cpython-lru_cache-weakref: Modules/zlib/algorithm.txt annotate

annotate Modules/zlib/algorithm.txt @ 87690:66c1c9f32567 default tip

Add use_weakref option to functools.lru_cache The use_weakref option makes lru_cache create weak references to the function arguments when creating the cache key, which prevents them from staying in the memory forever even after all other references to them are gone. This is a proof of concept for http://bugs.python.org/issue19859

author	Radomir Dopieralski <devel@sheep.art.pl>
date	Tue, 03 Dec 2013 16:02:48 +0100
parents	224aa49f22f1
children

rev	line source
36186 81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	1 1. Compression algorithm (deflate)
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	3 The deflation algorithm used by gzip (also zip and zlib) is a variation of
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	4 LZ77 (Lempel-Ziv 1977, see reference below). It finds duplicated strings in
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	5 the input data. The second occurrence of a string is replaced by a
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	6 pointer to the previous string, in the form of a pair (distance,
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	7 length). Distances are limited to 32K bytes, and lengths are limited
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	8 to 258 bytes. When a string does not occur anywhere in the previous
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	9 32K bytes, it is emitted as a sequence of literal bytes. (In this
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	10 description, `string' must be taken as an arbitrary sequence of bytes,
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	11 and is not restricted to printable characters.)
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	12
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	13 Literals or match lengths are compressed with one Huffman tree, and
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	14 match distances are compressed with another tree. The trees are stored
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	15 in a compact form at the start of each block. The blocks can have any
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	16 size (except that the compressed data for one block must fit in
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	17 available memory). A block is terminated when deflate() determines that
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	18 it would be useful to start another block with fresh trees. (This is
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	19 somewhat similar to the behavior of LZW-based _compress_.)
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	20
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	21 Duplicated strings are found using a hash table. All input strings of
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	22 length 3 are inserted in the hash table. A hash index is computed for
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	23 the next 3 bytes. If the hash chain for this index is not empty, all
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	24 strings in the chain are compared with the current input string, and
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	25 the longest match is selected.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	26
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	27 The hash chains are searched starting with the most recent strings, to
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	28 favor small distances and thus take advantage of the Huffman encoding.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	29 The hash chains are singly linked. There are no deletions from the
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	30 hash chains, the algorithm simply discards matches that are too old.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	31
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	32 To avoid a worst-case situation, very long hash chains are arbitrarily
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	33 truncated at a certain length, determined by a runtime option (level
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	34 parameter of deflateInit). So deflate() does not always find the longest
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	35 possible match but generally finds a match which is long enough.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	36
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	37 deflate() also defers the selection of matches with a lazy evaluation
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	38 mechanism. After a match of length N has been found, deflate() searches for
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	39 a longer match at the next input byte. If a longer match is found, the
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	40 previous match is truncated to a length of one (thus producing a single
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	41 literal byte) and the process of lazy evaluation begins again. Otherwise,
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	42 the original match is kept, and the next match search is attempted only N
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	43 steps later.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	44
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	45 The lazy match evaluation is also subject to a runtime parameter. If
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	46 the current match is long enough, deflate() reduces the search for a longer
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	47 match, thus speeding up the whole process. If compression ratio is more
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	48 important than speed, deflate() attempts a complete second search even if
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	49 the first match is already long enough.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	50
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	51 The lazy match evaluation is not performed for the fastest compression
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	52 modes (level parameter 1 to 3). For these fast modes, new strings
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	53 are inserted in the hash table only when no match was found, or
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	54 when the match is not too long. This degrades the compression ratio
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	55 but saves time since there are both fewer insertions and fewer searches.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	56
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	57
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	58 2. Decompression algorithm (inflate)
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	59
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	60 2.1 Introduction
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	61
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	62 The key question is how to represent a Huffman code (or any prefix code) so
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	63 that you can decode fast. The most important characteristic is that shorter
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	64 codes are much more common than longer codes, so pay attention to decoding the
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	65 short codes fast, and let the long codes take longer to decode.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	66
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	67 inflate() sets up a first level table that covers some number of bits of
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	68 input less than the length of longest code. It gets that many bits from the
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	69 stream, and looks it up in the table. The table will tell if the next
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	70 code is that many bits or less and how many, and if it is, it will tell
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	71 the value, else it will point to the next level table for which inflate()
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	72 grabs more bits and tries to decode a longer code.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	73
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	74 How many bits to make the first lookup is a tradeoff between the time it
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	75 takes to decode and the time it takes to build the table. If building the
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	76 table took no time (and if you had infinite memory), then there would only
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	77 be a first level table to cover all the way to the longest code. However,
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	78 building the table ends up taking a lot longer for more bits since short
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	79 codes are replicated many times in such a table. What inflate() does is
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	80 simply to make the number of bits in the first table a variable, and then
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	81 to set that variable for the maximum speed.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	82
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	83 For inflate, which has 286 possible codes for the literal/length tree, the size
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	84 of the first table is nine bits. Also the distance trees have 30 possible
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	85 values, and the size of the first table is six bits. Note that for each of
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	86 those cases, the table ended up one bit longer than the ``average'' code
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	87 length, i.e. the code length of an approximately flat code which would be a
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	88 little more than eight bits for 286 symbols and a little less than five bits
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	89 for 30 symbols.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	90
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	91
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	92 2.2 More details on the inflate table lookup
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	93
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	94 Ok, you want to know what this cleverly obfuscated inflate tree actually
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	95 looks like. You are correct that it's not a Huffman tree. It is simply a
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	96 lookup table for the first, let's say, nine bits of a Huffman symbol. The
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	97 symbol could be as short as one bit or as long as 15 bits. If a particular
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	98 symbol is shorter than nine bits, then that symbol's translation is duplicated
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	99 in all those entries that start with that symbol's bits. For example, if the
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	100 symbol is four bits, then it's duplicated 32 times in a nine-bit table. If a
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	101 symbol is nine bits long, it appears in the table once.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	102
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	103 If the symbol is longer than nine bits, then that entry in the table points
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	104 to another similar table for the remaining bits. Again, there are duplicated
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	105 entries as needed. The idea is that most of the time the symbol will be short
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	106 and there will only be one table look up. (That's whole idea behind data
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	107 compression in the first place.) For the less frequent long symbols, there
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	108 will be two lookups. If you had a compression method with really long
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	109 symbols, you could have as many levels of lookups as is efficient. For
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	110 inflate, two is enough.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	111
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	112 So a table entry either points to another table (in which case nine bits in
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	113 the above example are gobbled), or it contains the translation for the symbol
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	114 and the number of bits to gobble. Then you start again with the next
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	115 ungobbled bit.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	116
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	117 You may wonder: why not just have one lookup table for how ever many bits the
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	118 longest symbol is? The reason is that if you do that, you end up spending
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	119 more time filling in duplicate symbol entries than you do actually decoding.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	120 At least for deflate's output that generates new trees every several 10's of
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	121 kbytes. You can imagine that filling in a 2^15 entry table for a 15-bit code
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	122 would take too long if you're only decoding several thousand symbols. At the
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	123 other extreme, you could make a new table for every bit in the code. In fact,
86546 224aa49f22f1 Issue #18527: Upgrade internal copy of zlib to 1.2.8 Christian Heimes <christian@cheimes.de> parents: 36186 diff changeset	124 that's essentially a Huffman tree. But then you spend too much time
36186 81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	125 traversing the tree while decoding, even for short symbols.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	126
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	127 So the number of bits for the first lookup table is a trade of the time to
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	128 fill out the table vs. the time spent looking at the second level and above of
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	129 the table.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	130
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	131 Here is an example, scaled down:
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	132
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	133 The code being decoded, with 10 symbols, from 1 to 6 bits long:
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	134
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	135 A: 0
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	136 B: 10
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	137 C: 1100
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	138 D: 11010
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	139 E: 11011
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	140 F: 11100
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	141 G: 11101
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	142 H: 11110
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	143 I: 111110
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	144 J: 111111
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	145
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	146 Let's make the first table three bits long (eight entries):
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	147
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	148 000: A,1
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	149 001: A,1
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	150 010: A,1
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	151 011: A,1
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	152 100: B,2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	153 101: B,2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	154 110: -> table X (gobble 3 bits)
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	155 111: -> table Y (gobble 3 bits)
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	156
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	157 Each entry is what the bits decode as and how many bits that is, i.e. how
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	158 many bits to gobble. Or the entry points to another table, with the number of
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	159 bits to gobble implicit in the size of the table.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	160
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	161 Table X is two bits long since the longest code starting with 110 is five bits
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	162 long:
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	163
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	164 00: C,1
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	165 01: C,1
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	166 10: D,2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	167 11: E,2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	168
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	169 Table Y is three bits long since the longest code starting with 111 is six
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	170 bits long:
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	171
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	172 000: F,2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	173 001: F,2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	174 010: G,2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	175 011: G,2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	176 100: H,2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	177 101: H,2
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	178 110: I,3
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	179 111: J,3
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	180
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	181 So what we have here are three tables with a total of 20 entries that had to
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	182 be constructed. That's compared to 64 entries for a single table. Or
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	183 compared to 16 entries for a Huffman tree (six two entry tables and one four
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	184 entry table). Assuming that the code ideally represents the probability of
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	185 the symbols, it takes on the average 1.25 lookups per symbol. That's compared
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	186 to one lookup for the single table, or 1.66 lookups per symbol for the
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	187 Huffman tree.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	188
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	189 There, I think that gives you a picture of what's going on. For inflate, the
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	190 meaning of a particular symbol is often more than just a letter. It can be a
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	191 byte (a "literal"), or it can be either a length or a distance which
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	192 indicates a base value and a number of bits to fetch after the code that is
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	193 added to the base value. Or it might be the special end-of-block code. The
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	194 data structures created in inftrees.c try to encode all that information
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	195 compactly in the tables.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	196
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	197
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	198 Jean-loup Gailly Mark Adler
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	199 jloup@gzip.org madler@alumni.caltech.edu
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	200
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	201
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	202 References:
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	203
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	204 [LZ77] Ziv J., Lempel A., ``A Universal Algorithm for Sequential Data
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	205 Compression,'' IEEE Transactions on Information Theory, Vol. 23, No. 3,
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	206 pp. 337-343.
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	207
81fa47d2d477 Import zlib into Python tree. Martin v. Löwis <martin@v.loewis.de> parents: diff changeset	208 ``DEFLATE Compressed Data Format Specification'' available in
86546 224aa49f22f1 Issue #18527: Upgrade internal copy of zlib to 1.2.8 Christian Heimes <christian@cheimes.de> parents: 36186 diff changeset	209 http://tools.ietf.org/html/rfc1951

Mercurial > cpython-lru_cache-weakref

annotate Modules/zlib/algorithm.txt @ 87690:66c1c9f32567 default tip