Reversi - Complete Guide

⚫ What is Reversi?

Reversi, also widely known as Othello, is a two-player strategy game played on an 8x8 board with 64 double-sided discs — black on one side, white on the other. Players take turns placing discs on the board, and when you place a disc, any of your opponent's discs that are caught in a straight line between your newly placed disc and another disc of your color are flipped over to become yours. Lines can be horizontal, vertical, or diagonal, and a single move can flip discs in multiple directions at once.

The game's roots trace back to 1883, when Lewis Waterman published the original rules in England. The modern version, trademarked as Othello, was popularized by Goro Hasegawa in Japan in 1971 with the famous tagline "a minute to learn, a lifetime to master." That tagline holds up — Reversi's rules are simple, but the strategic depth is enormous.

📜 Rules

The game begins with four discs placed in the center of the board in a diagonal pattern — two black and two white, alternating. Black always moves first. On each turn, you must place a disc on an empty square such that at least one of your opponent's discs is flanked between your new disc and one of your existing discs along a straight line. All flanked opponent discs in every direction — horizontal, vertical, and diagonal — are flipped simultaneously.

If you cannot make a legal move (no placement would result in any flips), your turn is passed and your opponent moves again. The game ends when the board is completely full or when neither player can make a valid move. At that point, the player with the most discs of their color on the board wins. Ties are possible but rare.

On PlayVersusAI, you always play as black and move first, while the AI controls the white discs. Valid moves are highlighted on the board so you can always see your options.

🏆 Winning Strategies

Corners are king. The four corner squares are the most valuable positions on the board. Once a disc is placed in a corner, it can never be flipped — it's permanently yours. Corners also anchor edges, letting you build chains of stable discs that your opponent can't touch. Prioritize taking corners whenever the opportunity arises, and structure your play to create those opportunities.

Avoid the danger zones. The squares directly adjacent to corners — known as "C-squares" (edge-adjacent) and "X-squares" (diagonally adjacent) — are extremely dangerous to occupy when the neighboring corner is still empty. Playing on these squares often hands your opponent a free corner. As a rule, avoid C-squares and X-squares until the adjacent corner is already taken.

Disc minimization in the early game. This is counterintuitive but critical: in the opening and mid-game, you actually want fewer discs, not more. Having fewer discs means your opponent has more exposed pieces, which gives you more legal moves and more flexibility. Players who aggressively flip everything early often find themselves cornered with no good options later.

Mobility matters. The number of legal moves available to you at any given point is called your mobility. High mobility means more choices and more control. Try to maximize your own mobility while restricting your opponent's. If your opponent has no legal moves, they must pass — giving you back-to-back turns, which is a massive advantage.

Stable discs and the endgame. A stable disc is one that can never be flipped for the rest of the game — typically discs in corners, along filled edges, or in large connected groups anchored to corners. Building stable disc formations is a key long-term goal. In the endgame (roughly the last 15–20 moves), the strategy shifts entirely: disc count becomes everything. Flip as many discs as possible in those final moves to maximize your score.

🧠 How Our AI Works

The Reversi AI uses a minimax search algorithm with alpha-beta pruning to evaluate positions and choose the strongest move. It considers multiple factors when scoring a board state: a weighted positional map that assigns high value to corners and penalizes X-squares, mobility (how many legal moves each player has), stability (the count of discs that can never be flipped), and disc parity (which becomes the dominant factor in the endgame).

On AI Training difficulty, the engine uses iterative deepening with a strict 2-second time limit — it searches progressively deeper into the game tree, keeping the best result from the deepest completed search. This produces near-optimal play that will challenge even experienced Reversi players.

📊 Difficulty Levels

Each difficulty level adjusts how deeply the AI searches the game tree and whether it occasionally makes suboptimal moves to give you a more realistic opponent.

💡 Tips for Beginners

• Fight for corners at every opportunity — they're the foundation of a winning position and can never be taken back once claimed.
• Keep your disc count low in the early and mid-game. It feels wrong, but having fewer discs gives you more flexibility and more available moves.
• Maximize your mobility. Always look for moves that give you the most options on your next turn while restricting your opponent's choices.
• Never play on X-squares or C-squares when the adjacent corner is empty. These moves almost always hand your opponent a free corner.
• Think about the endgame from move one. Every disc you place early shapes the board for the final sequence of moves — plan ahead.

⚠️ Common Mistakes

• Disc maximization: Trying to have the most discs as early as possible is the single most common beginner mistake. It feels like winning, but it actually reduces your options and gives your opponent control of the board.
• Playing next to empty corners: The X-squares and C-squares adjacent to unclaimed corners are traps. Occupying them lets your opponent take the corner for free, gaining a permanent, unflippable advantage.
• Ignoring mobility: Focusing only on disc count while ignoring how many moves you have available leads to situations where you're forced into bad positions or have to pass entirely.
• Forgetting about stability: Not all discs are equal. A disc in the center of the board can be flipped multiple times throughout the game, while a stable disc anchored to a corner stays yours forever. Build stable formations whenever possible.