Chess

The game opens on a campaign rather than an empty board: sixteen stages spread across four places, each with its own rival. Pete plays on a park bench on a sunny afternoon, Marta runs a corner café under evening lamps, Viktor captains a late-night chess club, and The Grandmaster waits in a grand hall for the final. Every place mixes three kinds of stage. Odds games give the rival a handicap — Pete starts without his queen, Marta without a rook, Viktor without a knight, The Grandmaster without a pawn — so you learn to convert an advantage before you have to earn one. Puzzles ask for a mate in one, two or three moves from a set position. Endgames teach the techniques every player needs: mate with king and queen, mate with king and rook, and walking a pawn through to promotion before the moves run out. Each place ends with a full game against its rival at full strength for that place, and the engine resigns when it is clearly lost instead of making you chase its king around an empty board.

Each stage is worth up to three stars: one for winning, one for doing it without a hint, and one for doing it without taking a move back. Stars are kept per stage, so you can come back later and replay a stage for a better score, and the Stages map shows every stage, its stars and what is still locked. The difficulty climbs by what the rival can see, not by making it randomly stronger: the park rival looks one move ahead and leaves pieces hanging, the café owner looks two moves ahead, the club captain never walks into a capture, and The Grandmaster searches three moves deep with no blunders at all. Finish the sixteenth stage and the campaign is complete; free play against four engine levels stays open the whole time.

Chess needs no introduction, but a browser chess game does need one thing: an engine that actually plays chess. Move generation here is not "probably right" — it is checked against perft, the standard correctness test in computer chess, where a program counts every legal sequence to a given depth and the number has to match a published constant exactly. Six positions are used, chosen because between them they cover castling, en passant, promotion, pins and discovered check, and the test walks 11,024,485 positions in total. If a single rule were wrong anywhere, the count would not match. There is nothing to install and no account to make; it runs in the browser on desktop and on a phone.

The engine searches with alpha-beta and move ordering, and from level 3 up it adds a quiescence search. That last part matters more than depth does. Without it an engine stops counting in the middle of a trade and thinks it is winning because it just took a rook — and then loses its queen on the move it never looked at. Quiescence keeps following the captures until the position is quiet, which is why the stronger levels do not hand you free pieces.

There are four levels and they are not the same idea repeated with a bigger number. Level 1 searches one move ahead and then deliberately picks from among the moves close to its best one, so it makes the kind of mistakes a beginner makes and leaves pieces where you can take them; it is there to be beaten while you learn. Level 2 looks two moves ahead and plays a tidy, punishing game against loose pieces. Levels 3 and 4 search deeper and add quiescence, so tactics that worked earlier stop working and you have to make a real plan.

The board is built around not having to ask what just happened. Click a piece and its legal moves appear: a dot on a square you can move to, a ring around a piece you can take, so the two are different shapes rather than two shades of the same colour. The move you or the engine last played stays highlighted, and your king glows red the moment it is in check. Every move is written into a scrolling list in real chess notation, disambiguated properly — if two knights can reach the same square you get Nbd2 and Nfd2, not a guess. When a pawn reaches the far rank you are asked which piece you want, because there are positions where a knight is the right answer and quietly taking a queen would decide that for you.

The pieces are drawn rather than typed. Chess characters exist in Unicode, but every operating system draws them differently and some phones render them as flat blocks, so a game built on them looks like a different game depending on who opens it; these are drawn as shapes with distinct silhouettes that stay readable at phone size. Three buttons sit under the board: Hint draws an arrow showing a strong move, Take back rewinds to your turn so a single misclick does not cost you the game, and Stages opens the campaign map. On a campaign stage a hint or a take back is always allowed, it just costs that stage its matching star. Games end the way chess games end — checkmate, stalemate, the fifty-move rule, threefold repetition, or too little material left to mate — rather than dragging on forever in a dead position.