Match Pairs

How to play

Cards lie face down in a grid. On each turn you flip two. If they show the same symbol, the pair stays face up. If not, both turn back down after about 900 milliseconds, and you cannot flip anything else until they have. Find every pair in as few moves as you can.

One move is two flips. The fewest moves possible is one per pair, which you only get by luck. The timer starts on your first flip and pauses while the tab is hidden.

Pick a size from the Size menu:

Size	Grid	Cards	Pairs
Small	4 x 3	12	6
Medium	4 x 4	16	8
Large	5 x 4	20	10
Huge	6 x 6	36	18

Keyboard: Tab into the grid, then use the arrow keys to move between cards, Enter or Space to flip the focused card, and Home and End to jump to the first and last card. Only one card is in the Tab order at a time, so you do not have to tab through all 36.

Every symbol has its own colour, and each pair gets a card face tinted in that colour. Face-down cards have sky and violet stripes with a small diamond, and a matched pair gets a green ring and a tick. Colour is only a second cue. Each card also has a different shape, and its accessible name says “face down”, “face up” or “matched” along with the symbol, so the game never depends on colour alone.

How it works

The game is two small pieces: a deck builder and a state machine. Both are pure functions with no page code in them, which is what lets the test suite check the rules directly.

The deck. Each game picks the symbols it needs at random from a set of 18, puts two of each in a deck and shuffles it with a Fisher-Yates shuffle. That shuffle makes every ordering equally likely, provided the random numbers are uniform. In the browser they come from the crypto random generator rather than Math.random. The test suite feeds the shuffle thousands of seeded games and checks that each card position gets each symbol about equally often, and that the partner of a card is equally likely to be anywhere else in the grid. It is a sanity check, not a proof, but it would catch the usual shuffle mistakes.

The state machine. The game is always in one of three phases:

1. idle: nothing is waiting, so any face-down card can be flipped.
2. one-open: one card is face up, and the next flip completes a move.
3. resolving: a mismatch is on show, and flips are ignored until the delay ends.

Each state is immutable. A flip returns a new state, or the very same object if the flip is not allowed: flipping a card that is already face up, flipping a matched card, flipping during resolving, or an index outside the grid. After the delay the page asks the machine to resolve, which turns the mismatched pair back down. Winning is simply “pairs found equals total pairs”.

The tests also play whole games: a player who knows the deck finishes in exactly one move per pair on every size, random play always finishes, and a player who remembers cards seen so far needs fewer moves on average than one who does not.

Privacy and storage

Nothing is uploaded and there is no account. Your fewest moves and your best time for each size are saved in your browser’s local storage under one key, as small numbers inside a small JSON object. The time is kept in whole seconds, rounded down the same way the clock on the page counts, so the saved time matches what the clock showed. They are updated only when you win a game, and only if the result beats the old one. Clearing site data removes them.

Limits

• Fewest moves and best time are separate records, so they can come from different games.
• The timer measures time with the tab visible, so a game you leave open in another tab does not run up the clock.
• The shuffle is checked by statistical tests, which can show a bias but cannot prove there is none.
• The result you copy contains the game name, the board size, your moves and time and the page address, or a note that the game was unfinished.
• There are no scores shared between players and no multiplayer mode.

Everything runs locally in your browser.