Reversi, also known as Othello, is a strategy game played by two players: Black and White. It is played on an 8x8 board (usually Green in colour), called the Othello board. The two players place 64 discs, each of which is black on one side and white on the other. For convenience, each player begins with 32 discs but these do not belong to him and if his opponent runs out of discs, he is obliged to give him some. A disc is black if the black side is visible and white if the white face is on top.
The winner is the player who has more discs of his colour than his opponent at the end of the game. This will happen when neither of the two players has a legal move. Generally at this stage all 64 squares are occupied.
Figure 1 : the Initial Position
At the beginning of the game, two black discs are placed on e4 and d5 and
two white discs on d4 and e5 (see fig. 1).
Black always begins, and the two players subsequently take turns moving.
Fig. 2 : Black plays f5 ...
Fig. 3 : and flips e5 !
Fig. 4 : White f4, f6 or d6
Fig. 5 : if white plays d6.
At his turn, a player must place a disc of his colour on one of the empty squares of the othello board, adjacent to an opponent's disc. In addition, by placing his disc, he must flank one or several of his opponent's discs between the disc played and another disc of his own colour already on the board. He then flips to his colour all the discs which were flanked. The discs are neither removed from the Othello board nor moved from one square to another.
Black's first move may be, for example, to f5 (see figure 2). By playing f5, he flanks the white disc at e5 between the disc played and another black disc already on the board (here d5) ; he then flips this disc (see figure 3). Black could also have moved to e6, c4 or d3. However, these four black moves are perfectly symmetrical ; black needn't spend time thinking about his first move.
Now it's white's turn to move. He has three possible moves (see figure 4). Each possible move flips at least one opponent's disc. White may play f4, f6 or d6. Note that discs can be flanked in all eight directions. Furthermore, in each direction several discs may be flanked (see figures 6 and 7). All these flanked discs must be flipped.
Fig. 6 : Black plays c6 ...
Fig. 7 : resulting in this position.
Black has played to c6. He turns the discs at b6 (flanked by the disc at a6), b5 (flanked by a4), d7 (flanked by e8), c5 and c4 (flanked by c3). Note that neither d6 nor e6 are flipped due to the empty square at f6.
Fig. 8 : Black plays a5
Fig. 9 : c4 remains white.
There is no chain reaction : flipped discs may not be used to flip other discs on the same move. Thus, in figure 8, black moves to a5 :
The discs at b5 and c5 are flipped since they are flanked. At this point, even though c4 is flanked, it is not flipped (see figure 9). The reason for this is that it is not flanked between the disc played and another disc.
If, at your turn, you may not make a move to flip at least one opponent's disc according to these rules, you must pin your opponent's turn to play. But if a move is possible, you must play it.
Fig. 10 : The game is over !
The game is over when neither of the two players has a legal move.
Generally, this happens when all 64 squares are occupied. However, it is possible that some empty squares will remain where neither player may move : for example, if all the discs are the same colour after a turn, or in a position such as the one below (see figure 10).
Neither of the two players can play to b1 since no flipped discs are possible. In this case, we count discs to determine the final score. Empty squares are given to the winner by convention. In this game, white has 29 discs and black has 34, with one empty square. Thus black wins 35-29.