8puzzle is a very interesting and a well known problem in the field of Artificial Intelligence. It always has been an important subject in articles, books and become a part of course material in many universities. The 8puzzle is also known as the slidingblock puzzle or tilepuzzle and is meant for a single user. 8puzzle consists of 8 square tiles numbered 1 through 8 and one blank space on a 3 by 3 board. Moves of the puzzle are made by sliding an adjacent tile into the position occupied by the blank space, which has the effect of exchanging the positions of the tile and blank space. Only tiles that are horizontally or vertically adjacent (not diagonally adjacent) may be moved into the blank space. The objective of the game is to start from an initial configuration and end up in a configuration which the tiles are placed in ascending number order as shown in: figure 1 below.
Figure 1 Goal state of the 8puzzle
HOW TO PLAY
The game can be played using the arrow keys. Up arrow slides a tile up into the empty space, left arrow slides a tile left into the empty space and so on. You can choose to solve it automatically by program. It will display the animation sliding the tiles and solve the 8 puzzle in least possible moves using AI.METHODOLOGY
According to the authors Russel and Norvig in their book titled
Artificial Intelligence: A Modern Approach, the average solution for a
randomly generated 8puzzle instance is about 22 steps. This figure is
obtained by estimating the branching factor which is about 3 (when the
empty tile is in the middle, there are four possible moves; when it is
in a corner there are two; and when it is along an edge there are
three). This means that an exhaustive search to depth 22 would look at
about 322 ~ 3.1 x 1010 states. By keeping track of repeated states, it
is possible to cut this down by a factor of about 170,000, because there
are only 9!/2 = 181,440 distinct states that are reachable.ALGORITHM
After a preliminary investigation of the heuristic search strategies we
were able to figure out that A* algorithm is the best for the 8puzzle
problem. A* is the most widely used form of best first search algorithm
which is an instance of TreeSearch or GraphSearch where a best node is
expanded on the basis of an evaluation function f(n). Here, the
evaluation function of each node is calculated as a sum of two functions
g(n) and h(n) where, g(n) refers to the cost to reach the node n while
h(n) is the cost to get from node n to the goal. A* algorithm is
both optimal and complete. It is optimal in that sense h(n) is
admissible and is never overestimatedThe following pseudo code describes the A* algorithm:
function
A*(start,goal)
closedset := the empty set // The set of nodes
already evaluated.
openset := set containing the initial
node // The set of tentative nodes to be evaluated.
g_score[start] := 0 // Distance from
start along optimal path.
h_score[start] :=
heuristic_estimate_of_distance(start, goal)
f_score[start] := h_score[start] // Estimated total distance from
start to goal through y.
while
openset is not empty
x := the node in openset having the
lowest f_score[] value
if x = goal
return
reconstruct_path(came_from[goal])
remove x from openset
add x to closedset
foreach y in neighbor_nodes(x)
if y in closedset
continue
tentative_g_score := g_score[x] +
dist_between(x,y)
if y not in openset
add y to openset
tentative_is_better := true
elseif tentative_g_score <
g_score[y]
tentative_is_better := true
else
tentative_is_better := false
if tentative_is_better = true
came_from[y] := x
g_score[y] :=
tentative_g_score
h_score[y] :=
heuristic_estimate_of_distance(y, goal)
f_score[y] := g_score[y] +
h_score[y]
return failure
function reconstruct_path(current_node)
if came_from[current_node] is set
p =
reconstruct_path(came_from[current_node])
return (p + current_node)
else
return current_node

REFERENCE
1. Rich and Knight, Artificial Intelligence.3. A* search algorithm  Wikipedia, the free encyclopedia
4. Description and a solution strategy for the 8puzzle from http://www.8puzzle.com/
5. G. Novak, CS 381K: Heuristic Search: 8 Puzzle, 26 Sep 07
6. An appealing Java applet from www.permadi.com/java/puzzle8/
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