Reversi

The board, and how you win

The board is 8×8 — 64 squares — and 4 discs are already on it before anyone plays: 2 black and 2 white, crossed on the centre squares. Black always opens, and you pick which colour you take before the match starts.

You hold that colour until the match ends and the CPU holds the other one. There is no online play and no second player on the same device; the opponent here is always the CPU, and every match begins from that same centre position.

The match ends when neither side can move. Whoever holds more discs at that moment wins, an equal count is a draw, and both counts are on screen the whole way there. Those counts are the position rather than a target, and there is no clock beside them — the time you spend is recorded quietly, and it appears only as a total in your statistics.

Discs carry colour and nothing else: no letters and no numbers. That is why one board reads the same way in every language the app ships in.

Placing a disc, and what turns over

A disc goes on an empty square, and only where it traps at least one of the CPU’s discs between the disc you are placing and a disc you already hold. Every disc caught that way turns your colour, in every direction at once.

None of that is optional. You cannot turn fewer discs than the move traps, you cannot pick which of the trapped lines to take, and while you have a legal square you cannot decline to play.

• Tap a square with a dot. On your turn every square you may play carries one, and no other square is pressable — an illegal move is not something the game has to refuse.
• Lines run in all 8 directions: across, down and both diagonals. One move can close several of them at once, and all of them turn.
• A line only counts when a disc of your own closes it. A run of the CPU’s discs that reaches an empty square, or the edge of the board, turns nothing.
• No disc is ever removed and no square ever empties again. Each move adds exactly 1 disc, which is why a whole match is at most 60 moves long: 64 squares, less the 4 that were there at the start.
• The disc placed last carries a small mark, and the discs a move turned are drawn flipping over. The CPU’s reply is something you can find, not something you have to reconstruct.

Passing, and when the match ends

If you have no legal square, your turn is skipped by itself. There is no Pass button to press: a pass is what the rules did, not a decision you made. A short notice says which side was skipped — "No move for you — the CPU plays again" — and it stays silent, because it is information rather than a mistake.

A skipped turn is worth avoiding. It hands the other side two moves in a row, and a board can change a great deal in two moves. You cannot pass on purpose either: while a legal square exists, you have to play one.

When neither side can move, the match is over. Usually that means a full board, but a board with empty squares nobody may play is just as final, and it is settled the same way — count the discs, more wins, equal is a draw.

What you get at the end is a win, a loss or a draw and the two final counts. There is no score, no rating and no line about how you played. Losing says only that the CPU held more discs; starting the next match costs nothing.

Choosing your opponent, and your colour

The home screen asks you to Choose your opponent, and that is what the three difficulties are: three opponents playing the same rules on the same board. None of them sees anything you cannot, and all three are there from the first launch — no strength is unlocked by playing or by paying.

• Hard stops on a budget. It reads until it has visited 12,000 positions, then plays the best move of the deepest search it actually finished. No opening book sits behind it and no endgame is solved in advance: a stronger opponent was possible, one that makes you sit and wait for it was not.
• The reply is held back by about 450ms, and the search runs after that pause rather than during it — so a move appears at roughly 450ms plus however long the reading took, which for a Hard search that spends its whole budget measured around 180ms on the development machine. The pause is there so the CPU’s turn is visible as a turn, not to dress up the thinking.
• The search runs on your device and starts only when you place a disc. Nothing is being worked out while you look at the board.
• Black · first or White · second is the other choice on that screen. Black opens, so picking a colour is picking whether you move first or second; black is the default. It applies to the next match — a match already under way keeps the colour it started with.
• Records are kept per opponent: games played, wins, losses and draws, with your total play time underneath. There is no level list, no daily match and no streak, and nothing leaves your device.
• A match you walk away from is not counted as a loss. Starting it already counted it as played; only a match that reaches an ending becomes a win, a loss or a draw.

Undo, and the help the board gives you

Undo is unlimited and free. It steps back to your own last decision, so your move and the CPU’s reply come off the board together, along with any pass that happened between them. It never leaves you standing in the middle of the CPU’s turn, because that is not a position where you decided anything.

Undo is a take-back, not a re-roll. The CPU’s choice is fixed by the match’s seed and by how many moves have been made, so putting the board back and playing the same square brings the same reply — a golden test replays fixed matches move for move to keep it that way. What Undo buys you is the chance to try the other square and compare two real lines.

• Your legal moves are marked on the board, always. The first thing that trips people up in Reversi is reading the rule off the board — working out where you are even allowed to play — so the game answers that part and stops there.
• That is rule visibility, not advice. The dots say where a move is legal; they never say which of them is any good.
• There is no Hint. Nothing on this board is hidden: both colours, every empty square and both counts are in front of you. A suggested move could only be the best move, which would be the CPU playing your side as well as its own.
• Undo and the dots are the whole help system. Both work from the first launch, and no game feature is ever behind an ad or a purchase.
• A match in progress is saved, including whose turn it was — passes break the alternation, so the turn cannot be worked out from the discs afterwards. Undo history is not saved, so a resumed match starts from the position you left.

Sudoku has both Undo and Hint, Minesweeper has only Hint, and Reversi has only Undo. The shared rule is that every game gets free, unlimited help in a form that does not hollow it out — not that every game gets the same two buttons.

Corners, edges and the discs you keep

One rule decides where this game is won. A disc turns only when it is caught in a line closed at both ends, and a corner has no square beyond it in any direction — no line through a corner can ever be closed past it. A disc you put in a corner is yours for the rest of the match, and nothing in the rules can take it back.

• Edges are steadier than the middle for the same reason. A disc on an edge can only be caught along that edge, because on the outside of it there is no board to close a line from.
• The squares beside an empty corner are the dangerous ones, and the diagonal neighbour most of all. To take the corner the CPU needs a disc of yours next to it and one of its own further down the line — playing there is what supplies the first half.
• Holding more discs in the middle of a match is not the same as being ahead. Only the count at the moment neither side can move decides anything; the number before that is just where the board stands.
• Watch how many squares you will still be able to play, not only how many discs you hold. Normal weighs exactly that alongside the squares themselves — how many moves each side has — and the pass rule is why it matters: a side with nothing to play hands the other one a free turn.
• Easy hands corners over without noticing, which makes it the cheapest way to feel what holding one does to a board. Normal and Hard are not choosing at random, so a corner against them is something you have to arrange rather than something that falls to you.
• Use Undo to read a position out. Take the move back, play the other square, and compare what comes back — the reply to each is fixed, so both lines you are comparing are real.

Questions people ask