<s>
The	O
term	O
compressed	B-Data_Structure
data	I-Data_Structure
structure	I-Data_Structure
arises	O
in	O
the	O
computer	B-General_Concept
science	I-General_Concept
subfields	O
of	O
algorithms	O
,	O
data	B-General_Concept
structures	I-General_Concept
,	O
and	O
theoretical	O
computer	B-General_Concept
science	I-General_Concept
.	O
</s>
<s>
It	O
refers	O
to	O
a	O
data	B-General_Concept
structure	I-General_Concept
whose	O
operations	O
are	O
roughly	O
as	O
fast	O
as	O
those	O
of	O
a	O
conventional	O
data	B-General_Concept
structure	I-General_Concept
for	O
the	O
problem	O
,	O
but	O
whose	O
size	O
can	O
be	O
substantially	O
smaller	O
.	O
</s>
<s>
The	O
size	O
of	O
the	O
compressed	B-Data_Structure
data	I-Data_Structure
structure	I-Data_Structure
is	O
typically	O
highly	O
dependent	O
upon	O
the	O
entropy	O
of	O
the	O
data	O
being	O
represented	O
.	O
</s>
<s>
Important	O
examples	O
of	O
compressed	B-Data_Structure
data	I-Data_Structure
structures	I-Data_Structure
include	O
the	O
compressed	B-Algorithm
suffix	I-Algorithm
array	I-Algorithm
and	O
the	O
FM-index	B-Algorithm
,	O
both	O
of	O
which	O
can	O
represent	O
an	O
arbitrary	O
text	O
of	O
characters	O
T	O
for	O
pattern	B-Language
matching	I-Language
.	O
</s>
<s>
The	O
space	O
they	O
occupy	O
is	O
roughly	O
equal	O
to	O
the	O
size	O
of	O
the	O
text	O
T	O
in	O
entropy-compressed	O
form	O
,	O
such	O
as	O
that	O
obtained	O
by	O
Prediction	B-Algorithm
by	I-Algorithm
Partial	I-Algorithm
Matching	I-Algorithm
or	O
gzip	B-Application
.	O
</s>
<s>
Moreover	O
,	O
both	O
data	B-General_Concept
structures	I-General_Concept
are	O
self-indexing	O
,	O
in	O
that	O
they	O
can	O
reconstruct	O
the	O
text	O
T	O
in	O
a	O
random	O
access	O
manner	O
,	O
and	O
thus	O
the	O
underlying	O
text	O
T	O
can	O
be	O
discarded	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
they	O
simultaneously	O
provide	O
a	O
compressed	O
and	O
quickly	O
searchable	O
representation	O
of	O
the	O
text	O
T	O
.	O
They	O
represent	O
a	O
substantial	O
space	O
improvement	O
over	O
the	O
conventional	O
suffix	B-Architecture
tree	I-Architecture
and	O
suffix	B-Algorithm
array	I-Algorithm
,	O
which	O
occupy	O
many	O
times	O
more	O
space	O
than	O
the	O
size	O
of	O
T	O
.	O
They	O
also	O
support	O
searching	O
for	O
arbitrary	O
patterns	O
,	O
as	O
opposed	O
to	O
the	O
inverted	B-Algorithm
index	I-Algorithm
,	O
which	O
can	O
support	O
only	O
word-based	O
searches	O
.	O
</s>
<s>
In	O
addition	O
,	O
inverted	B-Algorithm
indexes	I-Algorithm
do	O
not	O
have	O
the	O
self-indexing	O
feature	O
.	O
</s>
<s>
An	O
important	O
related	O
notion	O
is	O
that	O
of	O
a	O
succinct	B-Data_Structure
data	I-Data_Structure
structure	I-Data_Structure
,	O
which	O
uses	O
space	O
roughly	O
equal	O
to	O
the	O
information-theoretic	O
minimum	O
,	O
which	O
is	O
a	O
worst-case	O
notion	O
of	O
the	O
space	O
needed	O
to	O
represent	O
the	O
data	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
size	O
of	O
a	O
compressed	B-Data_Structure
data	I-Data_Structure
structure	I-Data_Structure
depends	O
upon	O
the	O
particular	O
data	O
being	O
represented	O
.	O
</s>
<s>
When	O
the	O
data	O
are	O
compressible	O
,	O
as	O
is	O
often	O
the	O
case	O
in	O
practice	O
for	O
natural	O
language	O
text	O
,	O
the	O
compressed	B-Data_Structure
data	I-Data_Structure
structure	I-Data_Structure
can	O
occupy	O
space	O
very	O
close	O
to	O
the	O
information-theoretic	O
minimum	O
,	O
and	O
significantly	O
less	O
space	O
than	O
most	O
compression	O
schemes	O
.	O
</s>
