Let's begin this lesson with a look at
a common single player game called eight puzzle.
Eight puzzle is a sliding tile game.
The game board is a three by three square,
with number tiles in all but one of the cells.
The state of the game is modified by sliding
number tiles into the empty space from adjacent cells.
Thus moving the empty space to a new location.
there are four possible moves in, at most four possible moves.
Moving empty space up, down, left or right.
Obviously not all these moves are possible on all states.
States shown on the left and right here illustrate
the possible moves from the state shown in the center.
The tile could be moved down.
The emp, I'm sorry, the empty cell can be moved
down, the empty cell can be moved to the right.
The ultimate objective of the game is to place the tiles in order,
and position the empty square in the lower right cell, as shown here.
the game terminates after 8 moves or when all of
the tiles are in their right positions, whichever comes first.
In some versions of the game, partial credit
is given for states that approximate the ultimate goal.
With ten points being allocated for each numbered tile in the correct position,
and 20 points being allocated for having the empty tile in the correct position.
For example, the first state shown here is worth 40 points, since
four of the cells, the ones colored green are in their correct positions.
And the second stage on here is worth a hundred points since all of the tiles
are in their correct positions, and the empty
cell is in the lower right hand corner.
Now as with other games, it is possible to describe eight puzzle in GDL.
Here are the rules for three puzzle, its a version of
eight puzzle in a two by two board without partial credit.
Rules for eight puzzle with partial credit are, are very much the same,
except they're a little more verbose, and wouldn't easily fit on a page.
We won't go through the details of either rule set in
this lesson, but it may be worth your while to pause and
look over the rules and be sure you understand them.
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