<s>
In	O
information	O
theory	O
,	O
an	O
entropy	B-Algorithm
coding	I-Algorithm
(	O
or	O
entropy	B-Algorithm
encoding	I-Algorithm
)	O
is	O
any	O
lossless	B-Algorithm
data	I-Algorithm
compression	I-Algorithm
method	O
that	O
attempts	O
to	O
approach	O
the	O
lower	O
bound	O
declared	O
by	O
Shannon	B-Protocol
's	I-Protocol
source	I-Protocol
coding	I-Protocol
theorem	I-Protocol
,	O
which	O
states	O
that	O
any	O
lossless	B-Algorithm
data	I-Algorithm
compression	I-Algorithm
method	O
must	O
have	O
expected	O
code	O
length	O
greater	O
or	O
equal	O
to	O
the	O
entropy	O
of	O
the	O
source	O
.	O
</s>
<s>
More	O
precisely	O
,	O
the	O
source	B-Protocol
coding	I-Protocol
theorem	I-Protocol
states	O
that	O
for	O
any	O
source	O
distribution	O
,	O
the	O
expected	O
code	O
length	O
satisfies	O
,	O
where	O
is	O
the	O
number	O
of	O
symbols	O
in	O
a	O
code	O
word	O
,	O
is	O
the	O
coding	O
function	O
,	O
is	O
the	O
number	O
of	O
symbols	O
used	O
to	O
make	O
output	O
codes	O
and	O
is	O
the	O
probability	O
of	O
the	O
source	O
symbol	O
.	O
</s>
<s>
An	O
entropy	B-Algorithm
coding	I-Algorithm
attempts	O
to	O
approach	O
this	O
lower	O
bound	O
.	O
</s>
<s>
Two	O
of	O
the	O
most	O
common	O
entropy	B-Algorithm
coding	I-Algorithm
techniques	O
are	O
Huffman	B-General_Concept
coding	I-General_Concept
and	O
arithmetic	B-Algorithm
coding	I-Algorithm
.	O
</s>
<s>
If	O
the	O
approximate	O
entropy	O
characteristics	O
of	O
a	O
data	B-General_Concept
stream	I-General_Concept
are	O
known	O
in	O
advance	O
(	O
especially	O
for	O
signal	O
compression	O
)	O
,	O
a	O
simpler	O
static	O
code	O
may	O
be	O
useful	O
.	O
</s>
<s>
These	O
static	O
codes	O
include	O
universal	B-Algorithm
codes	I-Algorithm
(	O
such	O
as	O
Elias	B-Algorithm
gamma	I-Algorithm
coding	I-Algorithm
or	O
Fibonacci	B-Algorithm
coding	I-Algorithm
)	O
and	O
Golomb	B-Algorithm
codes	I-Algorithm
(	O
such	O
as	O
unary	B-Algorithm
coding	I-Algorithm
or	O
Rice	O
coding	O
)	O
.	O
</s>
<s>
Since	O
2014	O
,	O
data	O
compressors	O
have	O
started	O
using	O
the	O
asymmetric	B-General_Concept
numeral	I-General_Concept
systems	I-General_Concept
family	O
of	O
entropy	B-Algorithm
coding	I-Algorithm
techniques	O
,	O
which	O
allows	O
combination	O
of	O
the	O
compression	O
ratio	O
of	O
arithmetic	B-Algorithm
coding	I-Algorithm
with	O
a	O
processing	O
cost	O
similar	O
to	O
Huffman	B-General_Concept
coding	I-General_Concept
.	O
</s>
<s>
Besides	O
using	O
entropy	B-Algorithm
coding	I-Algorithm
as	O
a	O
way	O
to	O
compress	O
digital	O
data	O
,	O
an	O
entropy	O
encoder	O
can	O
also	O
be	O
used	O
to	O
measure	O
the	O
amount	O
of	O
similarity	O
between	O
streams	B-General_Concept
of	I-General_Concept
data	I-General_Concept
and	O
already	O
existing	O
classes	O
of	O
data	O
.	O
</s>
<s>
This	O
is	O
done	O
by	O
generating	O
an	O
entropy	O
coder/compressor	O
for	O
each	O
class	O
of	O
data	O
;	O
unknown	O
data	O
is	O
then	O
classified	B-General_Concept
by	O
feeding	O
the	O
uncompressed	O
data	O
to	O
each	O
compressor	O
and	O
seeing	O
which	O
compressor	O
yields	O
the	O
highest	O
compression	O
.	O
</s>
