505
506Deutsch Informational [Page 9]
507
508RFC 1951 DEFLATE Compressed Data Format Specification May 1996
509
510
511 BFINAL is set if and only if this is the last block of the data
512 set.
513
514 BTYPE specifies how the data are compressed, as follows:
515
516 00 - no compression
517 01 - compressed with fixed Huffman codes
518 10 - compressed with dynamic Huffman codes
519 11 - reserved (error)
520
521 The only difference between the two compressed cases is how the
522 Huffman codes for the literal/length and distance alphabets are
523 defined.
524
525 In all cases, the decoding algorithm for the actual data is as
526 follows:
527
528 do
529 read block header from input stream.
530 if stored with no compression
531 skip any remaining bits in current partially
532 processed byte
533 read LEN and NLEN (see next section)
534 copy LEN bytes of data to output
535 otherwise
536 if compressed with dynamic Huffman codes
537 read representation of code trees (see
538 subsection below)
539 loop (until end of block code recognized)
540 decode literal/length value from input stream
541 if value < 256
542 copy value (literal byte) to output stream
543 otherwise
544 if value = end of block (256)
545 break from loop
546 otherwise (value = 257..285)
547 decode distance from input stream
548
549 move backwards distance bytes in the output
550 stream, and copy length bytes from this
551 position to the output stream.
552 end loop
553 while not last block
554
555 Note that a duplicated string reference may refer to a string
556 in a previous block; i.e., the backward distance may cross one
557 or more block boundaries. However a distance cannot refer past
558 the beginning of the output stream. (An application using a
559
560
561
562Deutsch Informational [Page 10]
563
564RFC 1951 DEFLATE Compressed Data Format Specification May 1996
565
566
567 preset dictionary might discard part of the output stream; a
568 distance can refer to that part of the output stream anyway)
569 Note also that the referenced string may overlap the current
570 position; for example, if the last 2 bytes decoded have values
571 X and Y, a string reference with <length = 5, distance = 2>
572 adds X,Y,X,Y,X to the output stream.
573
574 We now specify each compression method in turn.
575
576 3.2.4. Non-compressed blocks (BTYPE=00)
577
578 Any bits of input up to the next byte boundary are ignored.
579 The rest of the block consists of the following information:
580
581 0 1 2 3 4...
582 +---+---+---+---+================================+
583 | LEN | NLEN |... LEN bytes of literal data...|
584 +---+---+---+---+================================+
585
586 LEN is the number of data bytes in the block. NLEN is the
587 one's complement of LEN.
588
589 3.2.5. Compressed blocks (length and distance codes)
590
591 As noted above, encoded data blocks in the "deflate" format
592 consist of sequences of symbols drawn from three conceptually
593 distinct alphabets: either literal bytes, from the alphabet of
594 byte values (0..255), or <length, backward distance> pairs,
595 where the length is drawn from (3..258) and the distance is
596 drawn from (1..32,768). In fact, the literal and length
597 alphabets are merged into a single alphabet (0..285), where
598 values 0..255 represent literal bytes, the value 256 indicates
599 end-of-block, and values 257..285 represent length codes
600 (possibly in conjunction with extra bits following the symbol
601 code) as follows:
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618Deutsch Informational [Page 11]
619
620RFC 1951 DEFLATE Compressed Data Format Specification May 1996
621
622
623 Extra Extra Extra
624 Code Bits Length(s) Code Bits Lengths Code Bits Length(s)
625 ---- ---- ------ ---- ---- ------- ---- ---- -------
626 257 0 3 267 1 15,16 277 4 67-82
627 258 0 4 268 1 17,18 278 4 83-98
628 259 0 5 269 2 19-22 279 4 99-114
629 260 0 6 270 2 23-26 280 4 115-130
630 261 0 7 271 2 27-30 281 5 131-162
631 262 0 8 272 2 31-34 282 5 163-194
632 263 0 9 273 3 35-42 283 5 195-226
633 264 0 10 274 3 43-50 284 5 227-257
634 265 1 11,12 275 3 51-58 285 0 258
635 266 1 13,14 276 3 59-66
636
637 The extra bits should be interpreted as a machine integer
638 stored with the most-significant bit first, e.g., bits 1110
639 represent the value 14.
640
641 Extra Extra Extra
642 Code Bits Dist Code Bits Dist Code Bits Distance
643 ---- ---- ---- ---- ---- ------ ---- ---- --------
644 0 0 1 10 4 33-48 20 9 1025-1536
645 1 0 2 11 4 49-64 21 9 1537-2048
646 2 0 3 12 5 65-96 22 10 2049-3072
647 3 0 4 13 5 97-128 23 10 3073-4096
648 4 1 5,6 14 6 129-192 24 11 4097-6144
649 5 1 7,8 15 6 193-256 25 11 6145-8192
650 6 2 9-12 16 7 257-384 26 12 8193-12288
651 7 2 13-16 17 7 385-512 27 12 12289-16384
652 8 3 17-24 18 8 513-768 28 13 16385-24576
653 9 3 25-32 19 8 769-1024 29 13 24577-32768
654
655 3.2.6. Compression with fixed Huffman codes (BTYPE=01)
656
657 The Huffman codes for the two alphabets are fixed, and are not
658 represented explicitly in the data. The Huffman code lengths
659 for the literal/length alphabet are:
660
661 Lit Value Bits Codes
662 --------- ---- -----
663 0 - 143 8 00110000 through
664 10111111
665 144 - 255 9 110010000 through
666 111111111
667 256 - 279 7 0000000 through
668 0010111
669 280 - 287 8 11000000 through
670 11000111
671
672
673
674Deutsch Informational [Page 12]
675
676RFC 1951 DEFLATE Compressed Data Format Specification May 1996
677
678
679 The code lengths are sufficient to generate the actual codes,
680 as described above; we show the codes in the table for added
681 clarity. Literal/length values 286-287 will never actually
682 occur in the compressed data, but participate in the code
683 construction.
684
685 Distance codes 0-31 are represented by (fixed-length) 5-bit
686 codes, with possible additional bits as shown in the table
687 shown in Paragraph 3.2.5, above. Note that distance codes 30-
688 31 will never actually occur in the compressed data.
689
690 3.2.7. Compression with dynamic Huffman codes (BTYPE=10)
691
692 The Huffman codes for the two alphabets appear in the block
693 immediately after the header bits and before the actual
694 compressed data, first the literal/length code and then the
695 distance code. Each code is defined by a sequence of code
696 lengths, as discussed in Paragraph 3.2.2, above. For even
697 greater compactness, the code length sequences themselves are
698 compressed using a Huffman code. The alphabet for code lengths
699 is as follows:
700
701 0 - 15: Represent code lengths of 0 - 15
702 16: Copy the previous code length 3 - 6 times.
703 The next 2 bits indicate repeat length
704 (0 = 3, ... , 3 = 6)
705 Example: Codes 8, 16 (+2 bits 11),
706 16 (+2 bits 10) will expand to
707 12 code lengths of 8 (1 + 6 + 5)
708 17: Repeat a code length of 0 for 3 - 10 times.
709 (3 bits of length)
710 18: Repeat a code length of 0 for 11 - 138 times
711 (7 bits of length)
712
713 A code length of 0 indicates that the corresponding symbol in
714 the literal/length or distance alphabet will not occur in the
715 block, and should not participate in the Huffman code
716 construction algorithm given earlier. If only one distance
717 code is used, it is encoded using one bit, not zero bits; in
718 this case there is a single code length of one, with one unused
719 code. One distance code of zero bits means that there are no
720 distance codes used at all (the data is all literals).
721
722 We can now define the format of the block:
723
724 5 Bits: HLIT, # of Literal/Length codes - 257 (257 - 286)
725 5 Bits: HDIST, # of Distance codes - 1 (1 - 32)
726 4 Bits: HCLEN, # of Code Length codes - 4 (4 - 19)
727
728
729
730Deutsch Informational [Page 13]
731
732RFC 1951 DEFLATE Compressed Data Format Specification May 1996
733
734
735 (HCLEN + 4) x 3 bits: code lengths for the code length
736 alphabet given just above, in the order: 16, 17, 18,
737 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15
738
739 These code lengths are interpreted as 3-bit integers
740 (0-7); as above, a code length of 0 means the
741 corresponding symbol (literal/length or distance code
742 length) is not used.
743
744 HLIT + 257 code lengths for the literal/length alphabet,
745 encoded using the code length Huffman code
746
747 HDIST + 1 code lengths for the distance alphabet,
748 encoded using the code length Huffman code
749
750 The actual compressed data of the block,
751 encoded using the literal/length and distance Huffman
752 codes
753
754 The literal/length symbol 256 (end of data),
755 encoded using the literal/length Huffman code
756
757 The code length repeat codes can cross from HLIT + 257 to the
758 HDIST + 1 code lengths. In other words, all code lengths form
759 a single sequence of HLIT + HDIST + 258 values.
760
761 3.3. Compliance
762
763 A compressor may limit further the ranges of values specified in
764 the previous section and still be compliant; for example, it may
765 limit the range of backward pointers to some value smaller than
766 32K. Similarly, a compressor may limit the size of blocks so that
767 a compressible block fits in memory.
768
769 A compliant decompressor must accept the full range of possible
770 values defined in the previous section, and must accept blocks of
771 arbitrary size.
772
7734. Compression algorithm details
774
775 While it is the intent of this document to define the "deflate"
776 compressed data format without reference to any particular
777 compression algorithm, the format is related to the compressed
778 formats produced by LZ77 (Lempel-Ziv 1977, see reference [2] below);
779 since many variations of LZ77 are patented, it is strongly
780 recommended that the implementor of a compressor follow the general
781 algorithm presented here, which is known not to be patented per se.
782 The material in this section is not part of the definition of the
783
784
785
786Deutsch Informational [Page 14]
787
788RFC 1951 DEFLATE Compressed Data Format Specification May 1996
789
790
791 specification per se, and a compressor need not follow it in order to
792 be compliant.
793
794 The compressor terminates a block when it determines that starting a
795 new block with fresh trees would be useful, or when the block size
796 fills up the compressor's block buffer.
797
798 The compressor uses a chained hash table to find duplicated strings,
799 using a hash function that operates on 3-byte sequences. At any
800 given point during compression, let XYZ be the next 3 input bytes to
801 be examined (not necessarily all different, of course). First, the
802 compressor examines the hash chain for XYZ. If the chain is empty,
803 the compressor simply writes out X as a literal byte and advances one
804 byte in the input. If the hash chain is not empty, indicating that
805 the sequence XYZ (or, if we are unlucky, some other 3 bytes with the
806 same hash function value) has occurred recently, the compressor
807 compares all strings on the XYZ hash chain with the actual input data
808 sequence starting at the current point, and selects the longest
809 match.
810
811 The compressor searches the hash chains starting with the most recent
812 strings, to favor small distances and thus take advantage of the
813 Huffman encoding. The hash chains are singly linked. There are no
814 deletions from the hash chains; the algorithm simply discards matches
815 that are too old. To avoid a worst-case situation, very long hash
816 chains are arbitrarily truncated at a certain length, determined by a
817 run-time parameter.
818
819 To improve overall compression, the compressor optionally defers the
820 selection of matches ("lazy matching"): after a match of length N has
821 been found, the compressor searches for a longer match starting at
822 the next input byte. If it finds a longer match, it truncates the
823 previous match to a length of one (thus producing a single literal
824 byte) and then emits the longer match. Otherwise, it emits the
825 original match, and, as described above, advances N bytes before
826 continuing.
827
828 Run-time parameters also control this "lazy match" procedure. If
829 compression ratio is most important, the compressor attempts a
830 complete second search regardless of the length of the first match.
831 In the normal case, if the current match is "long enough", the
832 compressor reduces the search for a longer match, thus speeding up
833 the process. If speed is most important, the compressor inserts new
834 strings in the hash table only when no match was found, or when the
835 match is not "too long". This degrades the compression ratio but
836 saves time since there are both fewer insertions and fewer searches.
837
838
839
840
841
842Deutsch Informational [Page 15]
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844RFC 1951 DEFLATE Compressed Data Format Specification May 1996
845
846
8475. References
848
849 [1] Huffman, D. A., "A Method for the Construction of Minimum
850 Redundancy Codes", Proceedings of the Institute of Radio
851 Engineers, September 1952, Volume 40, Number 9, pp. 1098-1101.
852
853 [2] Ziv J., Lempel A., "A Universal Algorithm for Sequential Data
854 Compression", IEEE Transactions on Information Theory, Vol. 23,
855 No. 3, pp. 337-343.
856
857 [3] Gailly, J.-L., and Adler, M., ZLIB documentation and sources,
858 available in ftp://ftp.uu.net/pub/archiving/zip/doc/
859
860 [4] Gailly, J.-L., and Adler, M., GZIP documentation and sources,
861 available as gzip-*.tar in ftp://prep.ai.mit.edu/pub/gnu/
862
863 [5] Schwartz, E. S., and Kallick, B. "Generating a canonical prefix
864 encoding." Comm. ACM, 7,3 (Mar. 1964), pp. 166-169.
865
866 [6] Hirschberg and Lelewer, "Efficient decoding of prefix codes,"
867 Comm. ACM, 33,4, April 1990, pp. 449-459.
868
8696. Security Considerations
870
871 Any data compression method involves the reduction of redundancy in
872 the data. Consequently, any corruption of the data is likely to have
873 severe effects and be difficult to correct. Uncompressed text, on
874 the other hand, will probably still be readable despite the presence
875 of some corrupted bytes.
876
877 It is recommended that systems using this data format provide some
878 means of validating the integrity of the compressed data. See
879 reference [3], for example.
880
8817. Source code
882
883 Source code for a C language implementation of a "deflate" compliant
884 compressor and decompressor is available within the zlib package at
885 ftp://ftp.uu.net/pub/archiving/zip/zlib/.
886
8878. Acknowledgements
888
889 Trademarks cited in this document are the property of their
890 respective owners.
891
892 Phil Katz designed the deflate format. Jean-Loup Gailly and Mark
893 Adler wrote the related software described in this specification.
894 Glenn Randers-Pehrson converted this document to RFC and HTML format.
895
896
897
898Deutsch Informational [Page 16]
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900RFC 1951 DEFLATE Compressed Data Format Specification May 1996
901
902
9039. Author's Address
904
905 L. Peter Deutsch
906 Aladdin Enterprises
907 203 Santa Margarita Ave.
908 Menlo Park, CA 94025
909
910 Phone: (415) 322-0103 (AM only)
911 FAX: (415) 322-1734
912 EMail: <ghost@aladdin.com>
913
914 Questions about the technical content of this specification can be
915 sent by email to:
916
917 Jean-Loup Gailly <gzip@prep.ai.mit.edu> and
918 Mark Adler <madler@alumni.caltech.edu>
919
920 Editorial comments on this specification can be sent by email to:
921
922 L. Peter Deutsch <ghost@aladdin.com> and
923 Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
924
925
926
927
928
929
930
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933
934
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1
2
3
4
5
6
7Network Working Group P. Deutsch
8Request for Comments: 1952 Aladdin Enterprises
9Category: Informational May 1996
10
11
12 GZIP file format specification version 4.3
13
14Status of This Memo
15
16 This memo provides information for the Internet community. This memo
17 does not specify an Internet standard of any kind. Distribution of
18 this memo is unlimited.
19
20IESG Note:
21
22 The IESG takes no position on the validity of any Intellectual
23 Property Rights statements contained in this document.
24
25Notices
26
27 Copyright (c) 1996 L. Peter Deutsch
28
29 Permission is granted to copy and distribute this document for any
30 purpose and without charge, including translations into other
31 languages and incorporation into compilations, provided that the
32 copyright notice and this notice are preserved, and that any
33 substantive changes or deletions from the original are clearly
34 marked.
35
36 A pointer to the latest version of this and related documentation in
37 HTML format can be found at the URL
38 <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>.
39
40Abstract
41
42 This specification defines a lossless compressed data format that is
43 compatible with the widely used GZIP utility. The format includes a
44 cyclic redundancy check value for detecting data corruption. The
45 format presently uses the DEFLATE method of compression but can be
46 easily extended to use other compression methods. The format can be
47 implemented readily in a manner not covered by patents.
48
49
50
51
52
53
54
55
56
57
58Deutsch Informational [Page 1]
59
60RFC 1952 GZIP File Format Specification May 1996
61
62
63Table 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 ......................................... 4
73 2.1. Overall conventions ....................................... 4
74 2.2. File format ............................................... 5
75 2.3. Member format ............................................. 5
76 2.3.1. Member header and trailer ........................... 6
77 2.3.1.1. Extra field ................................... 8
78 2.3.1.2. Compliance .................................... 9
79 3. References .................................................. 9
80 4. Security Considerations .................................... 10
81 5. Acknowledgements ........................................... 10
82 6. Author's Address ........................................... 10
83 7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
84 8. Appendix: Sample CRC Code .................................. 11
85
861. Introduction
87
88 1.1. Purpose
89
90 The purpose of this specification is to define a lossless
91 compressed data format that:
92
93 * Is independent of CPU type, operating system, file system,
94 and character set, and hence can be used for interchange;
95 * Can compress or decompress a data stream (as opposed to a
96 randomly accessible file) to produce another data stream,
97 using only an a priori bounded amount of intermediate
98 storage, and hence can be used in data communications or
99 similar structures such as Unix filters;
100 * Compresses data with efficiency comparable to the best
101 currently available general-purpose compression methods,
102 and in particular considerably better than the "compress"
103 program;
104 * Can be implemented readily in a manner not covered by
105 patents, and hence can be practiced freely;
106 * Is compatible with the file format produced by the current
107 widely used gzip utility, in that conforming decompressors
108 will be able to read data produced by the existing gzip
109 compressor.
110
111
112
113
114Deutsch Informational [Page 2]
115
116RFC 1952 GZIP File Format Specification May 1996
117
118
119 The data format defined by this specification does not attempt to:
120
121 * Provide random access to compressed data;
122 * Compress specialized data (e.g., raster graphics) as well as
123 the best currently available specialized algorithms.
124
125 1.2. Intended audience
126
127 This specification is intended for use by implementors of software
128 to compress data into gzip format and/or decompress data from gzip
129 format.
130
131 The text of the specification assumes a basic background in
132 programming at the level of bits and other primitive data
133 representations.
134
135 1.3. Scope
136
137 The specification specifies a compression method and a file format
138 (the latter assuming only that a file can store a sequence of
139 arbitrary bytes). It does not specify any particular interface to
140 a file system or anything about character sets or encodings
141 (except for file names and comments, which are optional).
142
143 1.4. Compliance
144
145 Unless otherwise indicated below, a compliant decompressor must be
146 able to accept and decompress any file that conforms to all the
147 specifications presented here; a compliant compressor must produce
148 files that conform to all the specifications presented here. The
149 material in the appendices is not part of the specification per se
150 and is not relevant to compliance.
151
152 1.5. Definitions of terms and conventions used
153
154 byte: 8 bits stored or transmitted as a unit (same as an octet).
155 (For this specification, a byte is exactly 8 bits, even on
156 machines which store a character on a number of bits different
157 from 8.) See below for the numbering of bits within a byte.
158
159 1.6. Changes from previous versions
160
161 There have been no technical changes to the gzip format since
162 version 4.1 of this specification. In version 4.2, some
163 terminology was changed, and the sample CRC code was rewritten for
164 clarity and to eliminate the requirement for the caller to do pre-
165 and post-conditioning. Version 4.3 is a conversion of the
166 specification to RFC style.
167
168
169
170Deutsch Informational [Page 3]
171
172RFC 1952 GZIP File Format Specification May 1996
173
174
1752. Detailed specification
176
177 2.1. Overall conventions
178
179 In the diagrams below, a box like this:
180
181 +---+
182 | | <-- the vertical bars might be missing
183 +---+
184
185 represents one byte; a box like this:
186
187 +==============+
188 | |
189 +==============+
190
191 represents a variable number of bytes.
192
193 Bytes stored within a computer do not have a "bit order", since
194 they are always treated as a unit. However, a byte considered as
195 an integer between 0 and 255 does have a most- and least-
196 significant bit, and since we write numbers with the most-
197 significant digit on the left, we also write bytes with the most-
198 significant bit on the left. In the diagrams below, we number the
199 bits of a byte so that bit 0 is the least-significant bit, i.e.,
200 the bits are numbered:
201
202 +--------+
203 |76543210|
204 +--------+
205
206 This document does not address the issue of the order in which
207 bits of a byte are transmitted on a bit-sequential medium, since
208 the data format described here is byte- rather than bit-oriented.
209
210 Within a computer, a number may occupy multiple bytes. All
211 multi-byte numbers in the format described here are stored with
212 the least-significant byte first (at the lower memory address).
213 For example, the decimal number 520 is stored as:
214
215 0 1
216 +--------+--------+
217 |00001000|00000010|
218 +--------+--------+
219 ^ ^
220 | |
221 | + more significant byte = 2 x 256
222 + less significant byte = 8
223
224
225
226Deutsch Informational [Page 4]
227
228RFC 1952 GZIP File Format Specification May 1996
229
230
231 2.2. File format
232
233 A gzip file consists of a series of "members" (compressed data
234 sets). The format of each member is specified in the following
235 section. The members simply appear one after another in the file,
236 with no additional information before, between, or after them.
237
238 2.3. Member format
239
240 Each member has the following structure:
241
242 +---+---+---+---+---+---+---+---+---+---+
243 |ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->)
244 +---+---+---+---+---+---+---+---+---+---+
245
246 (if FLG.FEXTRA set)
247
248 +---+---+=================================+
249 | XLEN |...XLEN bytes of "extra field"...| (more-->)
250 +---+---+=================================+
251
252 (if FLG.FNAME set)
253
254 +=========================================+
255 |...original file name, zero-terminated...| (more-->)
256 +=========================================+
257
258 (if FLG.FCOMMENT set)
259
260 +===================================+
261 |...file comment, zero-terminated...| (more-->)
262 +===================================+
263
264 (if FLG.FHCRC set)
265
266 +---+---+
267 | CRC16 |
268 +---+---+
269
270 +=======================+
271 |...compressed blocks...| (more-->)
272 +=======================+
273
274 0 1 2 3 4 5 6 7
275 +---+---+---+---+---+---+---+---+
276 | CRC32 | ISIZE |
277 +---+---+---+---+---+---+---+---+
278
279
280
281
282Deutsch Informational [Page 5]
283
284RFC 1952 GZIP File Format Specification May 1996
285
286
287 2.3.1. Member header and trailer
288
289 ID1 (IDentification 1)
290 ID2 (IDentification 2)
291 These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139
292 (0x8b, \213), to identify the file as being in gzip format.
293
294 CM (Compression Method)
295 This identifies the compression method used in the file. CM
296 = 0-7 are reserved. CM = 8 denotes the "deflate"
297 compression method, which is the one customarily used by
298 gzip and which is documented elsewhere.
299
300 FLG (FLaGs)
301 This flag byte is divided into individual bits as follows:
302
303 bit 0 FTEXT
304 bit 1 FHCRC
305 bit 2 FEXTRA
306 bit 3 FNAME
307 bit 4 FCOMMENT
308 bit 5 reserved
309 bit 6 reserved
310 bit 7 reserved
311
312 If FTEXT is set, the file is probably ASCII text. This is
313 an optional indication, which the compressor may set by
314 checking a small amount of the input data to see whether any
315 non-ASCII characters are present. In case of doubt, FTEXT
316 is cleared, indicating binary data. For systems which have
317 different file formats for ascii text and binary data, the
318 decompressor can use FTEXT to choose the appropriate format.
319 We deliberately do not specify the algorithm used to set
320 this bit, since a compressor always has the option of
321 leaving it cleared and a decompressor always has the option
322 of ignoring it and letting some other program handle issues
323 of data conversion.
324
325 If FHCRC is set, a CRC16 for the gzip header is present,
326 immediately before the compressed data. The CRC16 consists
327 of the two least significant bytes of the CRC32 for all
328 bytes of the gzip header up to and not including the CRC16.
329 [The FHCRC bit was never set by versions of gzip up to
330 1.2.4, even though it was documented with a different
331 meaning in gzip 1.2.4.]
332
333 If FEXTRA is set, optional extra fields are present, as
334 described in a following section.
335
336
337
338Deutsch Informational [Page 6]
339
340RFC 1952 GZIP File Format Specification May 1996
341
342
343 If FNAME is set, an original file name is present,
344 terminated by a zero byte. The name must consist of ISO
345 8859-1 (LATIN-1) characters; on operating systems using
346 EBCDIC or any other character set for file names, the name
347 must be translated to the ISO LATIN-1 character set. This
348 is the original name of the file being compressed, with any
349 directory components removed, and, if the file being
350 compressed is on a file system with case insensitive names,
351 forced to lower case. There is no original file name if the
352 data was compressed from a source other than a named file;
353 for example, if the source was stdin on a Unix system, there
354 is no file name.
355
356 If FCOMMENT is set, a zero-terminated file comment is
357 present. This comment is not interpreted; it is only
358 intended for human consumption. The comment must consist of
359 ISO 8859-1 (LATIN-1) characters. Line breaks should be
360 denoted by a single line feed character (10 decimal).
361
362 Reserved FLG bits must be zero.
363
364 MTIME (Modification TIME)
365 This gives the most recent modification time of the original
366 file being compressed. The time is in Unix format, i.e.,
367 seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this
368 may cause problems for MS-DOS and other systems that use
369 local rather than Universal time.) If the compressed data
370 did not come from a file, MTIME is set to the time at which
371 compression started. MTIME = 0 means no time stamp is
372 available.
373
374 XFL (eXtra FLags)
375 These flags are available for use by specific compression
376 methods. The "deflate" method (CM = 8) sets these flags as
377 follows:
378
379 XFL = 2 - compressor used maximum compression,
380 slowest algorithm
381 XFL = 4 - compressor used fastest algorithm
382
383 OS (Operating System)
384 This identifies the type of file system on which compression
385 took place. This may be useful in determining end-of-line
386 convention for text files. The currently defined values are
387 as follows:
388
389
390
391
392
393
394Deutsch Informational [Page 7]
395
396RFC 1952 GZIP File Format Specification May 1996
397
398
399 0 - FAT filesystem (MS-DOS, OS/2, NT/Win32)
400 1 - Amiga
401 2 - VMS (or OpenVMS)
402 3 - Unix
403 4 - VM/CMS
404 5 - Atari TOS
405 6 - HPFS filesystem (OS/2, NT)
406 7 - Macintosh
407 8 - Z-System
408 9 - CP/M
409 10 - TOPS-20
410 11 - NTFS filesystem (NT)
411 12 - QDOS
412 13 - Acorn RISCOS
413 255 - unknown
414
415 XLEN (eXtra LENgth)
416 If FLG.FEXTRA is set, this gives the length of the optional
417 extra field. See below for details.
418
419 CRC32 (CRC-32)
420 This contains a Cyclic Redundancy Check value of the
421 uncompressed data computed according to CRC-32 algorithm
422 used in the ISO 3309 standard and in section 8.1.1.6.2 of
423 ITU-T recommendation V.42. (See http://www.iso.ch for
424 ordering ISO documents. See gopher://info.itu.ch for an
425 online version of ITU-T V.42.)
426
427 ISIZE (Input SIZE)
428 This contains the size of the original (uncompressed) input
429 data modulo 2^32.
430
431 2.3.1.1. Extra field
432
433 If the FLG.FEXTRA bit is set, an "extra field" is present in
434 the header, with total length XLEN bytes. It consists of a
435 series of subfields, each of the form:
436
437 +---+---+---+---+==================================+
438 |SI1|SI2| LEN |... LEN bytes of subfield data ...|
439 +---+---+---+---+==================================+
440
441 SI1 and SI2 provide a subfield ID, typically two ASCII letters
442 with some mnemonic value. Jean-Loup Gailly
443 <gzip@prep.ai.mit.edu> is maintaining a registry of subfield
444 IDs; please send him any subfield ID you wish to use. Subfield
445 IDs with SI2 = 0 are reserved for future use. The following
446 IDs are currently defined:
447
448
449
450Deutsch Informational [Page 8]
451
452RFC 1952 GZIP File Format Specification May 1996
453
454
455 SI1 SI2 Data
456 ---------- ---------- ----
457 0x41 ('A') 0x70 ('P') Apollo file type information
458
459 LEN gives the length of the subfield data, excluding the 4
460 initial bytes.
461
462 2.3.1.2. Compliance
463
464 A compliant compressor must produce files with correct ID1,
465 ID2, CM, CRC32, and ISIZE, but may set all the other fields in
466 the fixed-length part of the header to default values (255 for
467 OS, 0 for all others). The compressor must set all reserved
468 bits to zero.
469
470 A compliant decompressor must check ID1, ID2, and CM, and
471 provide an error indication if any of these have incorrect
472 values. It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC
473 at least so it can skip over the optional fields if they are
474 present. It need not examine any other part of the header or
475 trailer; in particular, a decompressor may ignore FTEXT and OS
476 and always produce binary output, and still be compliant. A
477 compliant decompressor must give an error indication if any
478 reserved bit is non-zero, since such a bit could indicate the
479 presence of a new field that would cause subsequent data to be
480 interpreted incorrectly.
481
4823. References
483
484 [1] "Information Processing - 8-bit single-byte coded graphic
485 character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987).
486 The ISO 8859-1 (Latin-1) character set is a superset of 7-bit
487 ASCII. Files defining this character set are available as
488 iso_8859-1.* in ftp://ftp.uu.net/graphics/png/documents/
489
490 [2] ISO 3309
491
492 [3] ITU-T recommendation V.42
493
494 [4] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",
495 available in ftp://ftp.uu.net/pub/archiving/zip/doc/
496
497 [5] Gailly, J.-L., GZIP documentation, available as gzip-*.tar in
498 ftp://prep.ai.mit.edu/pub/gnu/
499
500 [6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table
501 Look-Up", Communications of the ACM, 31(8), pp.1008-1013.
502
503
504
505
506Deutsch Informational [Page 9]
507
508RFC 1952 GZIP File Format Specification May 1996
509
510
511 [7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal,
512 pp.118-133.
513
514 [8] ftp://ftp.adelaide.edu.au/pub/rocksoft/papers/crc_v3.txt,
515 describing the CRC concept.
516
5174. Security Considerations
518
519 Any data compression method involves the reduction of redundancy in
520 the data. Consequently, any corruption of the data is likely to have
521 severe effects and be difficult to correct. Uncompressed text, on
522 the other hand, will probably still be readable despite the presence
523 of some corrupted bytes.
524
525 It is recommended that systems using this data format provide some
526 means of validating the integrity of the compressed data, such as by
527 setting and checking the CRC-32 check value.
528
5295. Acknowledgements
530
531 Trademarks cited in this document are the property of their
532 respective owners.
533
534 Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler,
535 the related software described in this specification. Glenn
536 Randers-Pehrson converted this document to RFC and HTML format.
537
5386. Author's Address
539
540 L. Peter Deutsch
541 Aladdin Enterprises
542 203 Santa Margarita Ave.
543 Menlo Park, CA 94025
544
545 Phone: (415) 322-0103 (AM only)
546 FAX: (415) 322-1734
547 EMail: <ghost@aladdin.com>
548
549 Questions about the technical content of this specification can be
550 sent by email to:
551
552 Jean-Loup Gailly <gzip@prep.ai.mit.edu> and
553 Mark Adler <madler@alumni.caltech.edu>
554
555 Editorial comments on this specification can be sent by email to:
556
557 L. Peter Deutsch <ghost@aladdin.com> and
558 Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
559
560
561
562Deutsch Informational [Page 10]
563
564RFC 1952 GZIP File Format Specification May 1996
565
566
5677. Appendix: Jean-Loup Gailly's gzip utility
568
569 The most widely used implementation of gzip compression, and the
570 original documentation on which this specification is based, were
571 created by Jean-Loup Gailly <gzip@prep.ai.mit.edu>. Since this
572 implementation is a de facto standard, we mention some more of its
573 features here. Again, the material in this section is not part of
574 the specification per se, and implementations need not follow it to
575 be compliant.
576
577 When compressing or decompressing a file, gzip preserves the
578 protection, ownership, and modification time attributes on the local
579 file system, since there is no provision for representing protection
580 attributes in the gzip file format itself. Since the file format
581 includes a modification time, the gzip decompressor provides a
582 command line switch that assigns the modification time from the file,
583 rather than the local modification time of the compressed input, to
584 the decompressed output.
585
5868. Appendix: Sample CRC Code
587
588 The following sample code represents a practical implementation of
589 the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42
590 for a formal specification.)
591
592 The sample code is in the ANSI C programming language. Non C users
593 may find it easier to read with these hints:
594
595 & Bitwise AND operator.
596 ^ Bitwise exclusive-OR operator.
597 >> Bitwise right shift operator. When applied to an
598 unsigned quantity, as here, right shift inserts zero
599 bit(s) at the left.
600 ! Logical NOT operator.
601 ++ "n++" increments the variable n.
602 0xNNN 0x introduces a hexadecimal (base 16) constant.
603 Suffix L indicates a long value (at least 32 bits).
604
605 /* Table of CRCs of all 8-bit messages. */
606 unsigned long crc_table[256];
607
608 /* Flag: has the table been computed? Initially false. */
609 int crc_table_computed = 0;
610
611 /* Make the table for a fast CRC. */
612 void make_crc_table(void)
613 {
614 unsigned long c;
615
616
617
618Deutsch Informational [Page 11]
619
620RFC 1952 GZIP File Format Specification May 1996
621
622
623 int n, k;
624 for (n = 0; n < 256; n++) {
625 c = (unsigned long) n;
626 for (k = 0; k < 8; k++) {
627 if (c & 1) {
628 c = 0xedb88320L ^ (c >> 1);
629 } else {
630 c = c >> 1;
631 }
632 }
633 crc_table[n] = c;
634 }
635 crc_table_computed = 1;
636 }
637
638 /*
639 Update a running crc with the bytes buf[0..len-1] and return
640 the updated crc. The crc should be initialized to zero. Pre- and
641 post-conditioning (one's complement) is performed within this
642 function so it shouldn't be done by the caller. Usage example:
643
644 unsigned long crc = 0L;
645
646 while (read_buffer(buffer, length) != EOF) {
647 crc = update_crc(crc, buffer, length);
648 }
649 if (crc != original_crc) error();
650 */
651 unsigned long update_crc(unsigned long crc,
652 unsigned char *buf, int len)
653 {
654 unsigned long c = crc ^ 0xffffffffL;
655 int n;
656
657 if (!crc_table_computed)
658 make_crc_table();
659 for (n = 0; n < len; n++) {
660 c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
661 }
662 return c ^ 0xffffffffL;
663 }
664
665 /* Return the CRC of the bytes buf[0..len-1]. */
666 unsigned long crc(unsigned char *buf, int len)
667 {
668 return update_crc(0L, buf, len);
669 }
670
671
672
673
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1
2
3
4
5
6
7Network Working Group P. Deutsch
8Request for Comments: 1951 Aladdin Enterprises
9Category: Informational May 1996
10
11
12 DEFLATE Compressed Data Format Specification version 1.3
13
14Status of This Memo
15
16 This memo provides information for the Internet community. This memo
17 does not specify an Internet standard of any kind. Distribution of
18 this memo is unlimited.
19
20IESG Note:
21
22 The IESG takes no position on the validity of any Intellectual
23 Property Rights statements contained in this document.
24
25Notices
26
27 Copyright (c) 1996 L. Peter Deutsch
28
29 Permission is granted to copy and distribute this document for any
30 purpose and without charge, including translations into other
31 languages and incorporation into compilations, provided that the
32 copyright notice and this notice are preserved, and that any
33 substantive changes or deletions from the original are clearly
34 marked.
35
36 A pointer to the latest version of this and related documentation in
37 HTML format can be found at the URL
38 <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>.
39
40Abstract
41
42 This specification defines a lossless compressed data format that
43 compresses data using a combination of the LZ77 algorithm and Huffman
44 coding, with efficiency comparable to the best currently available
45 general-purpose compression methods. The data can be produced or
46 consumed, even for an arbitrarily long sequentially presented input
47 data stream, using only an a priori bounded amount of intermediate
48 storage. The format can be implemented readily in a manner not
49 covered by patents.
50
51
52
53
54
55
56
57
58Deutsch Informational [Page 1]
59
60RFC 1951 DEFLATE Compressed Data Format Specification May 1996
61
62
63Table 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 ............................ 4
72 2. Compressed representation overview ............................. 4
73 3. Detailed specification ......................................... 5
74 3.1. Overall conventions ....................................... 5
75 3.1.1. Packing into bytes .................................. 5
76 3.2. Compressed block format ................................... 6
77 3.2.1. Synopsis of prefix and Huffman coding ............... 6
78 3.2.2. Use of Huffman coding in the "deflate" format ....... 7
79 3.2.3. Details of block format ............................. 9
80 3.2.4. Non-compressed blocks (BTYPE=00) ................... 11
81 3.2.5. Compressed blocks (length and distance codes) ...... 11
82 3.2.6. Compression with fixed Huffman codes (BTYPE=01) .... 12
83 3.2.7. Compression with dynamic Huffman codes (BTYPE=10) .. 13
84 3.3. Compliance ............................................... 14
85 4. Compression algorithm details ................................. 14
86 5. References .................................................... 16
87 6. Security Considerations ....................................... 16
88 7. Source code ................................................... 16
89 8. Acknowledgements .............................................. 16
90 9. Author's Address .............................................. 17
91
921. Introduction
93
94 1.1. Purpose
95
96 The purpose of this specification is to define a lossless
97 compressed data format that:
98 * Is independent of CPU type, operating system, file system,
99 and character set, and hence can be used for interchange;
100 * Can be produced or consumed, even for an arbitrarily long
101 sequentially presented input data stream, using only an a
102 priori bounded amount of intermediate storage, and hence
103 can be used in data communications or similar structures
104 such as Unix filters;
105 * Compresses data with efficiency comparable to the best
106 currently available general-purpose compression methods,
107 and in particular considerably better than the "compress"
108 program;
109 * Can be implemented readily in a manner not covered by
110 patents, and hence can be practiced freely;
111
112
113
114Deutsch Informational [Page 2]
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116RFC 1951 DEFLATE Compressed Data Format Specification May 1996
117
118
119 * Is compatible with the file format produced by the current
120 widely used gzip utility, in that conforming decompressors
121 will be able to read data produced by the existing gzip
122 compressor.
123
124 The data format defined by this specification does not attempt to:
125
126 * Allow random access to compressed data;
127 * Compress specialized data (e.g., raster graphics) as well
128 as the best currently available specialized algorithms.
129
130 A simple counting argument shows that no lossless compression
131 algorithm can compress every possible input data set. For the
132 format defined here, the worst case expansion is 5 bytes per 32K-
133 byte block, i.e., a size increase of 0.015% for large data sets.
134 English text usually compresses by a factor of 2.5 to 3;
135 executable files usually compress somewhat less; graphical data
136 such as raster images may compress much more.
137
138 1.2. Intended audience
139
140 This specification is intended for use by implementors of software
141 to compress data into "deflate" format and/or decompress data from
142 "deflate" format.
143
144 The text of the specification assumes a basic background in
145 programming at the level of bits and other primitive data
146 representations. Familiarity with the technique of Huffman coding
147 is helpful but not required.
148
149 1.3. Scope
150
151 The specification specifies a method for representing a sequence
152 of bytes as a (usually shorter) sequence of bits, and a method for
153 packing the latter bit sequence into bytes.
154
155 1.4. Compliance
156
157 Unless otherwise indicated below, a compliant decompressor must be
158 able to accept and decompress any data set that conforms to all
159 the specifications presented here; a compliant compressor must
160 produce data sets that conform to all the specifications presented
161 here.
162
163 1.5. Definitions of terms and conventions used
164
165 Byte: 8 bits stored or transmitted as a unit (same as an octet).
166 For this specification, a byte is exactly 8 bits, even on machines
167
168
169
170Deutsch Informational [Page 3]
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172RFC 1951 DEFLATE Compressed Data Format Specification May 1996
173
174
175 which store a character on a number of bits different from eight.
176 See below, for the numbering of bits within a byte.
177
178 String: a sequence of arbitrary bytes.
179
180 1.6. Changes from previous versions
181
182 There have been no technical changes to the deflate format since
183 version 1.1 of this specification. In version 1.2, some
184 terminology was changed. Version 1.3 is a conversion of the
185 specification to RFC style.
186
1872. Compressed representation overview
188
189 A compressed data set consists of a series of blocks, corresponding
190 to successive blocks of input data. The block sizes are arbitrary,
191 except that non-compressible blocks are limited to 65,535 bytes.
192
193 Each block is compressed using a combination of the LZ77 algorithm
194 and Huffman coding. The Huffman trees for each block are independent
195 of those for previous or subsequent blocks; the LZ77 algorithm may
196 use a reference to a duplicated string occurring in a previous block,
197 up to 32K input bytes before.
198
199 Each block consists of two parts: a pair of Huffman code trees that
200 describe the representation of the compressed data part, and a
201 compressed data part. (The Huffman trees themselves are compressed
202 using Huffman encoding.) The compressed data consists of a series of
203 elements of two types: literal bytes (of strings that have not been
204 detected as duplicated within the previous 32K input bytes), and
205 pointers to duplicated strings, where a pointer is represented as a
206 pair <length, backward distance>. The representation used in the
207 "deflate" format limits distances to 32K bytes and lengths to 258
208 bytes, but does not limit the size of a block, except for
209 uncompressible blocks, which are limited as noted above.
210
211 Each type of value (literals, distances, and lengths) in the
212 compressed data is represented using a Huffman code, using one code
213 tree for literals and lengths and a separate code tree for distances.
214 The code trees for each block appear in a compact form just before
215 the compressed data for that block.
216
217
218
219
220
221
222
223
224
225
226Deutsch Informational [Page 4]
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228RFC 1951 DEFLATE Compressed Data Format Specification May 1996
229
230
2313. Detailed specification
232
233 3.1. Overall conventions In the diagrams below, a box like this:
234
235 +---+
236 | | <-- the vertical bars might be missing
237 +---+
238
239 represents one byte; a box like this:
240
241 +==============+
242 | |
243 +==============+
244
245 represents a variable number of bytes.
246
247 Bytes stored within a computer do not have a "bit order", since
248 they are always treated as a unit. However, a byte considered as
249 an integer between 0 and 255 does have a most- and least-
250 significant bit, and since we write numbers with the most-
251 significant digit on the left, we also write bytes with the most-
252 significant bit on the left. In the diagrams below, we number the
253 bits of a byte so that bit 0 is the least-significant bit, i.e.,
254 the bits are numbered:
255
256 +--------+
257 |76543210|
258 +--------+
259
260 Within a computer, a number may occupy multiple bytes. All
261 multi-byte numbers in the format described here are stored with
262 the least-significant byte first (at the lower memory address).
263 For example, the decimal number 520 is stored as:
264
265 0 1
266 +--------+--------+
267 |00001000|00000010|
268 +--------+--------+
269 ^ ^
270 | |
271 | + more significant byte = 2 x 256
272 + less significant byte = 8
273
274 3.1.1. Packing into bytes
275
276 This document does not address the issue of the order in which
277 bits of a byte are transmitted on a bit-sequential medium,
278 since the final data format described here is byte- rather than
279
280
281
282Deutsch Informational [Page 5]
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284RFC 1951 DEFLATE Compressed Data Format Specification May 1996
285
286
287 bit-oriented. However, we describe the compressed block format
288 in below, as a sequence of data elements of various bit
289 lengths, not a sequence of bytes. We must therefore specify
290 how to pack these data elements into bytes to form the final
291 compressed byte sequence:
292
293 * Data elements are packed into bytes in order of
294 increasing bit number within the byte, i.e., starting
295 with the least-significant bit of the byte.
296 * Data elements other than Huffman codes are packed
297 starting with the least-significant bit of the data
298 element.
299 * Huffman codes are packed starting with the most-
300 significant bit of the code.
301
302 In other words, if one were to print out the compressed data as
303 a sequence of bytes, starting with the first byte at the
304 *right* margin and proceeding to the *left*, with the most-
305 significant bit of each byte on the left as usual, one would be
306 able to parse the result from right to left, with fixed-width
307 elements in the correct MSB-to-LSB order and Huffman codes in
308 bit-reversed order (i.e., with the first bit of the code in the
309 relative LSB position).
310
311 3.2. Compressed block format
312
313 3.2.1. Synopsis of prefix and Huffman coding
314
315 Prefix coding represents symbols from an a priori known
316 alphabet by bit sequences (codes), one code for each symbol, in
317 a manner such that different symbols may be represented by bit
318 sequences of different lengths, but a parser can always parse
319 an encoded string unambiguously symbol-by-symbol.
320
321 We define a prefix code in terms of a binary tree in which the
322 two edges descending from each non-leaf node are labeled 0 and
323 1 and in which the leaf nodes correspond one-for-one with (are
324 labeled with) the symbols of the alphabet; then the code for a
325 symbol is the sequence of 0's and 1's on the edges leading from
326 the root to the leaf labeled with that symbol. For example:
327
328
329
330
331
332
333
334
335
336
337
338Deutsch Informational [Page 6]
339
340RFC 1951 DEFLATE Compressed Data Format Specification May 1996
341
342
343 /\ Symbol Code
344 0 1 ------ ----
345 / \ A 00
346 /\ B B 1
347 0 1 C 011
348 / \ D 010
349 A /\
350 0 1
351 / \
352 D C
353
354 A parser can decode the next symbol from an encoded input
355 stream by walking down the tree from the root, at each step
356 choosing the edge corresponding to the next input bit.
357
358 Given an alphabet with known symbol frequencies, the Huffman
359 algorithm allows the construction of an optimal prefix code
360 (one which represents strings with those symbol frequencies
361 using the fewest bits of any possible prefix codes for that
362 alphabet). Such a code is called a Huffman code. (See
363 reference [1] in Chapter 5, references for additional
364 information on Huffman codes.)
365
366 Note that in the "deflate" format, the Huffman codes for the
367 various alphabets must not exceed certain maximum code lengths.
368 This constraint complicates the algorithm for computing code
369 lengths from symbol frequencies. Again, see Chapter 5,
370 references for details.
371
372 3.2.2. Use of Huffman coding in the "deflate" format
373
374 The Huffman codes used for each alphabet in the "deflate"
375 format have two additional rules:
376
377 * All codes of a given bit length have lexicographically
378 consecutive values, in the same order as the symbols
379 they represent;
380
381 * Shorter codes lexicographically precede longer codes.
382
383
384
385
386
387
388
389
390
391
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394Deutsch Informational [Page 7]
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396RFC 1951 DEFLATE Compressed Data Format Specification May 1996
397
398
399 We could recode the example above to follow this rule as
400 follows, assuming that the order of the alphabet is ABCD:
401
402 Symbol Code
403 ------ ----
404 A 10
405 B 0
406 C 110
407 D 111
408
409 I.e., 0 precedes 10 which precedes 11x, and 110 and 111 are
410 lexicographically consecutive.
411
412 Given this rule, we can define the Huffman code for an alphabet
413 just by giving the bit lengths of the codes for each symbol of
414 the alphabet in order; this is sufficient to determine the
415 actual codes. In our example, the code is completely defined
416 by the sequence of bit lengths (2, 1, 3, 3). The following
417 algorithm generates the codes as integers, intended to be read
418 from most- to least-significant bit. The code lengths are
419 initially in tree[I].Len; the codes are produced in
420 tree[I].Code.
421
422 1) Count the number of codes for each code length. Let
423 bl_count[N] be the number of codes of length N, N >= 1.
424
425 2) Find the numerical value of the smallest code for each
426 code length:
427
428 code = 0;
429 bl_count[0] = 0;
430 for (bits = 1; bits <= MAX_BITS; bits++) {
431 code = (code + bl_count[bits-1]) << 1;
432 next_code[bits] = code;
433 }
434
435 3) Assign numerical values to all codes, using consecutive
436 values for all codes of the same length with the base
437 values determined at step 2. Codes that are never used
438 (which have a bit length of zero) must not be assigned a
439 value.
440
441 for (n = 0; n <= max_code; n++) {
442 len = tree[n].Len;
443 if (len != 0) {
444 tree[n].Code = next_code[len];
445 next_code[len]++;
446 }
447
448
449
450Deutsch Informational [Page 8]
451
452RFC 1951 DEFLATE Compressed Data Format Specification May 1996
453
454
455 }
456
457 Example:
458
459 Consider the alphabet ABCDEFGH, with bit lengths (3, 3, 3, 3,
460 3, 2, 4, 4). After step 1, we have:
461
462 N bl_count[N]
463 - -----------
464 2 1
465 3 5
466 4 2
467
468 Step 2 computes the following next_code values:
469
470 N next_code[N]
471 - ------------
472 1 0
473 2 0
474 3 2
475 4 14
476
477 Step 3 produces the following code values:
478
479 Symbol Length Code
480 ------ ------ ----
481 A 3 010
482 B 3 011
483 C 3 100
484 D 3 101
485 E 3 110
486 F 2 00
487 G 4 1110
488 H 4 1111
489
490 3.2.3. Details of block format
491
492 Each block of compressed data begins with 3 header bits
493 containing the following data:
494
495 first bit BFINAL
496 next 2 bits BTYPE
497
498 Note that the header bits do not necessarily begin on a byte
499 boundary, since a block does not necessarily occupy an integral
500 number of bytes.
501
502
503
504
505
506Deutsch Informational [Page 9]
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508RFC 1951 DEFLATE Compressed Data Format Specification May 1996
509
510
511 BFINAL is set if and only if this is the last block of the data
512 set.
513
514 BTYPE specifies how the data are compressed, as follows:
515
516 00 - no compression
517 01 - compressed with fixed Huffman codes
518 10 - compressed with dynamic Huffman codes
519 11 - reserved (error)
520
521 The only difference between the two compressed cases is how the
522 Huffman codes for the literal/length and distance alphabets are
523 defined.
524
525 In all cases, the decoding algorithm for the actual data is as
526 follows:
527
528 do
529 read block header from input stream.
530 if stored with no compression
531 skip any remaining bits in current partially
532 processed byte
533 read LEN and NLEN (see next section)
534 copy LEN bytes of data to output
535 otherwise
536 if compressed with dynamic Huffman codes
537 read representation of code trees (see
538 subsection below)
539 loop (until end of block code recognized)
540 decode literal/length value from input stream
541 if value < 256
542 copy value (literal byte) to output stream
543 otherwise
544 if value = end of block (256)
545 break from loop
546 otherwise (value = 257..285)
547 decode distance from input stream
548
549 move backwards distance bytes in the output
550 stream, and copy length bytes from this
551 position to the output stream.
552 end loop
553 while not last block
554
555 Note that a duplicated string reference may refer to a string
556 in a previous block; i.e., the backward distance may cross one
557 or more block boundaries. However a distance cannot refer past
558 the beginning of the output stream. (An application using a
559
560
561
562Deutsch Informational [Page 10]
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564RFC 1951 DEFLATE Compressed Data Format Specification May 1996
565
566
567 preset dictionary might discard part of the output stream; a
568 distance can refer to that part of the output stream anyway)
569 Note also that the referenced string may overlap the current
570 position; for example, if the last 2 bytes decoded have values
571 X and Y, a string reference with <length = 5, distance = 2>
572 adds X,Y,X,Y,X to the output stream.
573
574 We now specify each compression method in turn.
575
576 3.2.4. Non-compressed blocks (BTYPE=00)
577
578 Any bits of input up to the next byte boundary are ignored.
579 The rest of the block consists of the following information:
580
581 0 1 2 3 4...
582 +---+---+---+---+================================+
583 | LEN | NLEN |... LEN bytes of literal data...|
584 +---+---+---+---+================================+
585
586 LEN is the number of data bytes in the block. NLEN is the
587 one's complement of LEN.
588
589 3.2.5. Compressed blocks (length and distance codes)
590
591 As noted above, encoded data blocks in the "deflate" format
592 consist of sequences of symbols drawn from three conceptually
593 distinct alphabets: either literal bytes, from the alphabet of
594 byte values (0..255), or <length, backward distance> pairs,
595 where the length is drawn from (3..258) and the distance is
596 drawn from (1..32,768). In fact, the literal and length
597 alphabets are merged into a single alphabet (0..285), where
598 values 0..255 represent literal bytes, the value 256 indicates
599 end-of-block, and values 257..285 represent length codes
600 (possibly in conjunction with extra bits following the symbol
601 code) as follows:
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618Deutsch Informational [Page 11]
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620RFC 1951 DEFLATE Compressed Data Format Specification May 1996
621
622
623 Extra Extra Extra
624 Code Bits Length(s) Code Bits Lengths Code Bits Length(s)
625 ---- ---- ------ ---- ---- ------- ---- ---- -------
626 257 0 3 267 1 15,16 277 4 67-82
627 258 0 4 268 1 17,18 278 4 83-98
628 259 0 5 269 2 19-22 279 4 99-114
629 260 0 6 270 2 23-26 280 4 115-130
630 261 0 7 271 2 27-30 281 5 131-162
631 262 0 8 272 2 31-34 282 5 163-194
632 263 0 9 273 3 35-42 283 5 195-226
633 264 0 10 274 3 43-50 284 5 227-257
634 265 1 11,12 275 3 51-58 285 0 258
635 266 1 13,14 276 3 59-66
636
637 The extra bits should be interpreted as a machine integer
638 stored with the most-significant bit first, e.g., bits 1110
639 represent the value 14.
640
641 Extra Extra Extra
642 Code Bits Dist Code Bits Dist Code Bits Distance
643 ---- ---- ---- ---- ---- ------ ---- ---- --------
644 0 0 1 10 4 33-48 20 9 1025-1536
645 1 0 2 11 4 49-64 21 9 1537-2048
646 2 0 3 12 5 65-96 22 10 2049-3072
647 3 0 4 13 5 97-128 23 10 3073-4096
648 4 1 5,6 14 6 129-192 24 11 4097-6144
649 5 1 7,8 15 6 193-256 25 11 6145-8192
650 6 2 9-12 16 7 257-384 26 12 8193-12288
651 7 2 13-16 17 7 385-512 27 12 12289-16384
652 8 3 17-24 18 8 513-768 28 13 16385-24576
653 9 3 25-32 19 8 769-1024 29 13 24577-32768
654
655 3.2.6. Compression with fixed Huffman codes (BTYPE=01)
656
657 The Huffman codes for the two alphabets are fixed, and are not
658 represented explicitly in the data. The Huffman code lengths
659 for the literal/length alphabet are:
660
661 Lit Value Bits Codes
662 --------- ---- -----
663 0 - 143 8 00110000 through
664 10111111
665 144 - 255 9 110010000 through
666 111111111
667 256 - 279 7 0000000 through
668 0010111
669 280 - 287 8 11000000 through
670 11000111
671
672
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676RFC 1951 DEFLATE Compressed Data Format Specification May 1996
677
678
679 The code lengths are sufficient to generate the actual codes,
680 as described above; we show the codes in the table for added
681 clarity. Literal/length values 286-287 will never actually
682 occur in the compressed data, but participate in the code
683 construction.
684
685 Distance codes 0-31 are represented by (fixed-length) 5-bit
686 codes, with possible additional bits as shown in the table
687 shown in Paragraph 3.2.5, above. Note that distance codes 30-
688 31 will never actually occur in the compressed data.
689
690 3.2.7. Compression with dynamic Huffman codes (BTYPE=10)
691
692 The Huffman codes for the two alphabets appear in the block
693 immediately after the header bits and before the actual
694 compressed data, first the literal/length code and then the
695 distance code. Each code is defined by a sequence of code
696 lengths, as discussed in Paragraph 3.2.2, above. For even
697 greater compactness, the code length sequences themselves are
698 compressed using a Huffman code. The alphabet for code lengths
699 is as follows:
700
701 0 - 15: Represent code lengths of 0 - 15
702 16: Copy the previous code length 3 - 6 times.
703 The next 2 bits indicate repeat length
704 (0 = 3, ... , 3 = 6)
705 Example: Codes 8, 16 (+2 bits 11),
706 16 (+2 bits 10) will expand to
707 12 code lengths of 8 (1 + 6 + 5)
708 17: Repeat a code length of 0 for 3 - 10 times.
709 (3 bits of length)
710 18: Repeat a code length of 0 for 11 - 138 times
711 (7 bits of length)
712
713 A code length of 0 indicates that the corresponding symbol in
714 the literal/length or distance alphabet will not occur in the
715 block, and should not participate in the Huffman code
716 construction algorithm given earlier. If only one distance
717 code is used, it is encoded using one bit, not zero bits; in
718 this case there is a single code length of one, with one unused
719 code. One distance code of zero bits means that there are no
720 distance codes used at all (the data is all literals).
721
722 We can now define the format of the block:
723
724 5 Bits: HLIT, # of Literal/Length codes - 257 (257 - 286)
725 5 Bits: HDIST, # of Distance codes - 1 (1 - 32)
726 4 Bits: HCLEN, # of Code Length codes - 4 (4 - 19)
727
728
729
730Deutsch Informational [Page 13]
731
732RFC 1951 DEFLATE Compressed Data Format Specification May 1996
733
734
735 (HCLEN + 4) x 3 bits: code lengths for the code length
736 alphabet given just above, in the order: 16, 17, 18,
737 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15
738
739 These code lengths are interpreted as 3-bit integers
740 (0-7); as above, a code length of 0 means the
741 corresponding symbol (literal/length or distance code
742 length) is not used.
743
744 HLIT + 257 code lengths for the literal/length alphabet,
745 encoded using the code length Huffman code
746
747 HDIST + 1 code lengths for the distance alphabet,
748 encoded using the code length Huffman code
749
750 The actual compressed data of the block,
751 encoded using the literal/length and distance Huffman
752 codes
753
754 The literal/length symbol 256 (end of data),
755 encoded using the literal/length Huffman code
756
757 The code length repeat codes can cross from HLIT + 257 to the
758 HDIST + 1 code lengths. In other words, all code lengths form
759 a single sequence of HLIT + HDIST + 258 values.
760
761 3.3. Compliance
762
763 A compressor may limit further the ranges of values specified in
764 the previous section and still be compliant; for example, it may
765 limit the range of backward pointers to some value smaller than
766 32K. Similarly, a compressor may limit the size of blocks so that
767 a compressible block fits in memory.
768
769 A compliant decompressor must accept the full range of possible
770 values defined in the previous section, and must accept blocks of
771 arbitrary size.
772
7734. Compression algorithm details
774
775 While it is the intent of this document to define the "deflate"
776 compressed data format without reference to any particular
777 compression algorithm, the format is related to the compressed
778 formats produced by LZ77 (Lempel-Ziv 1977, see reference [2] below);
779 since many variations of LZ77 are patented, it is strongly
780 recommended that the implementor of a compressor follow the general
781 algorithm presented here, which is known not to be patented per se.
782 The material in this section is not part of the definition of the
783
784
785
786Deutsch Informational [Page 14]
787
788RFC 1951 DEFLATE Compressed Data Format Specification May 1996
789
790
791 specification per se, and a compressor need not follow it in order to
792 be compliant.
793
794 The compressor terminates a block when it determines that starting a
795 new block with fresh trees would be useful, or when the block size
796 fills up the compressor's block buffer.
797
798 The compressor uses a chained hash table to find duplicated strings,
799 using a hash function that operates on 3-byte sequences. At any
800 given point during compression, let XYZ be the next 3 input bytes to
801 be examined (not necessarily all different, of course). First, the
802 compressor examines the hash chain for XYZ. If the chain is empty,
803 the compressor simply writes out X as a literal byte and advances one
804 byte in the input. If the hash chain is not empty, indicating that
805 the sequence XYZ (or, if we are unlucky, some other 3 bytes with the
806 same hash function value) has occurred recently, the compressor
807 compares all strings on the XYZ hash chain with the actual input data
808 sequence starting at the current point, and selects the longest
809 match.
810
811 The compressor searches the hash chains starting with the most recent
812 strings, to favor small distances and thus take advantage of the
813 Huffman encoding. The hash chains are singly linked. There are no
814 deletions from the hash chains; the algorithm simply discards matches
815 that are too old. To avoid a worst-case situation, very long hash
816 chains are arbitrarily truncated at a certain length, determined by a
817 run-time parameter.
818
819 To improve overall compression, the compressor optionally defers the
820 selection of matches ("lazy matching"): after a match of length N has
821 been found, the compressor searches for a longer match starting at
822 the next input byte. If it finds a longer match, it truncates the
823 previous match to a length of one (thus producing a single literal
824 byte) and then emits the longer match. Otherwise, it emits the
825 original match, and, as described above, advances N bytes before
826 continuing.
827
828 Run-time parameters also control this "lazy match" procedure. If
829 compression ratio is most important, the compressor attempts a
830 complete second search regardless of the length of the first match.
831 In the normal case, if the current match is "long enough", the
832 compressor reduces the search for a longer match, thus speeding up
833 the process. If speed is most important, the compressor inserts new
834 strings in the hash table only when no match was found, or when the
835 match is not "too long". This degrades the compression ratio but
836 saves time since there are both fewer insertions and fewer searches.
837
838
839
840
841
842Deutsch Informational [Page 15]
843
844RFC 1951 DEFLATE Compressed Data Format Specification May 1996
845
846
8475. References
848
849 [1] Huffman, D. A., "A Method for the Construction of Minimum
850 Redundancy Codes", Proceedings of the Institute of Radio
851 Engineers, September 1952, Volume 40, Number 9, pp. 1098-1101.
852
853 [2] Ziv J., Lempel A., "A Universal Algorithm for Sequential Data
854 Compression", IEEE Transactions on Information Theory, Vol. 23,
855 No. 3, pp. 337-343.
856
857 [3] Gailly, J.-L., and Adler, M., ZLIB documentation and sources,
858 available in ftp://ftp.uu.net/pub/archiving/zip/doc/
859
860 [4] Gailly, J.-L., and Adler, M., GZIP documentation and sources,
861 available as gzip-*.tar in ftp://prep.ai.mit.edu/pub/gnu/
862
863 [5] Schwartz, E. S., and Kallick, B. "Generating a canonical prefix
864 encoding." Comm. ACM, 7,3 (Mar. 1964), pp. 166-169.
865
866 [6] Hirschberg and Lelewer, "Efficient decoding of prefix codes,"
867 Comm. ACM, 33,4, April 1990, pp. 449-459.
868
8696. Security Considerations
870
871 Any data compression method involves the reduction of redundancy in
872 the data. Consequently, any corruption of the data is likely to have
873 severe effects and be difficult to correct. Uncompressed text, on
874 the other hand, will probably still be readable despite the presence
875 of some corrupted bytes.
876
877 It is recommended that systems using this data format provide some
878 means of validating the integrity of the compressed data. See
879 reference [3], for example.
880
8817. Source code
882
883 Source code for a C language implementation of a "deflate" compliant
884 compressor and decompressor is available within the zlib package at
885 ftp://ftp.uu.net/pub/archiving/zip/zlib/.
886
8878. Acknowledgements
888
889 Trademarks cited in this document are the property of their
890 respective owners.
891
892 Phil Katz designed the deflate format. Jean-Loup Gailly and Mark
893 Adler wrote the related software described in this specification.
894 Glenn Randers-Pehrson converted this document to RFC and HTML format.
895
896
897
898Deutsch Informational [Page 16]
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900RFC 1951 DEFLATE Compressed Data Format Specification May 1996
901
902
9039. Author's Address
904
905 L. Peter Deutsch
906 Aladdin Enterprises
907 203 Santa Margarita Ave.
908 Menlo Park, CA 94025
909
910 Phone: (415) 322-0103 (AM only)
911 FAX: (415) 322-1734
912 EMail: <ghost@aladdin.com>
913
914 Questions about the technical content of this specification can be
915 sent by email to:
916
917 Jean-Loup Gailly <gzip@prep.ai.mit.edu> and
918 Mark Adler <madler@alumni.caltech.edu>
919
920 Editorial comments on this specification can be sent by email to:
921
922 L. Peter Deutsch <ghost@aladdin.com> and
923 Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
924
925
926
927
928
929
930
931
932
933
934
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939
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1
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3
4
5
6
7Network Working Group P. Deutsch
8Request for Comments: 1952 Aladdin Enterprises
9Category: Informational May 1996
10
11
12 GZIP file format specification version 4.3
13
14Status of This Memo
15
16 This memo provides information for the Internet community. This memo
17 does not specify an Internet standard of any kind. Distribution of
18 this memo is unlimited.
19
20IESG Note:
21
22 The IESG takes no position on the validity of any Intellectual
23 Property Rights statements contained in this document.
24
25Notices
26
27 Copyright (c) 1996 L. Peter Deutsch
28
29 Permission is granted to copy and distribute this document for any
30 purpose and without charge, including translations into other
31 languages and incorporation into compilations, provided that the
32 copyright notice and this notice are preserved, and that any
33 substantive changes or deletions from the original are clearly
34 marked.
35
36 A pointer to the latest version of this and related documentation in
37 HTML format can be found at the URL
38 <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>.
39
40Abstract
41
42 This specification defines a lossless compressed data format that is
43 compatible with the widely used GZIP utility. The format includes a
44 cyclic redundancy check value for detecting data corruption. The
45 format presently uses the DEFLATE method of compression but can be
46 easily extended to use other compression methods. The format can be
47 implemented readily in a manner not covered by patents.
48
49
50
51
52
53
54
55
56
57
58Deutsch Informational [Page 1]
59
60RFC 1952 GZIP File Format Specification May 1996
61
62
63Table 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 ......................................... 4
73 2.1. Overall conventions ....................................... 4
74 2.2. File format ............................................... 5
75 2.3. Member format ............................................. 5
76 2.3.1. Member header and trailer ........................... 6
77 2.3.1.1. Extra field ................................... 8
78 2.3.1.2. Compliance .................................... 9
79 3. References .................................................. 9
80 4. Security Considerations .................................... 10
81 5. Acknowledgements ........................................... 10
82 6. Author's Address ........................................... 10
83 7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
84 8. Appendix: Sample CRC Code .................................. 11
85
861. Introduction
87
88 1.1. Purpose
89
90 The purpose of this specification is to define a lossless
91 compressed data format that:
92
93 * Is independent of CPU type, operating system, file system,
94 and character set, and hence can be used for interchange;
95 * Can compress or decompress a data stream (as opposed to a
96 randomly accessible file) to produce another data stream,
97 using only an a priori bounded amount of intermediate
98 storage, and hence can be used in data communications or
99 similar structures such as Unix filters;
100 * Compresses data with efficiency comparable to the best
101 currently available general-purpose compression methods,
102 and in particular considerably better than the "compress"
103 program;
104 * Can be implemented readily in a manner not covered by
105 patents, and hence can be practiced freely;
106 * Is compatible with the file format produced by the current
107 widely used gzip utility, in that conforming decompressors
108 will be able to read data produced by the existing gzip
109 compressor.
110
111
112
113
114Deutsch Informational [Page 2]
115
116RFC 1952 GZIP File Format Specification May 1996
117
118
119 The data format defined by this specification does not attempt to:
120
121 * Provide random access to compressed data;
122 * Compress specialized data (e.g., raster graphics) as well as
123 the best currently available specialized algorithms.
124
125 1.2. Intended audience
126
127 This specification is intended for use by implementors of software
128 to compress data into gzip format and/or decompress data from gzip
129 format.
130
131 The text of the specification assumes a basic background in
132 programming at the level of bits and other primitive data
133 representations.
134
135 1.3. Scope
136
137 The specification specifies a compression method and a file format
138 (the latter assuming only that a file can store a sequence of
139 arbitrary bytes). It does not specify any particular interface to
140 a file system or anything about character sets or encodings
141 (except for file names and comments, which are optional).
142
143 1.4. Compliance
144
145 Unless otherwise indicated below, a compliant decompressor must be
146 able to accept and decompress any file that conforms to all the
147 specifications presented here; a compliant compressor must produce
148 files that conform to all the specifications presented here. The
149 material in the appendices is not part of the specification per se
150 and is not relevant to compliance.
151
152 1.5. Definitions of terms and conventions used
153
154 byte: 8 bits stored or transmitted as a unit (same as an octet).
155 (For this specification, a byte is exactly 8 bits, even on
156 machines which store a character on a number of bits different
157 from 8.) See below for the numbering of bits within a byte.
158
159 1.6. Changes from previous versions
160
161 There have been no technical changes to the gzip format since
162 version 4.1 of this specification. In version 4.2, some
163 terminology was changed, and the sample CRC code was rewritten for
164 clarity and to eliminate the requirement for the caller to do pre-
165 and post-conditioning. Version 4.3 is a conversion of the
166 specification to RFC style.
167
168
169
170Deutsch Informational [Page 3]
171
172RFC 1952 GZIP File Format Specification May 1996
173
174
1752. Detailed specification
176
177 2.1. Overall conventions
178
179 In the diagrams below, a box like this:
180
181 +---+
182 | | <-- the vertical bars might be missing
183 +---+
184
185 represents one byte; a box like this:
186
187 +==============+
188 | |
189 +==============+
190
191 represents a variable number of bytes.
192
193 Bytes stored within a computer do not have a "bit order", since
194 they are always treated as a unit. However, a byte considered as
195 an integer between 0 and 255 does have a most- and least-
196 significant bit, and since we write numbers with the most-
197 significant digit on the left, we also write bytes with the most-
198 significant bit on the left. In the diagrams below, we number the
199 bits of a byte so that bit 0 is the least-significant bit, i.e.,
200 the bits are numbered:
201
202 +--------+
203 |76543210|
204 +--------+
205
206 This document does not address the issue of the order in which
207 bits of a byte are transmitted on a bit-sequential medium, since
208 the data format described here is byte- rather than bit-oriented.
209
210 Within a computer, a number may occupy multiple bytes. All
211 multi-byte numbers in the format described here are stored with
212 the least-significant byte first (at the lower memory address).
213 For example, the decimal number 520 is stored as:
214
215 0 1
216 +--------+--------+
217 |00001000|00000010|
218 +--------+--------+
219 ^ ^
220 | |
221 | + more significant byte = 2 x 256
222 + less significant byte = 8
223
224
225
226Deutsch Informational [Page 4]
227
228RFC 1952 GZIP File Format Specification May 1996
229
230
231 2.2. File format
232
233 A gzip file consists of a series of "members" (compressed data
234 sets). The format of each member is specified in the following
235 section. The members simply appear one after another in the file,
236 with no additional information before, between, or after them.
237
238 2.3. Member format
239
240 Each member has the following structure:
241
242 +---+---+---+---+---+---+---+---+---+---+
243 |ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->)
244 +---+---+---+---+---+---+---+---+---+---+
245
246 (if FLG.FEXTRA set)
247
248 +---+---+=================================+
249 | XLEN |...XLEN bytes of "extra field"...| (more-->)
250 +---+---+=================================+
251
252 (if FLG.FNAME set)
253
254 +=========================================+
255 |...original file name, zero-terminated...| (more-->)
256 +=========================================+
257
258 (if FLG.FCOMMENT set)
259
260 +===================================+
261 |...file comment, zero-terminated...| (more-->)
262 +===================================+
263
264 (if FLG.FHCRC set)
265
266 +---+---+
267 | CRC16 |
268 +---+---+
269
270 +=======================+
271 |...compressed blocks...| (more-->)
272 +=======================+
273
274 0 1 2 3 4 5 6 7
275 +---+---+---+---+---+---+---+---+
276 | CRC32 | ISIZE |
277 +---+---+---+---+---+---+---+---+
278
279
280
281
282Deutsch Informational [Page 5]
283
284RFC 1952 GZIP File Format Specification May 1996
285
286
287 2.3.1. Member header and trailer
288
289 ID1 (IDentification 1)
290 ID2 (IDentification 2)
291 These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139
292 (0x8b, \213), to identify the file as being in gzip format.
293
294 CM (Compression Method)
295 This identifies the compression method used in the file. CM
296 = 0-7 are reserved. CM = 8 denotes the "deflate"
297 compression method, which is the one customarily used by
298 gzip and which is documented elsewhere.
299
300 FLG (FLaGs)
301 This flag byte is divided into individual bits as follows:
302
303 bit 0 FTEXT
304 bit 1 FHCRC
305 bit 2 FEXTRA
306 bit 3 FNAME
307 bit 4 FCOMMENT
308 bit 5 reserved
309 bit 6 reserved
310 bit 7 reserved
311
312 If FTEXT is set, the file is probably ASCII text. This is
313 an optional indication, which the compressor may set by
314 checking a small amount of the input data to see whether any
315 non-ASCII characters are present. In case of doubt, FTEXT
316 is cleared, indicating binary data. For systems which have
317 different file formats for ascii text and binary data, the
318 decompressor can use FTEXT to choose the appropriate format.
319 We deliberately do not specify the algorithm used to set
320 this bit, since a compressor always has the option of
321 leaving it cleared and a decompressor always has the option
322 of ignoring it and letting some other program handle issues
323 of data conversion.
324
325 If FHCRC is set, a CRC16 for the gzip header is present,
326 immediately before the compressed data. The CRC16 consists
327 of the two least significant bytes of the CRC32 for all
328 bytes of the gzip header up to and not including the CRC16.
329 [The FHCRC bit was never set by versions of gzip up to
330 1.2.4, even though it was documented with a different
331 meaning in gzip 1.2.4.]
332
333 If FEXTRA is set, optional extra fields are present, as
334 described in a following section.
335
336
337
338Deutsch Informational [Page 6]
339
340RFC 1952 GZIP File Format Specification May 1996
341
342
343 If FNAME is set, an original file name is present,
344 terminated by a zero byte. The name must consist of ISO
345 8859-1 (LATIN-1) characters; on operating systems using
346 EBCDIC or any other character set for file names, the name
347 must be translated to the ISO LATIN-1 character set. This
348 is the original name of the file being compressed, with any
349 directory components removed, and, if the file being
350 compressed is on a file system with case insensitive names,
351 forced to lower case. There is no original file name if the
352 data was compressed from a source other than a named file;
353 for example, if the source was stdin on a Unix system, there
354 is no file name.
355
356 If FCOMMENT is set, a zero-terminated file comment is
357 present. This comment is not interpreted; it is only
358 intended for human consumption. The comment must consist of
359 ISO 8859-1 (LATIN-1) characters. Line breaks should be
360 denoted by a single line feed character (10 decimal).
361
362 Reserved FLG bits must be zero.
363
364 MTIME (Modification TIME)
365 This gives the most recent modification time of the original
366 file being compressed. The time is in Unix format, i.e.,
367 seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this
368 may cause problems for MS-DOS and other systems that use
369 local rather than Universal time.) If the compressed data
370 did not come from a file, MTIME is set to the time at which
371 compression started. MTIME = 0 means no time stamp is
372 available.
373
374 XFL (eXtra FLags)
375 These flags are available for use by specific compression
376 methods. The "deflate" method (CM = 8) sets these flags as
377 follows:
378
379 XFL = 2 - compressor used maximum compression,
380 slowest algorithm
381 XFL = 4 - compressor used fastest algorithm
382
383 OS (Operating System)
384 This identifies the type of file system on which compression
385 took place. This may be useful in determining end-of-line
386 convention for text files. The currently defined values are
387 as follows:
388
389
390
391
392
393
394Deutsch Informational [Page 7]
395
396RFC 1952 GZIP File Format Specification May 1996
397
398
399 0 - FAT filesystem (MS-DOS, OS/2, NT/Win32)
400 1 - Amiga
401 2 - VMS (or OpenVMS)
402 3 - Unix
403 4 - VM/CMS
404 5 - Atari TOS
405 6 - HPFS filesystem (OS/2, NT)
406 7 - Macintosh
407 8 - Z-System
408 9 - CP/M
409 10 - TOPS-20
410 11 - NTFS filesystem (NT)
411 12 - QDOS
412 13 - Acorn RISCOS
413 255 - unknown
414
415 XLEN (eXtra LENgth)
416 If FLG.FEXTRA is set, this gives the length of the optional
417 extra field. See below for details.
418
419 CRC32 (CRC-32)
420 This contains a Cyclic Redundancy Check value of the
421 uncompressed data computed according to CRC-32 algorithm
422 used in the ISO 3309 standard and in section 8.1.1.6.2 of
423 ITU-T recommendation V.42. (See http://www.iso.ch for
424 ordering ISO documents. See gopher://info.itu.ch for an
425 online version of ITU-T V.42.)
426
427 ISIZE (Input SIZE)
428 This contains the size of the original (uncompressed) input
429 data modulo 2^32.
430
431 2.3.1.1. Extra field
432
433 If the FLG.FEXTRA bit is set, an "extra field" is present in
434 the header, with total length XLEN bytes. It consists of a
435 series of subfields, each of the form:
436
437 +---+---+---+---+==================================+
438 |SI1|SI2| LEN |... LEN bytes of subfield data ...|
439 +---+---+---+---+==================================+
440
441 SI1 and SI2 provide a subfield ID, typically two ASCII letters
442 with some mnemonic value. Jean-Loup Gailly
443 <gzip@prep.ai.mit.edu> is maintaining a registry of subfield
444 IDs; please send him any subfield ID you wish to use. Subfield
445 IDs with SI2 = 0 are reserved for future use. The following
446 IDs are currently defined:
447
448
449
450Deutsch Informational [Page 8]
451
452RFC 1952 GZIP File Format Specification May 1996
453
454
455 SI1 SI2 Data
456 ---------- ---------- ----
457 0x41 ('A') 0x70 ('P') Apollo file type information
458
459 LEN gives the length of the subfield data, excluding the 4
460 initial bytes.
461
462 2.3.1.2. Compliance
463
464 A compliant compressor must produce files with correct ID1,
465 ID2, CM, CRC32, and ISIZE, but may set all the other fields in
466 the fixed-length part of the header to default values (255 for
467 OS, 0 for all others). The compressor must set all reserved
468 bits to zero.
469
470 A compliant decompressor must check ID1, ID2, and CM, and
471 provide an error indication if any of these have incorrect
472 values. It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC
473 at least so it can skip over the optional fields if they are
474 present. It need not examine any other part of the header or
475 trailer; in particular, a decompressor may ignore FTEXT and OS
476 and always produce binary output, and still be compliant. A
477 compliant decompressor must give an error indication if any
478 reserved bit is non-zero, since such a bit could indicate the
479 presence of a new field that would cause subsequent data to be
480 interpreted incorrectly.
481
4823. References
483
484 [1] "Information Processing - 8-bit single-byte coded graphic
485 character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987).
486 The ISO 8859-1 (Latin-1) character set is a superset of 7-bit
487 ASCII. Files defining this character set are available as
488 iso_8859-1.* in ftp://ftp.uu.net/graphics/png/documents/
489
490 [2] ISO 3309
491
492 [3] ITU-T recommendation V.42
493
494 [4] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",
495 available in ftp://ftp.uu.net/pub/archiving/zip/doc/
496
497 [5] Gailly, J.-L., GZIP documentation, available as gzip-*.tar in
498 ftp://prep.ai.mit.edu/pub/gnu/
499
500 [6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table
501 Look-Up", Communications of the ACM, 31(8), pp.1008-1013.
502
503
504
505
506Deutsch Informational [Page 9]
507
508RFC 1952 GZIP File Format Specification May 1996
509
510
511 [7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal,
512 pp.118-133.
513
514 [8] ftp://ftp.adelaide.edu.au/pub/rocksoft/papers/crc_v3.txt,
515 describing the CRC concept.
516
5174. Security Considerations
518
519 Any data compression method involves the reduction of redundancy in
520 the data. Consequently, any corruption of the data is likely to have
521 severe effects and be difficult to correct. Uncompressed text, on
522 the other hand, will probably still be readable despite the presence
523 of some corrupted bytes.
524
525 It is recommended that systems using this data format provide some
526 means of validating the integrity of the compressed data, such as by
527 setting and checking the CRC-32 check value.
528
5295. Acknowledgements
530
531 Trademarks cited in this document are the property of their
532 respective owners.
533
534 Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler,
535 the related software described in this specification. Glenn
536 Randers-Pehrson converted this document to RFC and HTML format.
537
5386. Author's Address
539
540 L. Peter Deutsch
541 Aladdin Enterprises
542 203 Santa Margarita Ave.
543 Menlo Park, CA 94025
544
545 Phone: (415) 322-0103 (AM only)
546 FAX: (415) 322-1734
547 EMail: <ghost@aladdin.com>
548
549 Questions about the technical content of this specification can be
550 sent by email to:
551
552 Jean-Loup Gailly <gzip@prep.ai.mit.edu> and
553 Mark Adler <madler@alumni.caltech.edu>
554
555 Editorial comments on this specification can be sent by email to:
556
557 L. Peter Deutsch <ghost@aladdin.com> and
558 Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
559
560
561
562Deutsch Informational [Page 10]
563
564RFC 1952 GZIP File Format Specification May 1996
565
566
5677. Appendix: Jean-Loup Gailly's gzip utility
568
569 The most widely used implementation of gzip compression, and the
570 original documentation on which this specification is based, were
571 created by Jean-Loup Gailly <gzip@prep.ai.mit.edu>. Since this
572 implementation is a de facto standard, we mention some more of its
573 features here. Again, the material in this section is not part of
574 the specification per se, and implementations need not follow it to
575 be compliant.
576
577 When compressing or decompressing a file, gzip preserves the
578 protection, ownership, and modification time attributes on the local
579 file system, since there is no provision for representing protection
580 attributes in the gzip file format itself. Since the file format
581 includes a modification time, the gzip decompressor provides a
582 command line switch that assigns the modification time from the file,
583 rather than the local modification time of the compressed input, to
584 the decompressed output.
585
5868. Appendix: Sample CRC Code
587
588 The following sample code represents a practical implementation of
589 the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42
590 for a formal specification.)
591
592 The sample code is in the ANSI C programming language. Non C users
593 may find it easier to read with these hints:
594
595 & Bitwise AND operator.
596 ^ Bitwise exclusive-OR operator.
597 >> Bitwise right shift operator. When applied to an
598 unsigned quantity, as here, right shift inserts zero
599 bit(s) at the left.
600 ! Logical NOT operator.
601 ++ "n++" increments the variable n.
602 0xNNN 0x introduces a hexadecimal (base 16) constant.
603 Suffix L indicates a long value (at least 32 bits).
604
605 /* Table of CRCs of all 8-bit messages. */
606 unsigned long crc_table[256];
607
608 /* Flag: has the table been computed? Initially false. */
609 int crc_table_computed = 0;
610
611 /* Make the table for a fast CRC. */
612 void make_crc_table(void)
613 {
614 unsigned long c;
615
616
617
618Deutsch Informational [Page 11]
619
620RFC 1952 GZIP File Format Specification May 1996
621
622
623 int n, k;
624 for (n = 0; n < 256; n++) {
625 c = (unsigned long) n;
626 for (k = 0; k < 8; k++) {
627 if (c & 1) {
628 c = 0xedb88320L ^ (c >> 1);
629 } else {
630 c = c >> 1;
631 }
632 }
633 crc_table[n] = c;
634 }
635 crc_table_computed = 1;
636 }
637
638 /*
639 Update a running crc with the bytes buf[0..len-1] and return
640 the updated crc. The crc should be initialized to zero. Pre- and
641 post-conditioning (one's complement) is performed within this
642 function so it shouldn't be done by the caller. Usage example:
643
644 unsigned long crc = 0L;
645
646 while (read_buffer(buffer, length) != EOF) {
647 crc = update_crc(crc, buffer, length);
648 }
649 if (crc != original_crc) error();
650 */
651 unsigned long update_crc(unsigned long crc,
652 unsigned char *buf, int len)
653 {
654 unsigned long c = crc ^ 0xffffffffL;
655 int n;
656
657 if (!crc_table_computed)
658 make_crc_table();
659 for (n = 0; n < len; n++) {
660 c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
661 }
662 return c ^ 0xffffffffL;
663 }
664
665 /* Return the CRC of the bytes buf[0..len-1]. */
666 unsigned long crc(unsigned char *buf, int len)
667 {
668 return update_crc(0L, buf, len);
669 }
670
671
672
673
674Deutsch Informational [Page 12]
675
lib/std/compress/testdata/rfc1952.txt.gz created
Binary files /dev/null and b/lib/std/compress/testdata/rfc1952.txt.gz differ
lib/std/compress/zlib.zig+4-4
......@@ -107,21 +107,21 @@ fn testReader(data: []const u8, expected: []const u8) !void {
107107// https://tools.ietf.org/rfc/rfc1951.txt length=36944 bytes
108108// SHA256=5ebf4b5b7fe1c3a0c0ab9aa3ac8c0f3853a7dc484905e76e03b0b0f301350009
109109test "compressed data" {
110 const rfc1951_txt = @embedFile("rfc1951.txt");
110 const rfc1951_txt = @embedFile("testdata/rfc1951.txt");
111111
112112 // Compressed with compression level = 0
113113 try testReader(
114 @embedFile("rfc1951.txt.z.0"),
114 @embedFile("testdata/rfc1951.txt.z.0"),
115115 rfc1951_txt,
116116 );
117117 // Compressed with compression level = 9
118118 try testReader(
119 @embedFile("rfc1951.txt.z.9"),
119 @embedFile("testdata/rfc1951.txt.z.9"),
120120 rfc1951_txt,
121121 );
122122 // Compressed with compression level = 9 and fixed Huffman codes
123123 try testReader(
124 @embedFile("rfc1951.txt.fixed.z.9"),
124 @embedFile("testdata/rfc1951.txt.fixed.z.9"),
125125 rfc1951_txt,
126126 );
127127}