Online Reversi Game

A large lead in Reversi can disappear without a single disc leaving the board. Two players take turns adding black or white discs to an eight-by-eight board, turning enclosed opposing discs to their own colour. Occupied squares stay occupied, but their colour can change repeatedly. Winning depends on the discs showing your colour when play finishes, not the number you have flipped along the way.

Play opens with two discs of each colour in the centre, and Black moves first. A legal placement must trap at least one opposing disc in an unbroken straight line between the new disc and another disc of your colour. That line may be horizontal, vertical or diagonal. Simply touching an opposing disc is not enough, and every line enclosed by the placement must be flipped.

During the opening and middle game, keeping useful moves available often matters more than taking the largest immediate capture. Three positional ideas explain much of that tradeoff.

Mobility

The number of legal moves available to a player. Having several useful choices reduces the risk of being forced into a move that gives away a corner.

Stable discs

Discs that cannot be flipped for the rest of the game. A corner disc is stable because no opposing placement can enclose it from both ends.

Squares beside an empty corner

These can give the opponent a route into that corner, especially the diagonally neighbouring square. They are legal when they make a capture, but their safety depends on the surrounding position.

Taking a long row can leave more of your discs available for the opponent to bracket on the next turn. A smaller capture may preserve better choices, although taking fewer discs is not a universal rule either. As the board fills, the exact sequence of remaining moves and the discs it leaves at the finish become more important than a general preference for small captures.

A player with no legal move must pass, but a player who can move cannot choose to pass. One pass does not end the game or award a win. Play finishes only when neither player can move, which can happen with empty squares still on the board. The larger disc count wins, and equal counts produce a draw.

How to Use This Tool:

The default is Computer practice, with you playing Black against Strategic. Choose local play for two people sharing the same device.

1. Choose an Opponent. For computer practice, set Your side and Computer level. Those two settings are disabled for Two players on this device.
2. Enter a Seed or keep the default, reversi-1. Use 1 to 64 letters from A to Z in either case, digits, underscores or hyphens.
   Start game is disabled for an empty or invalid seed. Remove spaces, accented letters and other punctuation before starting.
3. Choose Start game. Black begins, so choosing White makes the computer take the opening move before your turn.
4. Select a marked square by clicking or tapping it. Gold rings identify the opposing discs that would flip, and the preview reports the number of discs and captured lines. A legal square is already previewed at the start of each human turn, but you can select another without changing the board.
   On the board, arrow keys move focus. Enter or Space selects a focused legal square for preview; moving focus alone does not change the selection. Tab reaches Place and the session controls.
5. Activate Place for the selected coordinate to commit the move. Captured discs flip together, and the computer replies automatically when applicable. Check To play before the next placement because a forced pass can leave the same colour to move again.

Pause holds the current board until you choose Resume. The game also pauses when the window loses focus or the tab becomes hidden. Restart begins again from the opening with the same setup and seed. There is no move clock.

Interpreting Results:

The preview counts opposing discs that will change colour, not the new disc you place. It confirms an immediate capture, not a strong move or a predicted win. The Black and White totals count discs currently on the board.

A completed game reports Black wins, White wins or A draw. Empty squares are not added to either total. Choosing End game instead produces Practice ended, with no winner even when one colour is ahead. Practice results have no monetary value.

Placements counts new discs played by both colours, excluding the four starting discs and all passes. Move history lists each placement, its flipped coordinates and the counts immediately afterward. Passes are listed separately. A computer reply can change the totals again before control returns to you.

Replay same seed starts another game from the opening. It does not undo a placement or play back the recorded moves. To reproduce computer replies, match the seed, rules and setup shown in Run details and repeat the same player placements. New seed changes the computer's random choices or tie-breaks, not the starting board, and may still produce the same moves. Seeds do not change two-player play.

Technical Details:

Bracketing is a straight-line test from the newly placed disc. Each direction is checked against the position before the move, so one placement can capture several lines without starting a chain reaction from the flipped discs.

Legal placements and turns

Disc-count arithmetic

Let f be the number of opposing discs captured by one placement. The moving player gains those discs plus the new disc, while the opponent loses only the captured discs. These updates apply before any reply.

Only one previously empty square becomes occupied per placement. With four discs already on the board, a game can contain at most 60 placements. Passes add none, and the game can finish sooner. All counts are whole discs, with no weighting or rounding in the final result.

Computer move selection

Casual uses a seeded choice among legal moves. Greedy considers only the number of discs flipped immediately and chooses among the moves tied for the largest capture. The seed settles those ties.

Strategic considers each candidate placement and every legal immediate reply by the opponent. It assigns the candidate the lowest evaluation among those replies, then chooses a candidate with the highest such evaluation. If the opponent has no reply, it evaluates the board immediately after its own placement without searching an additional turn. Equal evaluations are resolved using the seed.

For a position where play can continue, the practice evaluation H combines mobility with weighted disc ownership:

M is the computer's number of legal moves in the evaluated position, and R is the opponent's number. P adds the following weight for each computer disc and subtracts it for each opposing disc. Empty squares contribute zero.

A finished position replaces that positional evaluation with a win, loss or draw evaluation based on the disc difference:

D is the computer's disc count minus the opponent's. The sign function is 1 for a positive difference, 0 for a tie and −1 for a negative difference. These evaluations guide computer choices only; they are not displayed disc totals, win probabilities or player ratings.

The lookahead covers at most the candidate placement and one opponent reply, not the rest of the game. Square weights stay fixed, including the penalty beside a corner after that corner has been occupied. These are bounded practice policies, not rated playing strengths or a guarantee of best play.

Worked Examples:

FAQ:

References:

• The official rules of Othello, Sections 2–3: starting position and moves, United States Othello Association.
• Corners & stable discs, British Othello Federation.
• Frontiers and Walls, British Othello Federation.