<s>
A	O
dictionary	B-Algorithm
coder	I-Algorithm
,	O
also	O
sometimes	O
known	O
as	O
a	O
substitution	O
coder	O
,	O
is	O
a	O
class	O
of	O
lossless	B-Algorithm
data	I-Algorithm
compression	I-Algorithm
algorithms	O
which	O
operate	O
by	O
searching	O
for	O
matches	O
between	O
the	O
text	O
to	O
be	O
compressed	O
and	O
a	O
set	O
of	O
strings	O
contained	O
in	O
a	O
data	B-General_Concept
structure	I-General_Concept
(	O
called	O
the	O
'	O
dictionary	O
 '	O
)	O
maintained	O
by	O
the	O
encoder	O
.	O
</s>
<s>
When	O
the	O
encoder	O
finds	O
such	O
a	O
match	O
,	O
it	O
substitutes	O
a	O
reference	O
to	O
the	O
string	O
's	O
position	O
in	O
the	O
data	B-General_Concept
structure	I-General_Concept
.	O
</s>
<s>
Some	O
dictionary	B-Algorithm
coders	I-Algorithm
use	O
a	O
'	O
static	O
dictionary	O
 '	O
,	O
one	O
whose	O
full	O
set	O
of	O
strings	O
is	O
determined	O
before	O
coding	O
begins	O
and	O
does	O
not	O
change	O
during	O
the	O
coding	O
process	O
.	O
</s>
<s>
This	O
approach	O
is	O
most	O
often	O
used	O
when	O
the	O
message	O
or	O
set	O
of	O
messages	O
to	O
be	O
encoded	O
is	O
fixed	O
and	O
large	O
;	O
for	O
instance	O
,	O
an	O
application	B-Application
that	O
stores	O
the	O
contents	O
of	O
a	O
book	O
in	O
the	O
limited	O
storage	O
space	O
of	O
a	O
PDA	B-Application
generally	O
builds	O
a	O
static	O
dictionary	O
from	O
a	O
concordance	O
of	O
the	O
text	O
and	O
then	O
uses	O
that	O
dictionary	O
to	O
compress	O
the	O
verses	O
.	O
</s>
<s>
This	O
scheme	O
of	O
using	O
Huffman	B-General_Concept
coding	I-General_Concept
to	O
represent	O
indices	O
into	O
a	O
concordance	O
has	O
been	O
called	O
"	O
Huffword	B-Algorithm
"	O
.	O
</s>
<s>
In	O
LZ77	O
,	O
a	O
circular	B-Data_Structure
buffer	I-Data_Structure
called	O
the	O
"	O
sliding	O
window	O
"	O
holds	O
the	O
last	O
N	O
bytes	O
of	O
data	O
processed	O
.	O
</s>
<s>
LZW	B-Algorithm
is	O
similar	O
to	O
LZ78	O
,	O
but	O
,	O
the	O
dictionary	O
is	O
initialized	O
to	O
all	O
possible	O
symbols	O
.	O
</s>
<s>
Refer	O
to	O
LZW	B-Algorithm
for	O
how	O
this	O
is	O
handled	O
.	O
</s>
<s>
Enhancements	O
to	O
LZW	B-Algorithm
include	O
handing	O
symbol	O
sizes	O
other	O
than	O
8	O
bits	O
and	O
having	O
reserved	O
codes	O
to	O
reset	O
the	O
dictionary	O
and	O
to	O
indicate	O
end	O
of	O
data	O
.	O
</s>
