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1
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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]
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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
392
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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]
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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]
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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]
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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
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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
931
932
933
934
935
936
937
938
939
940
941
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