Reversi (Othello)

Goal: End the game with more discs of your color on the board than your opponent.

Board & Starting Position

Played on an 8×8 board. The game begins with 4 discs placed in the central 2×2 area in a diagonal same-color pattern: D4 white, E4 black, D5 black, E5 white (two black and two white discs on opposite diagonals).

Placement Rules

Black moves first, then players alternate. Each turn you must place one disc on an empty square such that the new disc and one of your existing discs lie on a straight line (horizontal, vertical, or diagonal) and sandwich at least one opponent disc. All sandwiched opponent discs immediately flip to your color.

Valid move criteria:

• Between the new disc and your existing disc, all squares must contain consecutive opponent discs
• A single move can flip discs in multiple directions (up to 8) simultaneously
• If a square cannot flip any opponent disc, you cannot place there

Passing

If you have no valid move on your turn, you must pass and your opponent plays again. If both players consecutively have no valid move, the game ends.

Winning

The game ends when neither player has a valid move (usually when the board is full, but it can also end early if remaining empty squares cannot be played by either side). The player with the most discs wins; equal counts result in a tie.

About the Dot Hints

The small dots shown on the board indicate all legal moves for the current turn under standard Reversi rules. They are not UI-imposed constraints — even without dots, the rules require every placement to sandwich at least one opponent disc. Dots simply help players quickly identify legal moves; they do not reduce your strategic choices, since every legal option is displayed.

1. Control the Corners (Most Important)

The 4 corners are the most valuable squares on the board — once occupied, they can never be flipped. Always look for opportunities to take corners, and avoid giving them to your opponent.

2. Avoid X-Squares Early

The 4 squares diagonally adjacent to corners (X-squares) are very dangerous in the opening. Placing there gives your opponent a path to the corner. Only play on an X-square when you are certain the opponent cannot exploit it.

3. Maximize Mobility

Try to keep more legal moves available than your opponent. Higher mobility means more control. If your opponent has no valid move, they must pass, possibly giving you multiple consecutive turns.

4. Build Stable Edges

Edge discs connected to a corner become stable (unflippable). Focus on extending chains from corners along the edges to consolidate territory.

5. Don't Chase Disc Count Early

In the opening and midgame, having fewer discs can actually be advantageous — fewer targets for your opponent to sandwich, higher mobility for you. Save big flips for the endgame.

6. Watch Parity

Reversi has an important parity principle: in each isolated empty region, the player who moves last usually gains an advantage. Plan moves so you are the last to play in a given region.

7. Forced Sequences

In the endgame, precise calculation can construct forced flipping chains that flip many discs at once. The hard AI performs a 6-ply search and demonstrates this tactic.

Reversi is a two-player abstract strategy board game invented by Englishman Lewis Waterman in 1883. It gained considerable popularity in England at the end of the 19th century. In 1898 the well-known German publisher Ravensburger began producing the game as one of its first titles.

In 1971, Japanese game enthusiast Goro Hasegawa adopted a fixed starting position based on the original rules and renamed it Othello, launched by Tsukuda Original Co. in Japan in 1973. The name references Shakespeare's play Othello, echoing the black-and-white coloring of the game pieces.

Difference between Reversi and Othello:

• Original Reversi (1883): The first 4 discs are placed alternately by players in the central 2×2 area, allowing 2 possible starting positions
• Modern Othello (1971): Fixed diagonal same-color starting position; rules are otherwise identical

Today the two names are used interchangeably. This implementation uses Othello's fixed starting position.

Awards & Recognition:

• 🏆 Mensa Select game list
• 🏆 World Othello Championship (WOC) held annually since 1977
• 🏆 National associations in dozens of countries

This digital implementation includes:

• ✅ 8×8 board with diagonal starting position following standard Othello rules
• ✅ Minimax + Alpha-Beta pruning AI with Easy / Medium / Hard difficulty levels
• ✅ Evaluation covering corners, X/C-squares, edges, mobility, stable discs, disc parity
• ✅ Undo / Redo for learning and experimentation
• ✅ Automatic pass detection and notification