Compressed Compact Suffix
Arrays
Veli Mäkinen
University of Helsinki
compact
Gonzalo Navarro
University of Chile
compress
Introduction
 We consider exact string matching on static
text.
 The task is to construct an index for the text
such that the occurrences of a given pattern
can be found efficiently.
 Well known optimal solution exists: build a
suffix tree over the text.
Introduction...
 The suffix-tree-based solution has a
weakness:
It takes too much spa
 In some applications the space usage is the
real bottleneck, not the search efficiency.
Introduction...
 During the last 10 years, many practical /
theoretical solutions with reduced space
complexities have been proposed.
 The work can roughly be divided into three
categories:
(1) Reducing constant factors
(2) Concrete optimization
(3) Abstract optimization
Reducing constant factors
 Suffix arrays (Manber & Myers 1990)
 Suffix cactuses (Kärkkäinen 1995)
 Sparse suffix trees (Kärkkäinen & Ukkonen
1996)
 Space-efficient suffix trees (Kurtz 1998)
 Enhanced suffix arrays (Abouelhoda &
Ohlebusch & Kurtz 2002)
Concrete optimization
 “¼ Minimizing automata”
 DAWGS (Blumer & Blumer & Haussler &
McConnel & Ehrenfeucht 1983)
 Compact DAWGS (Crochemore & Vérin
1997)
 Compact suffix arrays (Mäkinen 2000)
Abstract optimization
 Objective: Use as few space as possible to
support the functionality of a given abstract
definition of a data structure.
 Space is measured in bits and usually given
proportional to the entropy of the text.
Abstract optimization: Example
 A full text index for a given text T supports
the following operations:
- Exists(P): is P a substring of T?
- Count(P): how many times P occurs in T?
- Report(P): list occurrences of P in T.
Abstract optimization...
 Seminal work by Jacobson 1989: rankselect queries on bit-vectors.
 Rank-select-type structures for suffix trees
(Munro & Raman & Rao & Clark 1996-)
 Lempel-Ziv index (Kärkkäinen & Ukkonen
1996)
Abstract optimization...
 Compressed suffix arrays (Grossi & Vitter
2000, Sadakane 2000, 2002)
 FM-index (Ferragina & Manzini 2000)
 LZ-self-index (Navarro 2002)
 Space-optimal full-text indexes (Grossi &
Gupta & Vitter 2003, 2004)
This paper
 We use both concrete and abstract
optimization.
 We compress compact suffix array into a
succinct full-text index, supporting:
- Exists(P), Count(P) in O(|P| log |T|) time.
- Report(P) in O((|P|+occ)log |T|) time, where
occ is the number of occurrences.
This paper...
 Space requirement of our index is O(n(1+Hk
log n)) bits, where Hk=Hk(T) is the order-k
empirical entropy of T.
 Hk: “the average number of bits needed to
encode a symbol after seeing the k previous
ones, using a fixed codebook”.
This paper...
 In practice, the size of our index is 1.67 times
the text size including the text.
 Search times are comparable to compressed
suffix arrays that occupy O(H0 n) bits.
 Our index takes O(log n) times more space
than FM-index and the other space-optimal
indexes.
This paper...
 Simpler than the previous approaches and
more efficient in practice.
 No limitations on the alphabet size s:
- FM-index assumes constant alphabet.
- Some compressed suffix arrays assume
s=polylog(n).
Big picture
 Compact suffix array (CSA): some areas of a
suffix array are replaced by links to similar
areas.
 Compressed CSA (CCSA): We use the
conceptual structure of optimal CSA as such.
 We represent the links with respect to the
original suffix array.
Big picture...
 A bit-vector represents the boundaries of
areas replaced by links.
 Each area is represented by an integer
denoting the start of the linked area.
 Some additional structures are attached to
encode the text inside CCSA, etc.
Example: suffix array
sa
suffix
1: 12
$
2: 11
i$
3: 8
ippi$
4: 5
issippi$
5: 2
ississippi$
6: 1
mississippi$
7: 10
pi$
8: 9
ppi$
9: 7
sippi$
10: 4
sissippi$
11: 6
ssippi$
12: 3
ssissippi$
1 2 3 4 5 6 7 8 9 10 11 12
T= m i s s i s s i p p i $
Example: CSA
sa
csa
1: 12
1: (5,0,1)
2: 11
2: (1,0,1)
3: 8
3: (7,0,1)
4: 5
4: (9,0,2)
5: 2
6: 1
5: (4,1,1)
7: 10
6: (2,0,1)
8: 9
7: (6,0,1)
9: 7
8: (3,0,2)
10: 4
11: 6
12: 3
9: (8,0,2)
Example: CCSA
sa
ccsa
csa
1: 12
1: (5,0,1)
1: 6
1: 1
2: 11
2: (1,0,1)
2: 1
2: 1
3: 8
3: (7,0,1)
3: 8
3: 1
4: 5
4: (9,0,2)
4: 11
4: 1
5: 0
5: 2
6: 1
5: (4,1,1)
5: 5
6: 1
7: 10
6: (2,0,1)
6: 2
7: 1
8: 9
7: (6,0,1)
7: 7
8: 1
9: 7
8: (3,0,2)
8: 3
9: 1
10: 0
10: 4
11: 6
12: 3
9: (8,0,2)
9: 9
11: 1
12: 0
Example: CCSA...
ccsa
sa
1: 12
1: 6
1: 1
1: $
2: 11
2: 1
2: 1
2: i
3: 8
3: 8
3: 1
3: i
4: 5
4: 11
4: 1
4: i
5: 0
5: 2
1 2 3 4 5 6 7 8 9 10 11 12
m i s s i s s i p p i $
5: i
6: 1
5: 5
6: 1
7: 10
6: 2
7: 1
7: p
8: 9
7: 7
8: 1
8: p
9: 7
8: 3
9: 1
9: s
10: 0
10: s
11: 1
11: s
12: 0
12: s
10: 4
11: 6
12: 3
9: 9
6: m
Example: CCSA...
sa
ccsa
1: 12
1: 6
1: 1
1: $
1: 1
1: $
2: 11
2: 1
2: 1
2: i
2: 1
2: i
3: 8
3: 8
3: 1
3: m
3: 0
3: i
4: 5
4: 11
4: 1
4: p
4: 0
4: i
5: 0
5: s
5: 0
5: i
5: 2
6: 1
5: 5
6: 1
6: 1
6: m
7: 10
6: 2
7: 1
7: 1
7: p
8: 9
7: 7
8: 1
8: 0
8: p
9: 7
8: 3
9: 1
9: 1
9: s
10: 0
10: 0
10: s
11: 1
11: 0
11: s
12: 0
12: 0
12: s
10: 4
11: 6
12: 3
9: 9
Search on CCSA
 We simulate the standard binary search of
suffix array on CCSA.
 A sub-problem in the search is to compare
the pattern P against a suffix Tsa[i]...|T|.
 For this, we extract tsa[i] , tsa[i]+1 , tsa[i]+2 , ...,
tsa[i]+|P|-1, following the links of the CCSA.
Example: Search on CCSA
P=“isi” vs. Tsa[4]...|T| ?
Tsa[4]...|T| = i
ccsa
1: 6
1: 1
2: 1
2: 1
3: 8
4: 11
1: $
1: 1
2: i
2: 1
3: 1
3: m
3: 0
4: 1
4: p
4: 0
5: 0
5: s
5: 0
4
5: 5
6: 1
6: 1
6: 2
7: 1
7: 1
7: 7
8: 1
8: 0
8: 3
9: 1
9: 1
10: 0
10: 0
11: 1
11: 0
12: 0
12: 0
9: 9
2
Example: Search on CCSA
P=“isi” vs. Tsa[4]...|T| ?
Tsa[4]...|T| = i s
ccsa
1: 6
1: 1
1: $
1: 1
2: 1
2: 1
2: i
2: 1
3: 8
3: 1
3: m
3: 0
4: 11
4: 1
4: p
4: 0
5: 0
5: s
5: 0
5: 5
6: 1
6: 2
7: 1
7: 1
7: 7
8: 1
8: 0
8: 3
9: 1
9: 1
10: 0
10: 0
11: 1
11: 0
12: 0
12: 0
9: 9
9
6: 1
5
Example: Search on CCSA
P=“isi” vs. Tsa[4]...|T| ?
ccsa
1: 6
1: 1
1: $
1: 1
2: 1
2: 1
2: i
2: 1
3: 8
3: 1
3: m
3: 0
4: 11
4: 1
4: p
4: 0
5: s
5: 0
5: 0
Tsa[4]...|T| = i s s
>P
8
5: 5
6: 1
6: 1
6: 2
7: 1
7: 1
7: 7
8: 1
8: 0
8: 3
9: 1
9: 1
10: 0
10: 0
11: 1
11: 0
12: 0
12: 0
9: 9
5
Search on CCSA...
 To follow a link in constant time, we need
the operations rank(i) and selectprev(i) on
bit-vectors:
- rank(i) gives the number of 1’s upto
position i.
- selectprev(i) gives the position of the
previous 1 before position i.
Search on CCSA...
 Lemma [Jacobson 89, Munro et al. 96]: A
bit-vector of length n can be replaced with a
structure of size n+o(n) so that queries
rank(i) and selectprev(i) can be supported in
constant time.
Search on CCSA...
 Corollary: Existence and counting queries
can be supported by CCSA in time O(|P| log
|T|).
 Reporting queries can be supported by a
similar technique to access sampled suffixes.
Size of CCSA
 Overall we use O(n)+n’log n bits of space,
where n’ is the number of entries in the main
CCSA table.
 We show in the paper that n’ is also the
number of runs of symbols in the BurrowsWheeler transformed text.
 Finally, we show that n’· 2Hk n +sk.
Comparison: default settings
times |T|
FM
0.36
CSA 0.69
CCSA 1.67
LZ
1.5
Comparison: default settings...
times |T|
FM
0.36
CSA 0.69
CCSA 1.67
LZ
1.5
Comparison: same sample rate
times |T|
FM
0.41
CSA 0.58
CCSA 1.67
Comparison: same space
times |T|
FM
1.69
CSA 1.59
CCSA 1.67
LZ
1.5
Comparison: same space...
times |T|
FM
1.69
CSA 1.59
CCSA 1.67
LZ
1.5
Conclusion
 CCSA is much faster than the default
implementations of other small indexes in
reporting (except LZ-index).
 However, as the basic structure of the other
indexes takes less space, it is possible to
implement them using smaller sampling step
to make them occupy the same space as
CCSA and to work as efficiently.
Future
 In a subsequent work we have developed an
index (a cross between CCSA and FM-index)
taking O(Hk log s) bits of space supporting
counting queries in time O(|P|).
- optimal space/time on constant alphabet
- turns the exponential additive alphabet
factor of FM-index into a logarithmic
multiplicative factor.