Chess Game

A complete legal-move engine instead of a decorative board

Every move shown by the interface is generated from the current board, side to move, castling rights, en-passant target and move counters. A candidate move is then rejected if it leaves that side’s own king in check. That last legality filter matters for pinned pieces, discovered checks and en passant, where removing a pawn from a different square can unexpectedly open a rook or bishop line toward the king.

Check and checkmate use the same board state

A king is in check when an opposing pawn, knight, bishop, rook, queen or king attacks its square. The game does not allow a checked king to remain exposed after a move. When the side to move has no legal move, the engine distinguishes checkmate from stalemate by testing whether that king is currently attacked. A checkmated side loses; a stalemated position is recorded as a draw.

Castling has all of its transit restrictions

King-side and queen-side castling are available only while the corresponding rights remain intact, the king and rook are on their original squares, the spaces between them are clear and the king is not already in check. The king also cannot pass through or land on an attacked square. Moving the king removes both rights for that color, while moving or capturing an original rook removes the relevant side only.

En passant lasts for one reply

When a pawn advances two squares from its starting rank, the engine stores the square that an opposing pawn could enter by en passant. That target disappears after the next move if it is not used. Applying an en-passant capture removes the passed pawn from its original landing square before the king-safety test, so illegal discovered-check cases never appear as selectable moves.

Promotion offers four real choices

A pawn reaching the back rank may promote to a queen, rook, bishop or knight. The interface includes a promotion selector and maps a destination click to the selected legal promotion move. Queen is the default because it is the most common choice, but underpromotion remains available when a knight, rook or bishop is strategically better.

Draws are stored separately from wins

V0.96 detects stalemate, insufficient mating material, threefold repetition from the browser-local position history and the 50-move counter. Those endings are not forced into a win/loss record. A match draw contributes half a point to each side, while a normal Practice draw increments the specialist draw count.

Easy, Standard and Hard change search depth

All three computer levels receive the same complete public chess position because chess has no hidden pieces. Easy favors quick legal choices with deterministic seeded variation. Standard searches farther and evaluates material, piece development and checks. Hard searches one additional ply in the same rules engine. Difficulty never changes the player’s legal moves or gives the computer access to unavailable information.

Practice seeds make a game reproducible

A Practice session receives a seed that is written into the URL. The seed fixes color selection when Seeded random is chosen and fixes AI tie breaks between equivalent evaluations. Your own move choices still determine the game, but refresh and autosave return to the same session identity instead of silently creating a replacement. Fresh Practice intentionally creates another seed.

Undo is a study feature with a visible boundary

Undo restores the position from immediately before your previous move, including any computer reply that followed it. Because you have already seen later play, the game becomes Assisted. Hint also marks Assisted and selects a move from the strongest local evaluator. Restart keeps the same deterministic game identity but likewise marks the new attempt Assisted.

Clean wins mean no study aid was used

The Clean label is intentionally narrow. It does not claim a rating, percentile or tournament strength. A Clean win simply means the browser-local game reached a player victory without Hint, Undo or Restart. That distinction lets a learner use help freely without mixing assisted practice with unaided records.

Move history is readable without pretending to be formal PGN

The game log records a compact chess-like move label for every ply, including captures, castling, promotion, check and checkmate markers. It is designed for reviewing the current browser game rather than exporting tournament metadata. Board state, move history, repetition counts and match score are saved together so a restored position remains internally consistent.

Match to 3 makes longer practice possible

Practice Game is one game. Match to 3 carries a local score across deterministic games until either side reaches three points. A win is one point and a draw is half a point to each side. The next game may switch your seeded color when Seeded random is selected, while a fixed White or Black preference stays fixed.

Daily Chess is a specialist habit, not a Today’s 6 replacement

The separate Daily mode maps the date to a stable seed and player color, stores its own completion history and streak, and stays outside the six-game main Daily rotation. The frozen Today’s 6 lineup therefore remains untouched as new specialist games are added.