Reversi
Flip, surround, dominate the board.
Surround your opponent's discs to flip them to your colour. A minute to learn, a lifetime to master corners and mobility.
- Players
- 1 or 2 players
- Length
- 10 min
- Against the AI
- Two players on one device
15 days of full access, no account · No download
How to play
Goal of the game
To have the most discs of your colour when nobody can move any more. The board is 8 × 8 squares; the game starts with four discs in the centre.
How it works
- Black goes first. Every move must outflank at least one opposing disc: in a straight line (horizontal, vertical or diagonal), between the disc you place and another disc of your colour, with no empty square between them.
- All outflanked discs, in every direction, are flipped to your colour.
- If you have no legal move, you pass your turn. When neither player can move, the game ends and the discs are counted.
- The small dots show your legal moves (this option can be turned off).
About the AI
Iterative-deepening alpha-beta search with a null window, a transposition table with Zobrist keys and move ordering (stored move, corners, moves that reduce the opponent's mobility). The evaluation combines mobility, frontier discs, corners, X- and C-squares, edge stability, a square table and disc parity in the endgame. In the endgame, the AI calculates the game exactly down to the last disc: from 14 empty squares at Master level, 12 at Expert level. Lower levels search less deeply and add human-like inaccuracies.
AI levels
- BeginnerLooks only one move ahead and sometimes plays at random: perfect for learning.
- AmateurSees two moves ahead, starts to fear the corners, but still makes many mistakes.
- IntermediateSearches four moves ahead and solves the last ten squares exactly.
- ExpertDeep search of a second and a half, perfect endgame on twelve squares.
- MasterThree seconds of search and a perfect endgame on the last fourteen squares: nearly unbeatable.
