feat : shareable game links

[mateuskb]

What

Adds shareable links for the analysis page, plus a Share button in the toolbar.

Format	Notes
?pgn=<gzip+base64url>	Carries the game in the link itself
?lichessGameId=<id>	Already existed, now on the same loading and error path

Both accept &orientation=black.

The analysis page also writes the loaded game into the query string as you go, so copying the address bar shares it without pressing anything. Links that already name a source are left alone — ?lichessGameId=x resolves to the same game and is much nicer to share than a 1.5KB blob, so only games with no source param get one written (pasted PGNs, games opened from the local database, games finished on /play).

Why ?pgn= carries the whole game

Chesskit is a static export with no backend, so a self-contained link is the only format that cannot fail to resolve, and it behaves the same whatever the game's origin.

Compression uses the browser's CompressionStream, so no dependency is added. Measured on real games:

PGN	URL
Typical Chess.com game, 3120 chars	1521 chars
125-move annotated Lichess game, 7556 chars	3310 chars

There is a raw base64url fallback for browsers without CompressionStream, and a one character format tag so the encoding can change later without breaking links already shared.

Details worth flagging for review

The publishing effect terminates because encoding is deterministic. It reruns when it changes the query, so it only settles because CompressionStream zeroes the gzip MTIME field — the same PGN always yields the same param and the equality guard holds. I verified this at the byte level (1f8b08 00000000) across runs seconds apart. A param the component published is also recorded in a ref so the loading effect does not treat it as a new link and reload the game underneath the user.

The share link is built ahead of the click, not inside the handler. Compressing is async and Safari drops user activation across an await, so writing to the clipboard afterwards fails with NotAllowedError. Preparing it up front keeps the handler synchronous, matching how CopyPgnButton already works. This was a real failure observed in Safari, not a theoretical one.

Loading is gated on a share param actually being present, rather than running the loader on every mount and relying on it falling through every branch.

Safety

Malformed links land on an empty board instead of crashing, and in-flight work is discarded when the params change — including the decode path, which takes no AbortSignal and would otherwise be able to apply an older game over a newer one.

Decompression is bounded as it streams rather than checked afterwards. gzip reaches ratios near 1000:1, so a finished-string check is not a guard at all: a 271KB param inflates to 200MB and 1.1GB RSS in ~180ms, which takes the tab down long before any check on the result could refuse it. Decoding now reads the inflate stream chunk by chunk and cancels once the output passes the cap, and param length is capped so the work is bounded before inflation starts.

Testing

npm run lint passes, which type-checks as well. No test suite exists in the repo, so I exercised the compiled modules directly:

• Encode/decode, 13 cases: real Chess.com PGN round-trip, unicode player names, URL-safe alphabet, minimal games
• 6 malformed inputs ("", "1", "9abc", "1notbase64!!!", "1AAAA", "zzzz") all return undefined rather than throwing
• Decompression bomb sized to stay under the param cap, so the streaming guard is what has to catch it: 60MB payload rejected in 2ms with a 0.5MB heap delta

Verified manually in the browser for both formats.

History of this PR

Two things changed after this was first opened, both worth calling out rather than burying in the commit log:

• The bomb guard originally ran on the finished string, so the safety claim in the first version of this description did not actually hold. fix : bound decompression as it streams corrects it.
• A Chess.com format (?chessComUsername=&chessComGameId=) was included and has since been removed. Chess.com exposes no single-game endpoint, so it meant scanning monthly archives — bounded to six months at ~8.9MB for an active player, against 150+ archives unbounded — and it failed for older games. ?pgn= covers Chess.com games anyway, so the lookup was cost without benefit. src/lib/chessCom.ts is untouched by this PR.

Happy to split the address-bar commit out from the share-link commit if you would rather take them separately, or to squash the four commits into two if you prefer a clean history.

[yluom]

Dups #97 (another approch with compression lib)