Reversi

Reversi is a strategic board game played by two parties on an eight by eight square grid with pieces that have two distinct sides - coin-like markers with a light and a dark face are typical.

Mattel produces reversi equipment under the Othello™ brand.

Each of the two sides corresponds to one player; we will call them light and dark after the sides of Othello pieces, but "heads" versus "tails" is equally feasible so long as each marker has sufficiently distinctive sides.

Originally, Reversi didn't have a starting position to begin from. However, the rules for Othello state that the game begins with four markers placed in a square in the middle of the grid, two facing light-up (indicated by o in our diagrams), two pieces with the dark side up (indicated by *). The light player makes the first move.

   *o
   o*  (one of the possible starting positions)

Light must place a piece with the light side up on the grid, so that there is a straight (horizontal, vertical, or diagonal) line between the new piece and another light piece, with at least one dark piece between them. In the above situation, light has the following options indicated by dots:

   .
  .*o
   o*.
    .

After placing the piece, light is entitled to turn around all dark pieces between the new piece and the next light piece on a straight line, should one exist. All reversed pieces now show the light side, and light can use them in later moves -- unless dark was able to turn them back in the mean time.

If light decided to put a piece in the topmost location (all choices are equivalent at this time), one piece will be turned around:

   o
   oo
   o*

Now it is dark's turn. This player operates under the same rules, with the roles reversed: A dark piece is laid down so that one or more straight lines of light pieces can be turned around. Possiblities at this time:

   o.
   oo
  .o*

Dark takes the left alternative and is able to turn around one piece:

   o
   oo
  ***

Players take alternating turns. Should one player be unable to make a valid move it is the other player's turn again. When both players are unable to move, the game ends. This is the case when the grid has filled up, or when one player has no more pieces on the board. The player with more pieces available at the end wins.

Strategy

Great earnings are often quickly lost again. Before the end, positions are more important than sheer numbers. For example, the sides of the board are privileged, because pieces on the sides are harder to reverse. Most coveted are the corners because a piece residing there can never again be turned around.

References

• Play reversi online (http://www.gametheory.net/html/games.html)