<s>
Sokoban	B-Device
was	O
created	O
in	O
1981	O
by	O
Hiroyuki	O
Imabayashi	O
.	O
</s>
<s>
Sokoban	B-Device
was	O
a	O
hit	O
in	O
Japan	O
where	O
it	O
sold	O
more	O
than	O
400,000	O
copies	O
before	O
being	O
released	O
in	O
the	O
United	O
States	O
.	O
</s>
<s>
In	O
1988	O
,	O
Sokoban	B-Device
was	O
published	O
in	O
US	O
by	O
Spectrum	O
HoloByte	O
for	O
the	O
Commodore	O
64	O
,	O
IBM	B-Device
PC	I-Device
compatibles	O
,	O
Amiga	B-Device
,	O
and	O
Apple	B-Device
II	I-Device
as	O
Soko-Ban	B-Device
.	O
</s>
<s>
Implementations	O
of	O
Sokoban	B-Device
have	O
been	O
written	O
for	O
numerous	O
computer	B-Device
platforms	I-Device
,	O
including	O
almost	O
all	O
home	O
computer	O
and	O
personal	B-Device
computer	I-Device
systems	O
.	O
</s>
<s>
Different	O
versions	O
also	O
exist	O
for	O
video	B-Device
game	I-Device
consoles	I-Device
,	O
mobile	O
phones	O
,	O
graphic	B-Application
calculators	I-Application
,	O
digital	B-Device
cameras	I-Device
and	O
electronic	B-Device
organizers	I-Device
.	O
</s>
<s>
Sokoban	B-Device
can	O
be	O
studied	O
using	O
the	O
theory	O
of	O
computational	O
complexity	O
.	O
</s>
<s>
The	O
problem	O
of	O
solving	O
Sokoban	B-Device
puzzles	O
was	O
first	O
proved	O
to	O
be	O
NP-hard	O
.	O
</s>
<s>
This	O
is	O
of	O
interest	O
for	O
artificial	B-Application
intelligence	I-Application
(	O
AI	B-Application
)	O
research	O
because	O
solving	O
Sokoban	B-Device
can	O
be	O
compared	O
to	O
the	O
automated	B-Application
planning	I-Application
required	O
by	O
some	O
autonomous	O
robots	O
.	O
</s>
<s>
Sokoban	B-Device
is	O
difficult	O
not	O
only	O
because	O
of	O
its	O
large	O
branching	B-Data_Structure
factor	I-Data_Structure
,	O
but	O
also	O
because	O
of	O
its	O
large	O
search	B-Data_Structure
tree	I-Data_Structure
depth	O
.	O
</s>
<s>
Some	O
level	O
types	O
can	O
even	O
be	O
extended	O
indefinitely	O
,	O
with	O
each	O
iteration	B-Algorithm
requiring	O
an	O
exponentially	O
growing	O
number	O
of	O
moves	O
and	O
pushes	O
.	O
</s>
<s>
Skilled	O
human	O
players	O
rely	O
mostly	O
on	O
heuristics	B-Algorithm
and	O
are	O
usually	O
able	O
to	O
quickly	O
discard	O
a	O
great	O
many	O
futile	O
or	O
redundant	O
lines	O
of	O
play	O
by	O
recognizing	O
patterns	O
and	O
subgoals	O
,	O
thereby	O
drastically	O
reducing	O
the	O
search	O
effort	O
.	O
</s>
<s>
Some	O
Sokoban	B-Device
puzzles	O
can	O
be	O
solved	O
automatically	O
by	O
using	O
a	O
single-agent	O
search	O
algorithm	O
,	O
such	O
as	O
IDA*	B-Protocol
;	O
enhanced	O
by	O
several	O
techniques	O
that	O
make	O
use	O
of	O
domain-specific	O
knowledge	O
.	O
</s>
<s>
This	O
is	O
the	O
method	O
used	O
by	O
Rolling	O
Stone	O
,	O
a	O
Sokoban	B-Device
solver	O
developed	O
by	O
the	O
University	O
of	O
Alberta	O
GAMES	O
Group	O
.	O
</s>
<s>
However	O
,	O
the	O
more	O
complex	O
Sokoban	B-Device
levels	O
are	O
out	O
of	O
reach	O
even	O
for	O
the	O
best	O
automated	O
solvers	O
.	O
</s>
<s>
Several	O
puzzles	O
can	O
be	O
considered	O
variants	O
of	O
the	O
original	O
Sokoban	B-Device
game	O
in	O
the	O
sense	O
that	O
they	O
all	O
make	O
use	O
of	O
a	O
controllable	O
character	O
pushing	O
boxes	O
around	O
in	O
a	O
maze	B-Application
.	O
</s>
<s>
Alternative	O
tilings	O
:	O
In	O
the	O
standard	O
game	O
,	O
the	O
mazes	B-Application
are	O
laid	O
out	O
on	O
a	O
square	O
grid	O
.	O
</s>
<s>
Several	O
variants	O
apply	O
the	O
rules	O
of	O
Sokoban	B-Device
to	O
mazes	B-Application
laid	O
out	O
on	O
other	O
tilings	O
.	O
</s>
<s>
Multiple	O
pushers	O
:	O
In	O
the	O
variants	O
Multiban	B-Device
and	O
Interlock	O
,	O
the	O
player	O
can	O
control	O
multiple	O
characters	O
.	O
</s>
<s>
Standard	O
Sokoban	B-Device
puzzles	O
can	O
be	O
played	O
in	O
reverse	O
mode	O
,	O
and	O
the	O
reverse-mode	O
solutions	O
can	O
be	O
converted	O
to	O
solutions	O
for	O
the	O
standard-mode	O
puzzles	O
.	O
</s>
<s>
Therefore	O
,	O
reverse-mode	O
gameplay	O
can	O
also	O
be	O
instrumental	O
in	O
solving	O
standard	O
Sokoban	B-Device
puzzles	O
.	O
</s>
