Lossless compression II
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1 Lossless II D 44 R 52 B 81 C 84 D 86 R 82 A 85 A 87 A 83 R 88 A 8A B 89 A 8B Symbol Probability Range a 0.2 [0.0, 0.2) e 0.3 [0.2, 0.5) i 0.1 [0.5, 0.6) o 0.2 [0.6, 0.8) u 0.1 [0.8, 0.9)! 0.1 [0.9, 1.0) CSCI 470: Web Science Keith Vertanen
2 Overview Flavors of StaBc vs. Dynamic vs. AdapBve Lempel- Ziv- Welch (LZW) Fixed- length Variable- length paqern StaBsBcal coding ArithmeBc coding PredicBon by ParBal Match (PPM) 2
3 StaBc Compression models Predefined map for all text, e.g. ASCII, Morse Code Not opbmal: different texts = different stabsbcs Dynamic Generate model based on text Requires inibal pass before can start Must transmit the model (e.g. Huffman coding) AdapBve More accurate modeling = beqer Decoding must start from beginning (e.g. LZW) 3
4 LZW Lempel- Ziv- Welch (LZW) Basis for many popular formats e.g. PNG, 7zip, gzip, jar, PDF, compress, pkzip, GIF, TIFF Algorithm basics: Maintain a table of fixed- length s for variable- length paqerns in the input Built progressively by compressor and expander Table does not need to be transmiqed! Table is of fixed size Entries for all single characters in alphabet Entries for longer subs encountered 4
5 Details: LZW example 7- bit ASCII characters, 8- bit For real use, 8- bit ~12- bit Alphabet: 128 characters longer s Codeword: 2- digit hex value = single characters, e.g. 41 = A, 52 = R 80 = end of file 81- FF = longer s, e.g. 81 = AB, 88 = ABR Employs lookahead character to add s 5
6 input A B R A C A D A B R A B R A B R A matches Compressing D 44 Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c R 52 Ini6al set of character s 6
7 input A B R A C A D A B R A B R A B R A matches A 41 D 44 R 52 Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c Ini6al set of character s 7
8 input A B R A C A D A B R A B R A B R A matches A B D 44 R 52 BR 82 Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c Ini6al set of character s 8
9 input A B R A C A D A B R A B R A B R A matches A B R D 44 R 52 BR 82 RA 83 Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c Ini6al set of character s 9
10 input A B R A C A D A B R A B R A B R A matches A B R A D 44 R 52 BR 82 RA 83 AC 84 Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c Ini6al set of character s 10
11 input A B R A C A D A B R A B R A B R A matches A B R A C A D D 44 R 52 BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c Ini6al set of character s 11
12 input A B R A C A D A B R A B R A B R A matches A B R A C A D AB D 44 R 52 BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c ABR 88 Ini6al set of character s 12
13 input A B R A C A D A B R A B R A B R A matches A B R A C A D AB RA D 44 R 52 BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c ABR 88 Ini6al set of character s RAB 89 13
14 input A B R A C A D A B R A B R A B R A matches A B R A C A D AB RA BR D 44 R 52 BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c ABR 88 Ini6al set of character s RAB 89 BRA 8A 14
15 input A B R A C A D A B R A B R A B R A matches A B R A C A D AB RA BR ABR D 44 R 52 BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c ABR 88 Ini6al set of character s RAB 89 BRA 8A ABRA 8B 15
16 input A B R A C A D A B R A B R A B R A matches A B R A C A D AB RA BR ABR A D 44 BR 82 RA 83 AC 84 CA 85 AD 86 Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c R 52 Ini6al set of character s DA 87 ABR 88 RAB 89 BRA 8A ABRA 8B Input: 17 ASCII characters * 7 bits = 119 bits Output: 12 s * 8 bits = 96 bits Compression ratio: 82% 16
17 Compression data structure LZW uses two table operabons: Find longest- prefix match from current posibon Add entry with character added to longest match Trie data structure, "retrieval" Linked tree structure Path in tree defines Compressing Find longest s in table that is prefix of unscanned input Write of s Scan one character c ahead Associate next free with s + c 17
18 D 44 R 52 B 81 C 84 D 86 R 82 A 85 A 87 A 83 R 88 A 8A B 89 A 8B AD 86 BR 82 DA 87 RA 83 ABR 88 AC 84 RAB 89 CA 85 BRA 8A 18
19 Java implementabon, LZW private static final int R = 256; private static final int L = 4096; private static final int W = 12; // number of input chars // number of s = 2^W // width public static void compress() { String input = BinaryStdIn.readString(); // read input as TST<Integer> st = new TST<Integer>(); // trie data structure for (int i = 0; i < R; i++) st.put("" + (char) i, i); int code = R+1; // s for single chars // R is for EOF } while (input.length() > 0) { String s = st.longestprefixof(input); BinaryStdOut.write(st.get(s), W); // write W- bit for s int t = s.length(); if (t < input.length() && code < L) st.put(input.sub(0, t + 1), code++); // add new input = input.sub(t); } BinaryStdOut.write(R, W); // write last BinaryStdOut.close(); 19
20 input output Expansion Write the current val Read x from input D 44 Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s R 52 Ini6al set of character s val old = A x = 42 s = B c = B next code = 81 val new = B 20
21 input output A D 44 R 52 Ini6al set of character s Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = B x = 52 s = R c = R next code = 82 val new = R 21
22 input output A B D 44 R 52 Ini6al set of character s BR 82 Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = R x = 41 s = A c = A next code = 83 val new = A 22
23 input output A B R D 44 R 52 Ini6al set of character s BR 82 RA 83 Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = A x = 43 s = C c = C next code = 84 val new = C 23
24 input output A B R A D 44 R 52 Ini6al set of character s BR 82 RA 83 AC 84 Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = C x = 41 s = A c = A next code = 85 val new = A 24
25 input output A B R A C D 44 R 52 Ini6al set of character s BR 82 RA 83 AC 84 CA 85 Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = A x = 44 s = D c = D next code = 86 val new = D 25
26 input output A B R A C A D 44 R 52 Ini6al set of character s BR 82 RA 83 AC 84 CA 85 AD 86 Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = D x = 81 s = AB c = A next code = 87 val new = AB 26
27 input output A B R A C A D D 44 R 52 Ini6al set of character s BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = AB x = 83 s = RA c = R next code = 88 val new = RA 27
28 input output A B R A C A D AB D 44 R 52 Ini6al set of character s BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 ABR 88 Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = RA x = 82 s = BR c = B next code = 89 val new = BR 28
29 input output A B R A C A D AB RA D 44 R 52 Ini6al set of character s BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 ABR 88 RAB 89 Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = BR x = 88 s = ABR c = A next code = 8A = RA val new 29
30 input output A B R A C A D AB RA BR D 44 R 52 Ini6al set of character s BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 ABR 88 RAB 89 BRA 8A Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = ABR x = 41 s = A c = A next code = 8B = RA val new 30
31 input output A B R A C A D AB RA BR ABR D 44 R 52 Ini6al set of character s BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 ABR 88 RAB 89 BRA ABRA 8A 8B Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = A x = 80 s = c = next code = val new = 31
32 input output A B R A C A D AB RA BR ABR A D 44 R 52 Ini6al set of character s BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 ABR 88 RAB 89 BRA ABRA 8A 8B Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s val old = x = s = c = next code = val new = 32
33 Expansion data structure For a W- bit, look up value An array of size 2 W D 44 R 52 BR 82 RA 83 AC 84 CA 85 AD 86 DA 87 Expansion Write the current val Read x from input Set s to for x Set next unassigned to val+c where c is 1 st char in s Set val=s ABR 88 RAB 89 BRA ABRA 8A 8B 33
34 A tricky case: ABABABA input A B A B A B A matches Ini6al set of s 34
35 A tricky case: ABABABA input A B A B A B A matches A 41 Ini6al set of s 35
36 A tricky case: ABABABA input A B A B A B A matches A B BA 82 Ini6al set of s 36
37 A tricky case: ABABABA input A B A B A B A matches A B AB BA 82 ABA 83 Ini6al set of s 37
38 A tricky case: ABABABA input A B A B A B A matches A B AB ABA BA 82 ABA 83 NoBce right aler adding entry for "AB" the subsequent text had "AB" as a prefix. Ini6al set of s 38
39 A tricky case: expansion ABABABA input output Ini6al set of s 39
40 A tricky case: expansion ABABABA input output A Ini6al set of s 40
41 A tricky case: expansion ABABABA input output A B BA 82 Ini6al set of s 41
42 A tricky case: expansion ABABABA input output A B AB BA 82 We need to know the first character of 83 in order to enter 83 into the table! Fix: First character of 83 must be same as first leqer of current 81, i.e. "A". Ini6al set of s 42
43 A tricky case: expansion ABABABA input output A B AB ABA BA 82 ABA 83 Ini6al set of s 43
44 Java implementabon, LZW expansion private static final int R = 256; private static final int L = 4096; private static final int W = 12; // number of input chars // number of s = 2^W // width public static void expand() { String[] st = new String[L]; int i; // next available value // initialize symbol table with all 1- character s for (i = 0; i < R; i++) st[i] = "" + (char) i; st[i++] = ""; // (unused) lookahead for EOF int = BinaryStdIn.readInt(W); String val = st[]; } while (true) { BinaryStdOut.write(val); = BinaryStdIn.readInt(W); if ( == R) break; String s = st[]; if (i == ) s = val + val.charat(0); if (i < L) st[i++] = val + s.charat(0); val = s; } BinaryStdOut.close(); // special case hack 44
45 LZW decisions How big of a symbol table? How long is message? Whole message similar model? Many variabons What to do when symbol table fills up? Throw away and start over (e.g. GIF) Throw away when not effecbve (e.g. Unix compress) Many variabons Put longer subs in symbol table? Many variabons 45
46 Lempel- Ziv variants LZW variants LZ77, published Lempel & Ziv, 1977, not patented PNG LZ78, published Lempel & Ziv in 1978, patented LZW, Welch extension to LZ78, patented (expired 2003) GIF, TIFF, Pkzip Deflate = LZ77 variant + Huffman coding 7zip, gzip, jar, PDF 46
47 Some experiments Compressing Moby Dick Using Java programs for LZW and Huffman Different dicbonary sizes for LZW or reseqng when full Compared to zip and gzip 1,191,463 mobydick.txt 485,561 mobydick.gz 485,790 mobydick.zip 398,480 mobydick.xz 371,394 mobydick.bz2 667,651 mobydick.huff 597,060 mobydick.lzw 814,578 mobydick.huff.lzw 592,830 mobydick.lzw.huff 682,400 mobydick.lzw.reset 541,261 mobydick.lzw14 521,700 mobydick.lzw15 503,050 mobydick.lzw16 501,193 mobydick.lzw16.huff 521,700 mobydick.lzw17 514,393 mobydick.lzw18 571,548 mobydick.lzw20 47
48 Enter stabsbcal coding Natural language quite predictable ~ 1 bit of entropy per symbol Huffman coding sbll requires 1- bit min per symbol We're forced to use an integral number of bits DicBonary- based (LZW and friends) Memorizes sequences, but no nobon of frequency StaBsBcal coding Use long, specific context for predicbon P(A The_United_State_of_) =? Blend knowledge using contexts of different lengths AdapBve: model updates and change as text seen Olen aler every leqer! 48
49 MacKay, D.J.C. Informa6on Theory, Inference, and Learning Algorithms. 49
50 Guess the phrase 50
51 A simple unigram model of English ' </s> <sp> a b c d e f g h i j k l m n o p q r s t u v w x y z
52 From text to a real number ArithmeBc coding Message represented by real interval in [0, 1) More precise interval = specifies more bits Example e.g. [ , ) = "it was the best of Bmes" Or any number in that interval, Alphabet = {aeiou!} Transmission = eaii! Symbol Probability Range a 0.2 [0.0, 0.2) e 0.3 [0.2, 0.5) i 0.1 [0.5, 0.6) o 0.2 [0.6, 0.8) u 0.1 [0.8, 0.9)! 0.1 [0.9, 1.0) 52
53 Transmission = eaii! Bell, Cleary, WiKen. Text Compression. AEer seeing Range Symbol Probability Range [0.0, 1.0] a 0.2 [0.0, 0.2) e [0.2, 0.5) e 0.3 [0.2, 0.5) a [0.2, 0.26) i 0.1 [0.5, 0.6) i [0.23, 0.236) o 0.2 [0.6, 0.8) i [0.233, ) u 0.1 [0.8, 0.9)! [ , )! 0.1 [0.9, 1.0) Note: Encoder/decoder agree on symbol to terminate message, here we use! 53
54 Context modeling PredicBon by ParBal Match (PPM) P(next symbol) depends on previous symbol(s) Blending for dealing with 0- frequency problem Easy to build as adapbve Learn from the text as you go No trie to send as in Huffman coding 1984, but sbll compebbve for text Various variants, PPM- A/B/C/D/Z/* Implemented in 7- zip, open source packages, PPM for XML, PPM for executables, 54
55 PPM An alphabet A with q symbols Order = How many previous symbols to use 2 = condibon on 2 previous symbols, P(x n x n- 1, x n- 2 ) 1 = condibon on 1 previous symbol, P(x n x n- 1 ) 0 = condibon on current symbol, P(x n ) - 1 = uniform over the alphabet, P(x n ) = 1/q For a given order: Probability of next symbol based on counbng occurrences given prior context 55
56 Escape probabilibes Need weights to blend probabilibes Compute based on "escape" probability Allocate some mass in each order for when a lower- order model should make predicbon instead Method A: Add one to C 0 : count of characters seen in this context e o = 1 / (C 0 + 1) Method D: u = number of unique characters seen in this context e o = (u / 2) / C 0 56
57 Bell, Cleary, WiKen. Text Compression. 57
58 Lossless benchmarks year scheme bits /char 1967 ASCII Huffman LZ LZMW LZH move- to- front LZB gzip PPMC PPM Burrows- Wheeler BOA RK 1.89 Data using Calgary corpus 58
59 Compressing Moby Dick 1,191,463 mobydick.txt 485,561 mobydick.gz 485,790 mobydick.zip 398,480 mobydick.xz 371,394 mobydick.bz2 667,651 mobydick.huff 597,060 mobydick.lzw 814,578 mobydick.huff.lzw 592,830 mobydick.lzw.huff 682,400 mobydick.lzw.reset 541,261 mobydick.lzw14 521,700 mobydick.lzw15 503,050 mobydick.lzw16 501,193 mobydick.lzw16.huff 521,700 mobydick.lzw17 514,393 mobydick.lzw18 571,548 mobydick.lzw20 325,378 mobydick.ppmd 306,708 mobydick.zpaq 59
60 Summary DicBonary- based LZW and variants Memorizes sequences in data StaBsBcal coding Language model produces probabilibes Probability sequence defines point on [0, 1) Use arithmebc coding to convert to bits PredicBon by ParBal Match (PPM) 60
Lossless compression II
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