About this site

Why the site exists

Reversi is one of the easiest games in the world to learn and one of the hardest to find a straight answer about online. Search for the rules and you get pages that contradict each other on passing, on what happens when the board does not fill up, and on whether Reversi and Othello are the same game. Search for a board and you get an advert, a login and a download prompt.

This site is an attempt at the other thing: a board that works the moment the page loads, and rules pages that are correct because the code that plays the game generates them.

How the boards and diagrams are made

Nobody types out a board position on this site. Every diagram, including the one below, is produced by handing a list of moves to the rules engine and letting it replay them.

abcdefgh

12345678

That matters more than it sounds. If a diagram on this site shows a position, that position is reachable by legal play. If a page says a square is a legal move, the engine was asked. If a page quotes a disc count, the engine counted. A page here cannot drift away from the rules the board actually enforces, because there is only one set of rules in the codebase.

How the computer opponent works

In plain terms: the computer looks ahead through possible moves, scores each position it reaches, and picks the move that leads to the best score it can be sure of. A few details are worth knowing.

• The board is stored as numbers, not squares. Each colour is held as a pattern of bits, so the engine can work out every legal move on the board in a handful of arithmetic operations rather than by walking 64 squares. This is what makes it fast enough to search deeply in a browser.
• It scores positions on shape, not on discs. Corners, stability (discs that can never be flipped again) and mobility (how many legal moves each side has) carry far more weight than the disc count, which is nearly meaningless until the end.
• It prunes. Once a line of play is shown to be worse than one already found, the rest of that line is skipped. That is what lets it look many moves ahead without checking every possibility.
• It solves the ending exactly. Once few enough squares are left, the Hard level stops guessing and works the game out to the finish, so the last dozen moves are played perfectly. The search runs on a separate thread, so the board stays responsive while it thinks.
• The easy levels are deliberately imperfect. Beginner and Easy play weaker moves on purpose, so a new player can win.

How Reversi AI works goes through all of that in detail, with worked examples.

How the site is built

Static HTML, generated ahead of time. There is no server rendering pages on request and no framework shipped to your browser. Fonts, styles and images all come from this site rather than from another host; the one outside script is Google Analytics, which counts visits. The game itself runs entirely in your browser, so your moves never leave your device. The privacy page sets out exactly what is and is not collected, including the analytics.

Who writes it

Tim. I build small, focused web projects and write about the process at timmyships.com. I am not a titled Othello player and this site does not pretend otherwise. What it offers instead is care about accuracy: rules checked against the federation's published ruleset, positions generated by code rather than typed out, and a test suite behind the engine that produces them.

If something here is wrong, I would rather know. Corrections are welcome and get fixed.

Getting in touch

Email info@reversiboardgame.com, or see the contact page. For a bug on the board, paste the move list from beside the board into the email and the position can be reproduced exactly.

A note on the name

Othello is a registered trademark of MegaHouse Corporation and Othello, Co. This site is not affiliated with or endorsed by the trademark holders. "Reversi" is the public-domain name for the same game.

Where to start

If you have never played, how to play takes about five minutes. If you know the rules and lose a lot, try tips for beginners then strategy. If you just want a game, the board is on the home page and the computer is waiting on four strengths. The sitemap lists everything on the site.