Pocket Games
Reversi
Reversi takes a minute to learn and years to play badly, for a counterintuitive reason: having more discs in the middle of the game is almost always bad. Only the final count matters, and the road there runs through having fewer moves available to your opponent, not more discs.
The rules
An 8×8 board, black and white discs. You start with four discs in the middle, crossed. Black moves first.
Every move must surround: you place a disc so that one or more lines of enemy discs — horizontal, vertical or diagonal — are trapped between the disc you just placed and another disc of your colour. Every surrounded disc changes colour.
If you have no legal move you pass. The game ends when neither player can move, usually with the board full, and whoever has more discs wins.
On Pocket Games
Before you start you choose Against the bot or 2 Players: two players share the same phone, taking turns, and the board stays still. Against the bot you do not pick your colour: it is drawn at random for the first game, then it alternates with every new game, so one time you have black and move first, the next you have white.
On your turn the squares where a move is legal are marked with a translucent dot in your colour, and the last move played stays highlighted. If the player to move has no legal move, the pass is automatic: the message “No moves: turn skipped” appears and play goes on.
| Level | How it picks a move |
|---|---|
| Easy | at random among the legal moves |
| Medium | minimax search with alpha-beta pruning, 3 moves ahead |
| Hard | minimax search with alpha-beta pruning, 5 moves ahead |
Moves ahead count both its own and yours. Medium and Hard judge a position with a weight for every square (corners high, X and C squares negative) plus the difference in the number of moves available to each player.
- Against the bot, wins, draws and losses are counted separately for each level and shown on the start screen.
- In two-player mode no statistics are saved.
- If you leave halfway, the game is saved in both modes and waiting for you on the start screen.
- Moves cannot be undone.
Why having more discs is bad
This is the principle that overturns every beginner’s intuition, and it is called mobility.
Every disc you own is a disc your opponent can flip. Halfway through the game, a player who has flooded the board has an enormous number of targets and very few good moves: sooner or later they are forced to play next to a corner and hand it over. A player with few discs, all clustered in the middle, always has plenty of moves available and can pick the best one.
The practical rule, then: in the middle of the game, play to have few discs. Moves that flip a lot of discs at once are usually the worst ones. The count only matters at the end, and the last ten moves overturn it anyway.
Corners, edges and the squares to avoid
Corners are permanent. A disc in a corner can never be flipped, because there is nothing beyond it to close a surround. From there you build stable lines along the edges. Take three corners out of four and you almost always win.
The X squares. These are the four squares diagonally adjacent to the corners: occupying them is nearly always a disaster, because it gives your opponent the move that takes the corner. They are the most expensive mistake in the game.
The C squares. The ones next to the corners along the edges: dangerous for the same reason, though less lethal than the X squares.
Edges are not automatically good. A disc on the edge is harder to flip, but if it is not anchored to a corner it can certainly still go. Occupying a whole edge without holding the corners is a fragile position.
The endgame
In the last ten to fifteen moves the logic inverts: now discs really do count, and the game becomes a calculation. Two things to keep in mind:
- parity. Whoever plays last flips discs nobody can flip back. You want to enter the final phase with a number of empty squares that guarantees you the last move in every region of the board;
- closed regions. An empty area surrounded by your own discs is as good as a corner: only you will be able to play there.
Frequently asked questions
What happens if I cannot move?
The turn passes to your opponent on its own, with the message “No moves: turn skipped”. If neither player can move, the game ends even with the board not full, and whoever has more discs wins.
Should I flip as many discs as possible?
No, and that is mistake number one. Mid-game you want to flip few and keep plenty of moves available: the counting happens at the end.
How strong is the bot on Hard?
It looks five moves ahead and knows what corners are worth and how dangerous the squares next to them are: it will not hand you a corner by accident. When two moves look equal to it, it picks one at random, so its games do not always repeat. Against it, the mobility strategy is the surest way to win.
Can I leave a game halfway?
Yes. When you leave, the position is saved, both against the bot and in two-player mode, and the start screen offers the game in progress to continue. Only one is kept: starting a new game replaces the saved one.
Can I see my wins on another phone?
No. Wins, draws and losses against the bot stay on this device only and are not synced to your account.
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