Reversi

About this game

This is standard Othello. Black always moves first. A move is only legal if it flanks at least one line of opponent discs — meaning an unbroken run of the opponent's color, in a straight line from the placed disc, ending in one of your own discs — and every flanked disc flips to your color the moment you place. Legal squares are marked with a small dot before you move, so you always know where you can play. If a player has no legal move, their turn is skipped automatically and passed back to the opponent; the game ends only when neither player has a legal move anywhere on the board, at which point whoever has more discs wins — a tie is a draw.

The computer opponent runs a real alpha-beta search rather than picking randomly. Its evaluation weighs three things: a positional table that prizes corners heavily and penalizes the squares next to them (since those hand the corner away), the difference in how many legal moves each side has (mobility matters more than raw disc count for most of the game), and disc count itself, which is weighted more heavily as the board fills up and fewer empty squares remain. Easy searches shallowly with a lot of randomness mixed into its move choice, Medium searches deeper with a little randomness, and Hard runs iterative deepening under a short time budget so it always replies quickly rather than freezing the tab.

Undo steps back one full round-trip against the computer (its reply and your move together, so it stays your turn) and one move in pass-and-play — it also correctly skips back over any automatic passes. Your game in progress and your match score are both saved on this device, so closing the tab and coming back picks up exactly where you left off.