collin/mahjong

3f4c297b571ac5fc98d05833b005178f9011e077 / README.md

1	# 台灣麻將 — four-player mahjong, on one touchscreen or over the wire
2
3	Sixteen-tile Taiwanese mahjong for four people sitting around a single laptop.
4	All four hands are on screen at once, each rotated to face its own edge — put a
5	piece of card or plastic over your strip so the others can't see your tiles.
6	Every label is Chinese with English underneath; the tiles themselves stay
7	Chinese, because that is what the tiles say.
8
9	Nobody else around? 單人對局 Single player on the lobby gives you the
10	bottom seat and hands the other three to the computer — same rules, same
11	table, same scoring. See [computer players](#computer-players).
12
13	Friends elsewhere? 線上對戰 Online game puts everyone at the same table
14	from their own devices, and 手機派對 Party mode keeps the laptop as the
15	table while phones scan a QR to hold their own tiles — see
16	[playing online](#playing-online).
17
18	```
19	npm install
20	npm run dev # then open the browser full-screen (F11)
21	npm test # rules engine, the bots, and 200 randomly-played hands
22	```
23
24	## Screen layout
25
26	```
27	┌──── 對家 (rotated 180°) ────┐
28	上家 (rotated 90°) │ wall, and the discard pool │ 下家 (rotated −90°)
29	└──── 自家 (upright, near you) ┘
30	```
31
32	Seat 0 is the bottom edge, and play runs 0 → 1 (right) → 2 (top) → 3 (left),
33	which is the normal counter-clockwise 東南西北 order.
34
35	Each strip shows, from the player's edge inwards: nameplate (seat wind, 莊
36	marker, 聽 badge, chip count) → concealed hand → action buttons → melds and
37	flowers. Discards are not in the strips: they are thrown into the middle and
38	stay there for the hand, the way they would be on a table. (A phone has no
39	middle to throw into, so the compact layout keeps a discard row per seat — see
40	[on a phone](#on-a-phone).)
41
42	## Playing
43
44	- The tile you just drew is held apart from the sorted hand with a gold ring
45	and a 摸 label, instead of being sorted invisibly into it. It merges into the
46	hand once you discard. 補花 replacements and kong replacements are marked the
47	same way.
48	- The wall is drawn in the centre as a square of four staggered sides, laid
49	out like a `#` with the corners open, the way it's built on a real table. The
50	gold stack is the break point you draw from, stacks shrink to a single tile and
51	then vanish as they're used, and the dimmed tail is the 16-tile 底牌 that ends
52	the hand — which is also the end kong and flower replacements are taken from,
53	so the square is eaten from both directions at once. Only what is still
54	standing is drawn, and it is evened up over the four sides as it goes, which
55	is what lets every tile left in it be the size of a tile in your hand — see
56	[what is left of the wall](#what-is-left-of-the-wall).
57	- Discard — drag a tile out of your row and it comes up out of your hand and
58	follows your finger anywhere on the table; let go and it goes in, from where
59	you let go and at the speed you let go at. Dragging along the row is still
60	arranging your hand — which of the two you meant is decided once, from whether
61	the movement is mostly along the row or out of it, and then it sticks. Let go
62	back over your own hand and the tile goes back. Tapping a tile twice, or the
63	打出 button, still works and lobs it in for you.
64	- The discard pool — every tile thrown lands in the middle and stays there
65	until the hand ends, with real weight: it skids, turns, knocks the tiles
66	already there out of its way, and comes to rest against them. Nothing is ever
67	stacked on anything — tiles are solved as rectangles, so two lying at an angle
68	lean on each other rather than overlapping. The tile still to be claimed is
69	the lit one.
70	- Over the wall or across the table — which of the two ways a tile goes in is
71	not a setting, it is whatever the wall allows. At the start of a hand the
72	square is a solid barrier and a discard has to be lobbed over it, so the
73	flick's speed decides how far across the pool it lands. As the wall is eaten
74	away, gaps open in front of one seat and then another, and from then on that
75	seat can slide a tile in instead — flat across the cloth at exactly the speed
76	it was flicked. Which it will be is a ray cast at the stacks actually standing
77	there, so a gap off to one side counts, and a stack worn down to a single tile
78	is low enough to go over. A seat whose wall is still up says 牆未開 on its
79	nameplate, so a lob is never a mystery.
80	- Throw it badly and it ends up in somebody's tiles — the middle stops
81	exactly where their hand starts, so a tile that comes off the pool arrives in
82	their lap, and one that falls short lands out by their seat. Either way their
83	row takes the knock and rocks back, and with 報牌 on they tell you about it —
84	喂，小心點 if you are lucky, 是在哈囉 or 你牌品很差欸 if you are not, and
85	never the same line twice running. How hard it got there only decides how
86	loudly. Never your own edge: you cannot be barged by your own discard.
87	- Arranging — drag any tile in your hand to reorder it; 理牌 sorts it back
88	into suit order. Hands are never auto-sorted after the deal, so an arrangement
89	you set up survives draws, claims and kongs.
90	- Rules — the `?` on your nameplate opens a bilingual rules panel anchored to
91	your own edge of the table and rotated to face you.
92	- Claiming — after a discard, every seat that can claim gets 胡 / 槓 / 碰 /
93	吃 / 過 buttons on their own strip, and they resolve by priority (胡 > 槓/碰 >
94	吃; ties go to the player nearest the discarder in turn order).
95	The table never blocks on a claim. The next player gets a 摸牌 button and
96	may draw whenever they like, which shuts the window on anyone who hasn't
97	called — exactly like shouting 碰 before the next player picks up. A claim
98	already declared still stands, so calling in time always wins the tile. If
99	everyone answers first, play advances on its own without the extra tap.
100	The seat that draws next gets no separate 過 button, because for them the two
101	are the same move: picking up is how you decline. Their draw button reads
102	過．摸牌 while they have something they could have called instead. Wanting to
103	decline but leave the window open for everyone else is just waiting, which is
104	what not pressing anything already does.
105	- On your turn — 自摸, 暗槓 and 加槓 appear automatically when legal.
106	加槓 offers everyone else a 搶槓 chance.
107	- Where the buttons go — the strip is built from its edge inwards, so the
108	action bar holds a fixed height whether or not anything is in it. Otherwise
109	the melds above it shuffled along every time you lifted a tile
110	and 打出五萬 appeared, since a button naming a tile stands taller than a bare
111	過. Every button on the bar is given that one height for the same reason.
112	- When a hand ends — a big arrow drops onto the winner's edge of the table.
113	It lives inside their strip, so it turns with the seat and lands on the right
114	person wherever they are sitting, and 一炮多響 lights up every seat that
115	called. A 流局 has no winner to point at, so it turns the hands over instead:
116	who was 聽牌 and the tiles they were waiting on, the same reveal as at a real
117	table.
118	- Undo — a 復原 button in the middle of the table takes back the last
119	action, naming what it will undo (`復原玩家 2 打五萬`). It sits in the
120	centre rather than on a seat because whoever spots the mis-tap should be
121	able to reach it. Twenty actions deep, cleared when the next hand is dealt.
122	Snapshots live outside the saved state, so an undo does not survive a
123	refresh: what you can take back is what happened while everyone was still
124	watching it happen.
125	- Sound — on by default, muted from the 🔊 button in the middle of the
126	table or from settings. A dry clack as a tile goes down, and a distinct
127	two-note chime when a claim window opens, which is how a slow player notices
128	their 碰 is available before the next player draws it shut. Everything is
129	synthesised with a few oscillators (`src/game/sound.ts`) rather than sampled,
130	so there is nothing to load and the cue lands on the same frame as the tile.
131	Browsers refuse to start audio before the page has been clicked, so the first
132	sound anyone hears is the deal — triggered by the button that starts it.
133	- 報牌 (voice) — a second switch under sound calls the game out loud: 碰,
134	吃, 槓, 胡了, 自摸, and the name of every tile as it is discarded — 三條,
135	五萬, 東風. A flower says 補花 and then which one. A new call cuts off one
136	still being spoken; at table pace the newest is the only one that matters.
137	See [the voice pack](#the-voice-pack) for where the audio comes from.
138	- Settings — 設定 from the lobby, or from the game-over panel: player names,
139	底 / 台 / starting chips, the house-rule switches below, and sound. Kept in
140	`localStorage` separately from the save, so they carry over to the next game.
141	Stakes are locked once a game is under way — they'd otherwise rewrite chips
142	already won.
143
144	## Playing online
145
146	Two networked modes, both built on [antics](https://antics.gg) — a room
147	service for web games with no backend of your own: one client (the antics
148	host) runs the engine, publishes the whole `GameState` as room state after
149	every move, and plays intents the other devices send as events. The engine was
150	already a pure JSON state machine, so nothing in `src/game/` knows the network
151	exists — the same trick `autoplay.ts` plays, stretched over a wire. All of it
152	lives in `src/net/`.
153
154	- 線上對戰 Online game — everyone on their own device. Whoever opens the
155	room picks 線上對戰 and gets a seat lobby; friends who open the same link
156	land in it, sit down, and the host deals. Empty seats go to the computer,
157	which runs on the host client — that is what stands in for a server on the
158	classic (free) tier. Each screen draws the same full table, turned so your
159	own seat is the bottom edge; other people's tiles are face down. Joining
160	mid-game takes over a vacant or computer seat, so a dropped friend can
161	reload and carry on.
162	- 手機派對 Party mode — the laptop stays the table, exactly like hotseat,
163	and each seat's corner of the felt wears a QR. Scanning one loads a
164	controller on the phone: just that hand, face up, with the claim buttons —
165	and the throw. Flick a tile up off the top of the phone and it sails in from
166	your edge of the common screen, at the speed and angle you let go of it
167	(`NetThrow`, mapped into table coordinates by `TablePool.throwFromNet`). The
168	seat's tiles go face down on the shared screen and its QR hides; hold the 📱
169	tag on the nameplate to show it again for a player who lost theirs. A phone
170	that leaves hands the seat back to the laptop. Seats never lock: a second
171	phone scanning the same QR joins the same hand — two people playing one
172	hand, whoever acts first acts.
173
174	Design notes, in the order they bit:
175
176	- Seats are claims on a shared map, `seats: {ids, name}[]` in room state,
177	arbitrated by the host. Your own hand's arrangement never goes over the wire
178	— the state carries a multiset and each device keeps its own order
179	(`src/net/handOrder.ts`), because a round-trip inside a drag gesture is lag
180	you can feel.
181	- The host can change. antics re-elects when the host leaves; every other
182	client already mirrors the full state, so the new host starts its engine
183	from what it was just watching and the game carries on.
184	- A hidden host must keep hosting. Browsers suspend requestAnimationFrame
185	in background tabs, and the antics SDK flushes state writes off rAF — so a
186	party screen somebody tabbed away from would silently stop answering.
187	`src/net/clock.ts` feeds the SDK's `__anticsClock__` injection point from a
188	Web Worker, whose timers are not throttled.
189	- Fairness is social, not cryptographic. The classic tier shares the whole
190	state with every client, wall and hands included — a friend with devtools
191	open can cheat. The antics sim tier would fix that, and costs money.
192
193	### Hosting on antics
194
195	```
196	npm run build && node scripts/antics-deploy.mjs # keyless — a fresh link each deploy
197	node scripts/antics-deploy.mjs --project <id> # stable /p/ link (needs `npx antics-cli login`)
198	```
199
200	The deploy script packs the 57 voice clips into one `voice/pack.json` (an
201	antics bundle holds at most 50 files; `sound.ts` prefers the pack when it is
202	there). Keyless deploys are pinned to their content hash — share a new link
203	after every deploy — and keyless rooms hold 8 players and expire after 24
204	hours. Deploying under a project gives a share link that follows updates.
205
206	The QR links point at the game's own URL (`/raw/<hash>/?room=…&seat=…`), not
207	the antics room page: the room page forwards only the parameters it knows
208	about to the game, and `seat` is not one of them — and a phone is better off
209	without the chrome anyway.
210
211	## On a phone
212
213	The table assumes four people sitting around a screen lying flat, and about
214	770px in both directions before the middle is worth looking at. A phone has
215	neither, so under `(max-height: 620px), (max-width: 820px)` it switches to the
216	compact layout: nothing is rotated, everything reads upright for the one person
217	holding it, the other three seats shrink to cards showing what is public about
218	them, and the depth that frees up goes to your hand, which is allowed to wrap.
219
220	There is no centre square there — the middle collapses to a one-line bar showing
221	what the wall square was telling you anyway — so there is nowhere to throw a tile
222	to. The compact layout therefore keeps a discard row per seat, and discarding
223	is tap-twice or 打出, exactly as it was. The pool and the throw are a full-size
224	feature.
225
226	The breakpoint is written down once, in `COMPACT_QUERY` in `src/ui/compact.ts`,
227	which sets a `compact` class on `<html>` for the CSS to key off.
228
229	## Computer players
230
231	Single player seats you at the bottom and gives seats 1-3 to the computer.
232	Their tiles go face down and their 聽 / 過水 badges come off, since both would
233	give the hand away; everything public — the pool, melds, flowers, chips — stays
234	exactly as it is, and everyone turns their hand over when the hand ends. They
235	throw their tiles in like anyone else, and under the same rule: a computer seat
236	whose wall is open slides them across, and one still walled in lobs them over.
237
238	The engine does not know the bots exist. `AutoPlay` (`src/game/autoplay.ts`)
239	watches the same state the screen does, and when the seat being waited on
240	belongs to the computer it calls the identical method the button would have
241	called — `discard`, `respond`, `declareConcealedKong`. So scoring, sound,
242	saving, 過水 and 包牌 all work without a special case, and a bot cannot make a
243	move a person could not. It never acts for you and never closes your claim
244	window: if the table is waiting on you, it waits.
245
246	How they play (`src/game/bot.ts`) is one idea applied everywhere. A hand is
247	judged at its resting size — the (5 − melds) × 3 + 1 tiles you hold between a
248	discard and your next draw — by its 向聽 first and by its 進張 count second.
249
250	- Discard — try every distinct tile, keep the one that leaves the best
251	resting hand. Ties go to whatever is hardest to build on: a lone honour with
252	most of its copies already gone, before a lone 五萬 that still has neighbours
253	to meet.
254	- 碰 / 吃 — only when the hand that comes out the other side is strictly
255	closer to home. Melding costs concealment and flexibility, so a claim that
256	merely holds the 向聽 steady is declined — which is why the bots pass on
257	pungs that would narrow a two-sided wait to a single tile.
258	- 槓 — judged more kindly: it pays 台 and fetches a replacement, so standing
259	still is good enough.
260	- 胡 — always taken.
261
262	What they watch you do (`src/game/danger.ts`) is the other half. Efficiency
263	alone throws whatever is fastest and pays for it; these bots read the table
264	first, off the face-up table only — discard rows, exposed melds, the 過水 locks
265	the nameplates already show. Two separate questions:
266
267	- How close does each seat look? Turns taken (a hand is usually settled
268	inside ten goes each, long before the wall looks finished, so counting the
269	wall is the wrong clock), melds exposed, and what they have been throwing
270	lately. Nobody discards 五條 out of a hand that still needs shaping — it comes
271	out once the shape is done, so late middle tiles are the tell. A 過水 lock is
272	a confession: to be locked out of a tile you had to have been able to win on
273	it.
274	- How likely is this tile the one they want? An honour can only be caught by
275	a pair or a triplet; a 五萬 sits in the middle of every run through it. A tile
276	they discarded themselves was not their tile when it went down — not proof
277	now, but the best evidence there is. A tile with no copies left unseen cannot
278	be a pair or triplet wait at all. Two melds in one suit means stay out of that
279	suit. And two dragon pungs down means the third dragon is not going anywhere
280	near the table, because 包牌 bills the feeder for the entire hand.
281
282	Whether any of that changes the discard depends on the bot's own hand. At 聽牌
283	it pushes — nothing short of 包牌 is worth breaking a ready hand for. Two or
284	three away with somebody live across the table and it will give up a whole 向聽
285	step to throw something safe, which is what folding is.
286
287	The estimate is checked against ground truth in the tests rather than assumed:
288	over the tiles bots actually considered, the ones the model rated below 0.2 deal
289	in 0% of the time and the ones above 0.8 deal in 10.5%, cleanly monotonic in
290	between. Switching the read on cuts deal-ins by about 13% over a few hundred
291	hands, takes hands from ~30 discards to ~38, and moves the draw rate from
292	almost nothing to about one hand in ten — which is what a table where people
293	stop feeding each other actually looks like.
294
295	What they know is still only what they can see. `unseenFor` counts the four
296	copies of each tile and subtracts the bot's own hand plus every discard and
297	exposed meld on the table. Neither module ever reads an opponent's concealed
298	tiles or looks at the wall.
299
300	There is no difficulty setting, and the reading stops at the discard: claims are
301	still judged purely on speed, and a bot will take a 碰 that walks it into
302	trouble.
303
304	向聽 lives in `src/game/shanten.ts`, apart from `hu.ts`, because the two want
305	different things: the win check has to be exact, and this one has to be fast —
306	it runs a few hundred times for every discard a bot considers. The tests hold
307	them against each other over random hands, since they work in completely
308	different ways and any disagreement is a bug in the fast one.
309
310	Undo behaves differently against the computer: rewinding into the middle of its
311	turn would only hand the move straight back to it, so one press goes back to
312	the last point you had a decision to make, computer replies and all.
313
314	## Saving
315
316	The game state is plain data, so a save is just its JSON in `localStorage`,
317	rewritten after every move. Close the lid, refresh, or run the battery flat and
318	nothing is lost — the lobby offers 繼續對局 Resume above the two ways to
319	start a fresh one, showing whether it was a solo or a hotseat game, the round,
320	hand number, chip counts and when it was saved. Which seats the computer holds
321	is part of the state, so a save resumes as the kind of game it was. A
322	save is validated before it is offered (four players, 144 tiles accounted for),
323	so a truncated or hand-edited one is ignored rather than loaded into a broken
324	table. `VERSION` in `save.ts` retires old saves if the state shape changes.
325
326	## Rules implemented
327
328	- 144 tiles (four of each suit/honour, eight flowers), 16-tile hands, dealer
329	draws the 17th.
330	- 補花 at the deal and on every drawn flower, replacements from the back of the
331	wall.
332	- 吃 only from 上家; 碰/槓/胡 from anyone. 明槓, 暗槓, 加槓, 搶槓, 槓上開花.
333	- 流局 when 16 tiles remain (`Rules.wallReserve`); the dealer keeps the deal on a
334	draw or on a dealer win (連莊), otherwise the deal passes and the round wind
335	advances every four passes. A full 四圈 game is 16 dealer passes.
336
337	### House rules (switches in `Rules`, all on the settings screen)
338
339	- 過水 `sacredDiscard` (on) — pass on a tile you could have won with and it
340	is dead to you until your own next draw; the seat shows a 過水 badge listing
341	what it is locked out of, so the missing 胡 button is never a mystery. A draw
342	from either end of the wall lifts it. `sacredClearedByClaim` (off) decides
343	whether taking a 吃 / 碰 / 槓 lifts it too — tables genuinely differ.
344	- 一炮多響 `multipleWinners` (off) — one discard pays out to every seat that
345	calls on it, each settled separately against the discarder. With it off the
346	tile goes to the caller nearest the discarder. Either way the table now waits
347	for other seats that can win before settling, so the nearest seat wins the
348	tile rather than the quickest hand — and the 摸牌 button still closes the
349	window on anyone dithering.
350	- 包牌 `liability` (on) — feeding the pung that completes a visible 大三元
351	or 大四喜 makes the feeder answer for the whole hand, in place of all three
352	payers. Only the seat that fed the last of those sets is on the hook, and
353	only if it came off a discard: a hand that assembled them itself, or closed
354	the set with a 暗槓, has nobody to blame.
355
356	### 台 scoring (`src/game/tai.ts`)
357
358	自摸 1 · 門清 1 · 門清自摸 +1 · 全求人 2 · 平胡 2 · 五門齊 2 · 正花 1 each ·
359	花槓 2 · 八仙過海 8 · 圈風 / 門風 1 each · 三元牌 1 each · 小三元 4 · 大三元 8 ·
360	小四喜 8 · 大四喜 16 · 碰碰胡 4 · 混一色 4 · 清一色 8 · 字一色 16 ·
361	三暗刻 2 / 四暗刻 5 / 五暗刻 8 · 獨聽 · 單釣 1 · 搶槓 1 · 槓上開花 1 ·
362	海底撈月 1 · 河底撈魚 1 · 天胡 16 · 地胡 16 · 人胡 8 · 莊家 1 (連N拉N → 2N+1)
363
364	Ambiguous hands are decomposed every legal way and scored at the best reading.
365
366	Payment (`Game.settle`): one unit is `底 + 台 × 台值`
367	(`DEFAULT_RULES` = 底 3, 台 1, 100 chips each).
368	放槍一家付 — the discarder alone pays one unit; on 自摸 all three pay one unit
369	each. 拉莊 is billed to the dealer alone when the dealer is a payer, and added
370	to the whole hand when the dealer wins.
371
372	House rules vary a lot; the tai table and payments are plain data/functions in
373	`tai.ts` and `types.ts` if yours differ.
374
375	## The voice pack
376
377	A mahjong table only ever says about fifty things — 42 tile names and a handful
378	of calls — so the whole vocabulary is rendered ahead of time into
379	`public/voice/` (~330 kB of mp3) rather than left to whatever speech synthesis
380	the browser happens to have. On Linux that is usually espeak-ng, which is
381	intelligible but sounds like a modem; and on a machine with no Chinese voice at
382	all the feature would silently do nothing. Shipping the audio makes playback
383	instant, identical everywhere, and lets a call be cut off mid-word when the
384	next one lands, all through the same Web Audio graph as the chimes.
385
386	`scripts/voice.mjs` is the authoring step, not part of `npm run build`. It needs
387	[piper](https://github.com/OHF-Voice/piper1-gpl) and ffmpeg on PATH:
388
389	```
390	node scripts/voice.mjs # → public/voice/*.mp3 + manifest.json
391	PIPER_MODEL=/path/to/voice.onnx node scripts/voice.mjs # a different voice
392	```
393
394	The wording lives in that script, and some of it is deliberately not the bare
395	tile character: 東 alone is a direction where 東風 is the tile, and the dragons
396	are 紅中 / 發財 / 白板 the way they are actually called — which also gives the
397	phonemiser enough context to get the tone right, since 中 on its own is as
398	likely to come out zhòng.
399
400	The clips here were rendered with piper's `zh_CN-huayan-medium`. If you
401	redistribute this app, check that voice's model card in
402	[piper-voices](https://huggingface.co/rhasspy/piper-voices) for the terms
403	attached to it, the same way you would the tile art below — or re-render the
404	pack with a voice whose terms suit you, which is a single command.
405
406	## Tile art
407
408	`public/tiles/tiles.svg` is the postmodern tileset from
409	[gnome-mahjongg](https://gitlab.gnome.org/GNOME/gnome-mahjongg), extracted from
410	the installed binary's GResource — a 43×2 sprite sheet (second row is the
411	highlighted variant, used for a lifted tile). `public/tiles/back.png` is the
412	blank tile from its smooth theme, tinted jade, since a solitaire game has no
413	face-down art of its own.
414
415	That art is GPL-2.0-or-later. Fine for playing at home; if you ever
416	distribute this app, either honour the GPL or swap the two files for art of your
417	own — `SPRITE_COL` in `tiles.ts` is the only mapping that would need updating.
418
419	Known gaps and house rules that aren't implemented yet are listed in
420	[TODO.md](TODO.md).
421
422	## Layout
423
424	```
425	public/tiles/ sprite sheet + tile back
426	src/game/tiles.ts tile codes, wall, shuffle, sprite mapping
427	src/game/hu.ts hand decomposition, 聽 detection, wait shapes
428	src/game/shanten.ts 向聽 / 進張 counting, for the computer players
429	src/game/bot.ts what a computer player discards and claims (pure)
430	src/game/danger.ts reading the table: who looks ready, which tiles are hot
431	src/game/autoplay.ts when it does it, and how long it appears to think
432	src/game/tai.ts 台 scoring
433	src/game/engine.ts state machine: deal, turns, claim resolution, settlement
434	src/game/ctl.ts TableCtl — the surface the UI drives, local or networked
435	src/game/wall.ts what is left of the square, and what still blocks a throw
436	src/net/session.ts the antics room: host duties, seats, intents, migration
437	src/net/table.ts TableCtl over the wire: mirror + intents + hand overlay
438	src/net/protocol.ts what crosses the wire, in one place
439	src/net/handOrder.ts your own arrangement, kept on your own device
440	src/net/clock.ts worker-driven SDK clock, so a hidden host keeps hosting
441	src/ui/Controller.tsx the phone at the party table: your tiles and the throw
442	src/ui/OnlineLobby.tsx seats and the deal button, before an online game
443	scripts/antics-deploy.mjs dist/ → antics.gg, voice pack and all
444	src/table/physics.ts the discard pool: rectangles with weight, pure and testable
445	src/table/geometry.ts the table measured — colliders, launch points, throw gate
446	src/table/pool.ts game state ⇄ tiles in the middle, by diffing the discards
447	src/ui/ TileView, Hand (drag and throw), Seat, Center, Pool, Help
448	```
449
450	## The tiles in the middle
451
452	The pool is a small rigid-body solver (`src/table/physics.ts`) that knows nothing
453	about mahjong, the DOM, or the clock: fixed 1/120s steps, no randomness of its
454	own, and everything in table pixels. Given the same throws it produces the same
455	pile every time, which is what lets a refresh mid-hand come back to the pile you
456	had rather than a freshly scattered one — positions are derived from the hand
457	number and each tile's place in its thrower's discards, never saved.
458
459	It is drawn on a canvas that spans the whole table, not just the centre.
460	That is deliberate: `.slot` and `.seat` both clip their own contents, which is
461	why a tile can't be animated out of a strip as an element. On the canvas there is
462	nothing to clip it, so a tile lifted out of a hand crosses the table in one
463	piece.
464
465	Nothing about the square is calculated twice. Its size lives entirely in CSS
466	(`--ws`, `--wd`), so `geometry.ts` measures it instead of restating that
467	arithmetic: `WallRing` already renders every stack as a real element tagged with
468	its index, and the colliders are those elements' bounding rects. That one
469	mapping is what makes the wall a physical object — the thing a thrown tile
470	bounces off, and the thing the throw gate casts its ray at.
471
472	## What is left of the wall
473
474	Four walls of eighteen full-size stacks want about 560px a side. No ordinary
475	window has that between the top and bottom strips, and the tile is not allowed
476	to shrink to make it fit — a tile in the wall is the same tile you play with, so
477	drawing it smaller was a visible lie.
478
479	What saves it is that nobody ever sees a whole wall: four hands come off the
480	front before the table is on screen, so barely half of it is left by then. So
481	only the stacks still standing are drawn, and they are laid out around a ring
482	cut to the middle rather than to eighteen — `ringLayout` in `game/wall.ts`
483	measures how many stacks a side can take (`WallRing` renders one hidden cell and
484	asks CSS how big it came out) and shares what is left over the four sides in
485	proportion. Every seat keeps a wall in front of it instead of the remainder
486	piling into one corner, and the order runs all the way round, so the break point
487	is still at the head of it and the 底牌 tail still at the end.
488
489	Shuffling the wall along as it is eaten costs nothing to look at, because every
490	stack shows the same green back — and it is what people do to a half-eaten wall
491	anyway.
492
493	`pool.ts` never listens for events. It diffs the discards against what it is
494	already drawing, keyed by `seat:index` — and those keys never shift, because
495	discards are only ever pushed and a claim only ever pops the one on top. So a
496	key that appeared is a tile to throw in and a key that went is a tile to take
497	off, which makes undo, 下一局, a claim and resuming a saved game all the same
498	code path. The engine has no idea any of it exists, and the save format did not
499	change.