Flipside
Reversi / Othello — ink-wash dojo edition
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How to Play Flipside
Flipside is a faithful browser recreation of the classic board game Reversi, also sold under the brand name Othello. The game is played on an 8×8 grid. Two players take turns placing discs — one side plays Black, the other plays White. Black always moves first.
To place a disc, you must choose an empty cell that outflanks at least one of your opponent's discs. Outflanking means your new disc and one of your existing discs form a straight line — horizontally, vertically, or diagonally — with one or more opponent discs sandwiched between them. Every sandwiched opponent disc then flips to your color.
If no legal placement exists on your turn, your turn is automatically passed. If neither player can move, the game ends and the player with more discs on the board wins. A tie is possible if both sides finish with 32 discs each.
Controls: Click or tap any highlighted cell to place your disc. The hint overlay (toggled with the H key or the Hints button) draws a faint ring on every legal cell. On Easy and Medium difficulty, the Undo button rewinds one full move — it takes back the AI's reply and your own last disc, returning the board to the position you faced before your last placement — giving you a chance to reconsider your strategy. Hard mode disables undo to match competitive play conditions.
About the AI Opponent
Flipside's AI uses the Minimax algorithm with alpha-beta pruning. On Easy, the AI looks four moves ahead; on Medium, six moves; on Hard, eight moves. In all cases the AI evaluates positions using a positional weight table that assigns high value to corners (100 points each), moderate value to the safe edge cells (5 or 10 points), and a −25 penalty to every cell that touches a corner — the X-squares diagonally adjacent to it and the two edge cells beside it — because occupying one typically hands your opponent that corner on their next move. Interior cells score 1–5 points each.
In practice this means the Easy AI plays correct moves but makes occasional positional errors, the Medium AI is a genuine challenge, and the Hard AI will consistently exploit poor corner play and punish early X-square occupancy.
Strategy Tips for Flipside
Reversi rewards long-term positional thinking over raw disc count. A common beginner mistake is to maximize disc flips early, which can leave you with a bloated disc count that gives the opponent many valid moves to flip large swaths of your board on a single turn. The best players often hold fewer discs than their opponent through most of the mid-game.
- Grab corners at every opportunity. Corner discs can never be flipped because no sandwich can pass through a board edge. A captured corner anchors a permanent chain of discs along the adjacent edges, often generating a cascade of stable pieces that can never change color for the rest of the game.
- Never occupy an X-square unless the corner is already yours. An X-square (diagonally adjacent to a corner, e.g. b2, b7, g2, g7) gives your opponent an immediate route to claim the corner. This is the single most common error in beginner play and the reason the AI's positional weight table assigns a −25 penalty to these cells.
- Control mobility, not disc count. Limit the number of legal moves available to your opponent. Fewer opponent moves means they have less flexibility to exploit your position. The concept is called "mobility" in competitive Reversi and is worth more than raw disc count through the mid-game.
- Edges and C-squares. Edge cells (not adjacent to corners) are moderately stable because they can only be approached from one side. C-squares (adjacent to an edge corner, e.g. a2, b1) share some of this stability and are far safer than X-squares, though still inferior to holding the corner itself.
- Parity matters at the end. In the late game, having the last move in an empty region ("parity") often determines which player gets to choose strategically beneficial placements rather than being forced to fill residual cells. When the board is nearly full, count the empty regions and aim to control parity in the larger ones.
Frequently Asked Questions
How does flipping work in Flipside?
When you place a disc, every enemy disc sandwiched between your new disc and any other of your discs — horizontally, vertically, or diagonally — flips to your color instantly. All eight directions are checked simultaneously, so a single well-chosen placement can flip large numbers of discs across multiple lines at once. You must flip at least one disc; placements that would capture nothing are illegal and cannot be selected. The hint overlay shows you every cell where a legal placement is currently available.
What are Black's opening moves in Flipside?
Flipside always starts from the standard four-disc cross — White on d4 and e5, Black on e4 and d5 — and Black moves first. That leaves Black exactly four legal openings: d3, c4, f5 and e6, each of which flips a single disc. Because the starting position is symmetric, all four are the same game rotated or mirrored, so none of them is stronger than the others — the real opening choices begin with White's reply. With hints on, the board marks all four for you.
How does the AI opponent work?
Flipside's AI uses the Minimax algorithm with alpha-beta pruning to search the game tree to a fixed depth. On Easy, it searches 4 plies (half-moves) ahead; Medium searches 6; Hard searches 8. Each leaf position is evaluated using a positional weight table: corners score +100, the safe edge cells +10 or +5, every cell touching a corner — the X-square diagonal to it and the two edge cells beside it — scores −25, and interior cells score 1 to 5. Mobility — the difference in legal move counts between the two players — is also factored in, worth +5 per move advantage. Alpha-beta pruning cuts branches that cannot improve on the best result found so far, making Hard-mode depth-8 search feasible in the browser in under two seconds.
Can I undo a move in Flipside?
Yes, on Easy and Medium difficulty the Undo button (or Ctrl+Z) rewinds one full move and hands the turn back to you — it takes back the AI's reply and your own last disc, restoring the exact position you faced before your last placement. This lets you experiment with alternate lines without restarting. If a pass let the AI move twice in a row, all of those replies are taken back together, so it is always genuinely your turn again; undo is unavailable while the AI is still thinking. Hard mode disables undo entirely — consistent with tournament play, where taking back moves is not permitted — and the Undo button is greyed out as soon as a Hard game begins. Undo stops once you have no earlier turn of your own left to return to.
Why are corner positions worth so much in Reversi?
The four corner cells — a1, a8, h1, h8 in standard notation — are the only cells on the board that can never be flipped once captured. This is because a disc can only be outflanked if a straight line of opponent discs separates it from one of your discs, and board edges prevent any line from extending through a corner. Once you hold a corner, every disc in the adjacent row and column that you place afterward becomes progressively more stable. Competitive players assess board position largely in terms of stable disc count — discs that cannot change color for the rest of the game — and corners are the starting point of almost every stable configuration. This is why Flipside's AI weights corner cells at 100 and penalizes X-squares at −25: giving up an X-square almost always costs you a corner on the opponent's next turn.
