collin/mahjong

95126effc7a7d363a91f49efda2491719e116ecf / 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 and
5	each face down until its player taps it — the piece of card everybody used to
6	lay over their strip, done by the table itself. Every label is Chinese with
7	English underneath; the tiles themselves stay Chinese, because that is what the
8	tiles say.
9
10	開始 is the only button on the lobby (繼續對局 above it when there is a
11	save), and it says nothing else, because the table says the rest — three strips
12	offering 坐下 and a QR in the middle of the felt. It deals
13	you in at the bottom edge with the computer on the other three, on this device,
14	without a word said to any server. Everything people call a mode is that table
15	with different hands changing hands, one at a time, mid-game:
16
17	- somebody sits down at this screen and **taps a hand the computer has been
18	playing** — the tiles turn over and are theirs;
19	- somebody scans a seat's QR and takes that hand onto their phone;
20	- somebody opens the invite link from three cities away and takes one from
21	their own laptop, with the table turned so their seat is the near edge.
22
23	Each of those swaps one seat and redeals nothing, and each undoes: a hand goes
24	back to the computer with a tap, and one that loses its device gets given back
25	to the computer on its own so the table never stops. See
26	[one table](#one-table), [computer players](#computer-players) and
27	[playing online](#playing-online).
28
29	```
30	npm install
31	npm run dev # then open the browser full-screen (F11)
32	npm test # rules engine, the bots, and 200 randomly-played hands
33	```
34
35	## Screen layout
36
37	```
38	┌──── 對家 (rotated 180°) ────┐
39	上家 (rotated 90°) │ wall, and the discard pool │ 下家 (rotated −90°)
40	└──── 自家 (upright, near you) ┘
41	```
42
43	Seat 0 is the bottom edge, and play runs 0 → 1 (right) → 2 (top) → 3 (left),
44	which is the normal counter-clockwise 東南西北 order.
45
46	Each strip shows, from the player's edge inwards: nameplate (seat wind, 莊
47	marker, 聽 badge, chip count) → concealed hand → action buttons → melds and
48	flowers. Discards are not in the strips: they are thrown into the middle and
49	stay there for the hand, the way they would be on a table. (A phone has no
50	middle to throw into, so the compact layout keeps a discard row per seat — see
51	[on a phone](#on-a-phone).)
52
53	## Playing
54
55	- Your hand is face down at a shared screen, and a tap turns it over. It
56	covers itself again when the hand is dealt and when your turn ends — the two
57	moments the tiles stop being needed, and without them a hand turned over once
58	stays turned over all evening, which is the same as never having covered it.
59	蓋牌 beside 理牌 puts it back sooner. While it is covered the only button the
60	strip has is 看牌: every other one names a tile — 打出五萬, 吃 with these two
61	— and would say through the card what is under it. The 看牌 of whoever the
62	table is waiting on wears a gold ring, since whose go it is was never a
63	secret. Nothing is covered where the screen is playing one hand, which is a
64	screen that belongs to one player — three computers and you, or your own
65	tiles on your own device.
66	- The tile you just drew is held apart from the sorted hand with a gold ring
67	and a 摸 label, instead of being sorted invisibly into it. It merges into the
68	hand once you discard. 補花 replacements and kong replacements are marked the
69	same way.
70	- The wall is drawn in the centre as a square of four staggered sides, laid
71	out like a `#` the way it is built on a real table — every corner covered by
72	one wall carrying past the end of the next. All 144 tiles, at the size of the
73	tiles in your hand, because they are the same tiles. The gold stack is the
74	break point you draw from, and where that is depends on what the dealer threw:
75	stacks shrink to a single tile and then vanish as they are used, so the gap
76	opens wherever the dice opened the wall and grows from there. The 16-tile 底牌
77	tail at the other end is where kong and flower replacements come from, so the
78	square is eaten from both directions at once. See
79	[擲骰](#擲骰--where-the-wall-is-broken).
80	- Discard — drag a tile out of your row and it comes up out of your hand and
81	follows your finger anywhere on the table; let go and it goes in, from where
82	you let go and at the speed you let go at. Dragging along the row is still
83	arranging your hand — which of the two you meant is decided once, from whether
84	the movement is mostly along the row or out of it, and then it sticks. Let go
85	back over your own hand and the tile goes back. Tapping a tile twice, or the
86	打出 button, still works and lobs it in for you.
87	- The discard pool — every tile thrown lands in the middle and stays there
88	until the hand ends, with real weight: it skids, turns, knocks the tiles
89	already there out of its way, and comes to rest against them. Nothing is ever
90	stacked on anything — tiles are solved as rectangles, so two lying at an angle
91	lean on each other rather than overlapping. The tile still to be claimed is
92	the lit one.
93	- Over the wall or across the table — which of the two ways a tile goes in is
94	not a setting, it is whatever the wall allows. At the start of a hand the
95	square is a solid barrier and a discard has to be lobbed over it, so the
96	flick's speed decides how far across the pool it lands. As the wall is eaten
97	away, gaps open in front of one seat and then another, and from then on that
98	seat can slide a tile in instead — flat across the cloth at exactly the speed
99	it was flicked. Which it will be is a ray cast at the stacks actually standing
100	there, so a gap off to one side counts, and a stack worn down to a single tile
101	is low enough to go over. A seat whose wall is still up says 牆未開 on its
102	nameplate, so a lob is never a mystery.
103	- Throw it badly and it ends up in somebody's tiles — the middle stops
104	exactly where their hand starts, so a tile that comes off the pool arrives in
105	their lap, and one that falls short lands out by their seat. Either way their
106	row takes the knock and rocks back, and with 報牌 on they tell you about it —
107	喂，小心點 if you are lucky, 是在哈囉 or 你牌品很差欸 if you are not, and
108	never the same line twice running. How hard it got there only decides how
109	loudly. Never your own edge: you cannot be barged by your own discard.
110	- Arranging — drag any tile in your hand to reorder it; 理牌 sorts it back
111	into suit order. Hands are never auto-sorted after the deal, so an arrangement
112	you set up survives draws, claims and kongs.
113	- Rules — the `?` on your nameplate opens a bilingual rules panel anchored to
114	your own edge of the table and rotated to face you.
115	- Claiming — after a discard, every seat that can claim gets 胡 / 槓 / 碰 /
116	吃 / 過 buttons on their own strip, and they resolve by priority (胡 > 槓/碰 >
117	吃; ties go to the player nearest the discarder in turn order).
118	The table never blocks on a claim. The next player gets a 摸牌 button and
119	may draw whenever they like, which shuts the window on anyone who hasn't
120	called — exactly like shouting 碰 before the next player picks up. A claim
121	already declared still stands, so calling in time always wins the tile. If
122	everyone answers first, play advances on its own without the extra tap.
123	The seat that draws next gets no separate 過 button, because for them the two
124	are the same move: picking up is how you decline. Their button says 摸牌
125	either way; what they would be giving up by pressing it is the 碰 or the 吃
126	sitting next to it on the bar, so it does not have to be spelled out on the
127	tile as well. Wanting to decline but leave the window open for everyone else
128	is just waiting, which is what not pressing anything already does. (On a
129	phone the button still reads 過．摸牌 — a phone's bar is a row with room to
130	write on, and the hand it belongs to is not on the same screen as the table.)
131	- On your turn — 自摸, 暗槓 and 加槓 appear automatically when legal.
132	加槓 offers everyone else a 搶槓 chance.
133	- Where the buttons go — on the same row as your tiles, at the right-hand end
134	of it, where the tile you are about to throw already is. The bar is out of the
135	strip's flow, so a button arriving moves nothing: 打出 appearing when you
136	lift a tile used to shove the melds and flowers above it up the strip. Every
137	button on the bar is given one height for the same reason. A bar with more on
138	it than the room beside the hand stacks up the strip in tile-wide lines, most
139	urgent call at the bottom, rather than running off the end of it. And the room
140	for the first line is in the tile size itself: a strip is sized to hold sixteen
141	tiles, the slot it keeps for the tile it is about to draw, and one button —
142	which is why the tiles down the sides of a short window are the size they are.
143	- What the buttons say — the call is written down the tile the way the
144	character on a tile's face is, one on top of the next in the middle of the
145	face, with the English along the bottom edge. Two characters stacked are the
146	size a tile's own character is; two side by side had to be shrunk to fit
147	across it. A call too long for the height (電腦代打) takes a second column to
148	the left of the first, which is where the next column of vertical Chinese
149	goes. 打出 names no tile: the one it would throw is lifted out of the row
150	beside it wearing a gold ring, so saying 五萬 on the button as well only
151	crowds the face.
152	- Whose go it is — a screen with four people round it is the one place a
153	game can go quiet without anybody noticing: three of them are watching the
154	table and the fourth is looking at their phone. So the table says it loudly.
155	The strip of the seat being waited on lights from its inner edge, their name
156	goes gold at the far end of it, and seven seconds of gold drain along the
157	edge that faces the middle — from both ends towards the centre, because two
158	of the four strips are upside down from wherever you are sitting. **Nothing
159	happens when it reaches zero.** No tile is thrown for you and no seat is
160	skipped; the line the bar leaves behind breathes, and the table goes on
161	waiting. It is a nudge, not a rule. Said on a covered hand too — whose turn
162	it is was never one of the things the card is hiding — and only where the
163	screen is playing more than one hand, since a screen playing one has nobody
164	to tell. A phone at the table gets the same seven seconds
165	across its top, a gold rim edge to edge (red for a claim window), and one
166	short buzz as the game comes round to it.
167	- On the keys — ← and → walk the selection along your hand and space throws
168	the tile that is lifted. How long you hold space is how hard it goes: the
169	tile draws back out of the hand while the key is down and leaves when you let
170	go, so a tap slides it in and a long hold sends it across the square. Escape
171	puts it back. The wall still decides whether it can be slid or has to be
172	lobbed, exactly as with a finger.
173	- When a hand ends — a big arrow drops onto the winner's edge of the table.
174	It lives inside their strip, so it turns with the seat and lands on the right
175	person wherever they are sitting, and 一炮多響 lights up every seat that
176	called. A 流局 has no winner to point at, so it turns the hands over instead:
177	who was 聽牌 and the tiles they were waiting on, the same reveal as at a real
178	table.
179	- The gear — beside the `?` on every nameplate, and it opens that player's
180	own copy of the table's controls: undo, sound, full screen, 設定. Anchored in
181	their strip and turned to face their edge, like the rules panel, so what you
182	open reads the right way up to you and to nobody else. These used to be a bar
183	in the middle of the table, which is the one place that belongs to everybody
184	— the pile lands there and it is upside down to two of the four.
185	- Undo — 復原 in that panel takes back the last action, naming what it will
186	undo (`復原玩家 2 打五萬`). It is in all four panels rather than only the
187	seat that made the mistake, because whoever spots the mis-tap should be able
188	to reach it. Twenty actions deep, cleared when the next hand is dealt.
189	Snapshots live outside the saved state, so an undo does not survive a
190	refresh: what you can take back is what happened while everyone was still
191	watching it happen.
192	- Sound — on by default, muted from the 🔊 button behind any seat's gear, or
193	from settings. A dry clack as a tile goes down, and a distinct
194	two-note chime when a claim window opens, which is how a slow player notices
195	their 碰 is available before the next player draws it shut. Everything is
196	synthesised with a few oscillators (`src/game/sound.ts`) rather than sampled,
197	so there is nothing to load and the cue lands on the same frame as the tile.
198	Browsers refuse to start audio before the page has been clicked, so the first
199	sound anyone hears is the deal — triggered by the button that starts it.
200	- 報牌 (voice) — a second switch under sound calls the game out loud: 碰,
201	吃, 槓, 胡了, 自摸, and the name of every tile as it is discarded — 三條,
202	五萬, 東風. A flower says 補花 and then which one. A new call cuts off one
203	still being spoken; at table pace the newest is the only one that matters.
204	See [the voice pack](#the-voice-pack) for where the audio comes from.
205	- 方位音 (sound by seat) — a third switch, on by default. Every noise comes
206	out of the edge its seat is sitting at: 上家 to your left, 下家 to your right,
207	and 對家 across the table, where it is quieter and has the top taken off it —
208	a metre of air and three people's arms do not carry 12 kHz. So you can tell
209	whose 碰 that was without looking up, which is the whole point of hearing it.
210	Tiles out in the middle are placed properly rather than by edge: the physics
211	knows to the pixel where a tile landed, and that is where the knock comes
212	from. A device holding one hand turns the table so that hand is at the
213	bottom, and the sound turns with it. See [audio, by seat](#audio-by-seat).
214	- Settings — 設定 from the lobby, or from the game-over panel: player names,
215	底 / 台 / starting chips, the house-rule switches below, and sound. Kept in
216	`localStorage` separately from the save, so they carry over to the next game.
217	Stakes are locked once a game is under way — they'd otherwise rewrite chips
218	already won.
219
220	## One table
221
222	There were three buttons on this lobby — 單人對局, 四人同桌, 線上對戰 — and
223	they were never three games. Every party table played its unclaimed seats
224	hotseat-style at the screen already; every hotseat table would have taken a
225	phone if it had had a room to put it in; every online table filled its empty
226	chairs with the same computer players the solo game is made of. They differed
227	in who was holding which hand, and that is not something a lobby should have to
228	know before the tiles are dealt.
229
230	So there is one button, and the question it used to ask is answered
231	continuously instead. A hand is played by one of three things — **the computer,
232	somebody at this screen, or a device of its own** — and any of them becomes any
233	other while the hand is in progress, with nothing redealt and nobody consulted
234	but the person doing it.
235
236	\| \| played by \| becomes a person here \| becomes a phone \|
237	\|---\|---\|---\|---\|
238	\| 電腦 \| the bots in `autoplay.ts` \| tap the strip — 坐下 \| its QR, from its gear \|
239	\| 這裡 \| this screen, face down under a tap \| — \| its QR, from its gear \|
240	\| 手機 \| whoever scanned in \| they leave → the computer \| — \|
241
242	Nothing touches the network until a phone is wanted. A table of people and
243	computers is four hands and an engine; there is nothing for a server to do, and
244	`src/net/` is never constructed. The room is opened by the first thing that
245	needs one — the 用手機拿牌 in a seat's gear — and when it opens it *publishes
246	the hand in progress* rather than dealing over the top of it
247	(`NetSession.publishTable`). The engine is the page's own either way:
248	`NetTable` is a wrapper that goes on around a live `Game`, not a second one, so
249	opening a room costs a `structuredClone` and changes nothing on screen except
250	that the QRs start working.
251
252	Each seat's QR is its own, and lives in that seat's gear beside the sound
253	and the undo, carrying `?room=…&seat=N`. Scanning it lands on that chair
254	without anybody having to say which one they are in: the hand leaves the shared
255	screen for good — face down there, not under a card that lifts — and the phone
256	gets it face up with the claim buttons and the throw. There is a QR for the
257	whole table in the middle of the felt as well, but only while the table is
258	still being sat down at; the moment the first tile is thrown it steps aside,
259	because from then on the middle is where the tiles go.
260
261	**A hand that loses its device is kept for fifteen seconds and then given to
262	the computer** (`EMPTY_SEAT_GRACE`). A phone locking itself looks exactly like
263	a phone leaving for good and only one of them means it, so the nameplate shows
264	⏳ and the table waits; when the wait is up the computer picks the tiles up and
265	play carries on. Nothing is lost either way — coming back takes the seat
266	straight off the computer again, and so does one tap on it at the table. The
267	same countdown covers the screen closing its lid: the room records which device
268	the table is being played at (`screen` in room state), and hands with neither a
269	device nor a table behind them go the same way.
270
271	The save follows the table. Whichever device is running the game writes
272	`localStorage` (`engine.persist = authority`; every mirror has it off, and no
273	business writing it), so 繼續對局 restores here and the room, if one is wanted
274	again, opens behind it exactly as it did the first time.
275
276	Covering is arithmetic, not a mode. A hand starts face down when this
277	device is playing more than one of them — which is the definition of a screen
278	several people are sitting round. One hand is one person, whether they are
279	playing three computers or holding their own tiles on their own phone, and
280	covering your own tiles from yourself is a tap in the way.
281
282	## Playing online
283
284	Every device that is not the table is on one room relay of our own:
285	`server/rooms.ts`,
286	a couple hundred lines of Node on `ws`. The relay knows nothing about mahjong:
287	rooms with codes, host election by join order, one bag of shared state only the
288	host may write, and events fanned out to everyone else. One client — the
289	host — runs the engine, publishes the whole `GameState` as room state after
290	every move, and plays intents the other devices send as events. The engine was
291	already a pure JSON state machine, so nothing in `src/game/` knows the network
292	exists — the same trick `autoplay.ts` plays, stretched over a wire. The client
293	end of the wire is `src/net/room.ts`; the rest of `src/net/` is the game
294	riding it.
295
296	- A device that holds one hand shows that hand. A phone shows just the
297	hand, face up, with the claim buttons — and the throw: flick a tile up off
298	the top of the phone and it sails in from your edge of the common screen, at
299	the speed and angle you let go of it (`NetThrow`, mapped into table
300	coordinates by `TablePool.throwFromNet`). Anything bigger shows the whole
301	table instead, turned so that seat is the bottom edge, with everyone else's
302	tiles face down. Same room, same seat map; the only thing deciding between
303	them is how much screen there is.
304	- A device that holds no hand yet gets the seat picker, and every seat is
305	offered — including the ones the computer is playing, which is the ordinary
306	way in, and the ones somebody already holds, which is how two people play one
307	hand off two devices. A seat's own QR names its chair and skips the picker.
308	- The device the table was opened on goes on showing the table: the felt,
309	the middle, the wall, and every hand no phone has taken, covered-with-a-tap
310	like a hotseat game. Hold the 📱 tag on a nameplate to show the table's QR
311	again for a player who lost theirs. With sound on, the phone in each player's
312	hand is what says their calls and the table goes quiet for that seat — see
313	[audio, by seat](#audio-by-seat).
314
315	Design notes, in the order they bit:
316
317	- Seats are claims on a shared map, `seats: {ids, name}[]` in room state,
318	arbitrated by the host. Your own hand's arrangement never goes over the wire
319	— the state carries a multiset and each device keeps its own order
320	(`src/net/handOrder.ts`), because a round-trip inside a drag gesture is lag
321	you can feel.
322	- The host can change. The relay re-elects — the earliest joiner still
323	connected — when the host leaves; every other client already mirrors the
324	full state, so the new host starts its engine from what it was just watching
325	and the game carries on.
326	- A hidden host must keep hosting. Browsers suspend requestAnimationFrame
327	and throttle timers in background tabs, so nothing in `src/net/` runs on a
328	loop: state writes flush on a microtask, everything inbound arrives over the
329	WebSocket (whose delivery is not throttled), and dead connections are the
330	server's ping loop's problem. A table screen somebody tabbed away from
331	keeps answering.
332	- Identity is a UUID in sessionStorage, chosen by the device and taken at
333	its word by the relay. The seat map is keyed on it, so a reload or a wifi
334	blip walks back into its own seat. No auth — this is a home server for one
335	table of friends.
336	- Fairness is social, not cryptographic. Room state carries the whole
337	game, wall and hands included — a friend with devtools open can cheat.
338	Moving the engine into the relay would fix that; the server is ours now, so
339	only the work stands in the way (TODO).
340
341	### Running it
342
343	```
344	npm run dev # vite, with the relay on /ws of the same origin
345	npm run build && npm run serve # production: dist/ + relay, one process
346	```
347
348	The relay always lives at `/ws` on the page's own origin. In development
349	`vite.config.ts` attaches it to vite's server, so `npm run dev` is the whole
350	stack; in production `server/index.ts` serves the built `dist/` and the relay
351	from one port (`PORT`, default 8080), listening on the LAN by default so
352	phones on the same wifi can scan straight in. Anything further away wants TLS
353	in front — a reverse proxy with a certificate — since phones only get camera
354	and fullscreen on https.
355
356	Rooms live in memory. A restart drops them; the next visitor on an old link
357	recreates the room empty, and a host mid-game republishes its state on the
358	next move. Joining a room writes `?room=` back into the address bar, so the
359	page URL — the host's included — is the invite link.
360
361	A QR pointing at `localhost` is a QR only the host's machine can scan, so the
362	server always gives the invite links a public face: `PUBLIC_URL` if set, else
363	it runs its own rsgrok tunnel (the house ngrok replacement) — spawned
364	the moment the server knows its port, the https URL read off the tunnel-up
365	line, no `:4040` inspection API (another agent may own that port). Every QR
366	and invite link wears that origin instead of the page's. If the tunnel dies,
367	or `rsgrok` is not on the PATH (`RSGROK_BIN` points elsewhere), links fall
368	back to the page's own address and the tunnel is retried on later joins.
369
370	The tunnel always asks for the same subdomain — `mahjong-table`, or whatever
371	`TUNNEL_NAME` says — so restarts land back on the URL people already have
372	instead of burning a fresh name each run. A name someone else already holds
373	kills that first attempt before it prints a URL; the retry then goes out
374	nameless and takes whatever it is given.
375
376	## On a phone
377
378	The table assumes four people sitting around a screen lying flat, and about
379	770px in both directions before the middle is worth looking at. A phone has
380	neither, so under `(max-height: 620px), (max-width: 820px)` it switches to the
381	compact layout: nothing is rotated, everything reads upright for the one person
382	holding it, the other three seats shrink to cards showing what is public about
383	them, and the depth that frees up goes to your hand, which is allowed to wrap.
384
385	There is no centre square there — the middle collapses to a one-line bar showing
386	what the wall square was telling you anyway — so there is nowhere to throw a tile
387	to. The compact layout therefore keeps a discard row per seat, and discarding
388	is tap-twice or 打出, exactly as it was. The pool and the throw are a full-size
389	feature.
390
391	The breakpoint is written down once, in `COMPACT_QUERY` in `src/ui/compact.ts`,
392	which sets a `compact` class on `<html>` for the CSS to key off.
393
394	## Computer players
395
396	Single player seats you at the bottom and gives seats 1-3 to the computer.
397	Their tiles go face down and their 聽 / 過水 badges come off, since both would
398	give the hand away; everything public — the pool, melds, flowers, chips — stays
399	exactly as it is, and everyone turns their hand over when the hand ends. They
400	throw their tiles in like anyone else, and under the same rule: a computer seat
401	whose wall is open slides them across, and one still walled in lobs them over.
402
403	The engine does not know the bots exist. `AutoPlay` (`src/game/autoplay.ts`)
404	watches the same state the screen does, and when the seat being waited on
405	belongs to the computer it calls the identical method the button would have
406	called — `discard`, `respond`, `declareConcealedKong`. So scoring, sound,
407	saving, 過水 and 包牌 all work without a special case, and a bot cannot make a
408	move a person could not. It never acts for you and never closes your claim
409	window: if the table is waiting on you, it waits.
410
411	How they play (`src/game/bot.ts`) is one idea applied everywhere. A hand is
412	judged at its resting size — the (5 − melds) × 3 + 1 tiles you hold between a
413	discard and your next draw — by its 向聽 first and by its 進張 count second.
414
415	- Discard — try every distinct tile, keep the one that leaves the best
416	resting hand. Ties go to whatever is hardest to build on: a lone honour with
417	most of its copies already gone, before a lone 五萬 that still has neighbours
418	to meet.
419	- 碰 / 吃 — only when the hand that comes out the other side is strictly
420	closer to home. Melding costs concealment and flexibility, so a claim that
421	merely holds the 向聽 steady is declined — which is why the bots pass on
422	pungs that would narrow a two-sided wait to a single tile.
423	- 槓 — judged more kindly: it pays 台 and fetches a replacement, so standing
424	still is good enough.
425	- 胡 — always taken.
426
427	What they watch you do (`src/game/danger.ts`) is the other half. Efficiency
428	alone throws whatever is fastest and pays for it; these bots read the table
429	first, off the face-up table only — discard rows, exposed melds, the 過水 locks
430	the nameplates already show. Two separate questions:
431
432	- How close does each seat look? Turns taken (a hand is usually settled
433	inside ten goes each, long before the wall looks finished, so counting the
434	wall is the wrong clock), melds exposed, and what they have been throwing
435	lately. Nobody discards 五條 out of a hand that still needs shaping — it comes
436	out once the shape is done, so late middle tiles are the tell. A 過水 lock is
437	a confession: to be locked out of a tile you had to have been able to win on
438	it.
439	- How likely is this tile the one they want? An honour can only be caught by
440	a pair or a triplet; a 五萬 sits in the middle of every run through it. A tile
441	they discarded themselves was not their tile when it went down — not proof
442	now, but the best evidence there is. A tile with no copies left unseen cannot
443	be a pair or triplet wait at all. Two melds in one suit means stay out of that
444	suit. And two dragon pungs down means the third dragon is not going anywhere
445	near the table, because 包牌 bills the feeder for the entire hand.
446
447	Whether any of that changes the discard depends on the bot's own hand. At 聽牌
448	it pushes — nothing short of 包牌 is worth breaking a ready hand for. Two or
449	three away with somebody live across the table and it will give up a whole 向聽
450	step to throw something safe, which is what folding is.
451
452	The estimate is checked against ground truth in the tests rather than assumed:
453	over the tiles bots actually considered, the ones the model rated below 0.2 deal
454	in 0% of the time and the ones above 0.8 deal in 10.5%, cleanly monotonic in
455	between. Switching the read on cuts deal-ins by about 13% over a few hundred
456	hands, takes hands from ~30 discards to ~38, and moves the draw rate from
457	almost nothing to about one hand in ten — which is what a table where people
458	stop feeding each other actually looks like.
459
460	What they know is still only what they can see. `unseenFor` counts the four
461	copies of each tile and subtracts the bot's own hand plus every discard and
462	exposed meld on the table. Neither module ever reads an opponent's concealed
463	tiles or looks at the wall.
464
465	There is no difficulty setting, and the reading stops at the discard: claims are
466	still judged purely on speed, and a bot will take a 碰 that walks it into
467	trouble.
468
469	向聽 lives in `src/game/shanten.ts`, apart from `hu.ts`, because the two want
470	different things: the win check has to be exact, and this one has to be fast —
471	it runs a few hundred times for every discard a bot considers. The tests hold
472	them against each other over random hands, since they work in completely
473	different ways and any disagreement is a bug in the fast one.
474
475	Undo behaves differently against the computer: rewinding into the middle of its
476	turn would only hand the move straight back to it, so one press goes back to
477	the last point you had a decision to make, computer replies and all.
478
479	## Saving
480
481	The game state is plain data, so a save is just its JSON in `localStorage`,
482	rewritten after every move. Close the lid, refresh, or run the battery flat and
483	nothing is lost — the lobby offers 繼續對局 Resume above 開始, showing the
484	round, hand number, chip counts and when it was saved. Which seats the computer
485	holds is part of the state, so a save comes back with the same hands in the
486	same places, and everybody who was on a phone scans back in exactly as they did
487	the first time — a room is opened around a resumed table the same way it is
488	opened around a fresh one, when somebody wants one. It is written by whichever
489	device is running the game (`engine.persist = authority`), so the mirrors on
490	everyone else's screens never fight it for the slot. A
491	save is validated before it is offered (four players, 144 tiles accounted for),
492	so a truncated or hand-edited one is ignored rather than loaded into a broken
493	table. `VERSION` in `save.ts` retires old saves if the state shape changes.
494
495	## Rules implemented
496
497	- 144 tiles (four of each suit/honour, eight flowers), 16-tile hands, dealer
498	draws the 17th.
499	- 補花 at the deal and on every drawn flower, replacements from the back of the
500	wall.
501	- 吃 only from 上家; 碰/槓/胡 from anyone. 明槓, 暗槓, 加槓, 搶槓, 槓上開花.
502	- 流局 when 16 tiles remain (`Rules.wallReserve`); the dealer keeps the deal on a
503	draw or on a dealer win (連莊), otherwise the deal passes and the round wind
504	advances every four passes. A full 四圈 game is 16 dealer passes.
505
506	### House rules (switches in `Rules`, all on the settings screen)
507
508	- 過水 `sacredDiscard` (on) — pass on a tile you could have won with and it
509	is dead to you until your own next draw; the seat shows a 過水 badge listing
510	what it is locked out of, so the missing 胡 button is never a mystery. A draw
511	from either end of the wall lifts it. `sacredClearedByClaim` (off) decides
512	whether taking a 吃 / 碰 / 槓 lifts it too — tables genuinely differ.
513	- 一炮多響 `multipleWinners` (off) — one discard pays out to every seat that
514	calls on it, each settled separately against the discarder. With it off the
515	tile goes to the caller nearest the discarder. Either way the table now waits
516	for other seats that can win before settling, so the nearest seat wins the
517	tile rather than the quickest hand — and the 摸牌 button still closes the
518	window on anyone dithering.
519	- 包牌 `liability` (on) — feeding the pung that completes a visible 大三元
520	or 大四喜 makes the feeder answer for the whole hand, in place of all three
521	payers. Only the seat that fed the last of those sets is on the hook, and
522	only if it came off a discard: a hand that assembled them itself, or closed
523	the set with a 暗槓, has nobody to blame.
524
525	### 台 scoring (`src/game/tai.ts`)
526
527	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 ·
528	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 ·
529	小四喜 8 · 大四喜 16 · 碰碰胡 4 · 混一色 4 · 清一色 8 · 字一色 16 ·
530	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 ·
531	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)
532
533	Ambiguous hands are decomposed every legal way and scored at the best reading.
534
535	Payment (`Game.settle`): one unit is `底 + 台 × 台值`
536	(`DEFAULT_RULES` = 底 3, 台 1, 100 chips each).
537	放槍一家付 — the discarder alone pays one unit; on 自摸 all three pay one unit
538	each. 拉莊 is billed to the dealer alone when the dealer is a payer, and added
539	to the whole hand when the dealer wins.
540
541	House rules vary a lot; the tai table and payments are plain data/functions in
542	`tai.ts` and `types.ts` if yours differ.
543
544	## Audio, by seat
545
546	A sound at a mahjong table comes from somewhere. You know whose 碰 that was
547	because it came from your left, and you know a tile has been thrown hard at the
548	far corner without watching it go. Both of those are cheap to keep, and both
549	are gone the moment everything comes out of the middle of one screen.
550
551	Every cue is a seat's. `SoundCue` carries a `seat` alongside the kind and
552	the tile — the seat that threw, called, or won — and leaves it off the things
553	that belong to the table rather than to anybody: the deal, a 流局, an undo. The
554	engine says whose only because a sound has to come from somewhere; what that
555	means in the room is entirely the UI's business, as it always was.
556
557	A seat becomes a place. `App.tsx` turns a seat into a *position round this
558	screen* — 0 the near edge, 1 the right, 2 across, 3 the left — because that is
559	what the ear wants and the seat number is not: a device holding one hand turns
560	the table so that hand is at the bottom, and the sound has to turn with it.
561	`game/sound.ts` gives each position a bus of its own: a pan, a gain, and a
562	lowpass that opens all the way for the near edge and closes to 5 kHz for the
563	far one. Distance is two things at once — quieter, and with the top taken off
564	— because between you and 對家 there is a metre of air, a wall of tiles, and
565	three people's arms, none of which carry treble. Tiles out in the middle skip
566	the edges entirely and are placed continuously, panned by where on the felt
567	they actually landed; the physics knows that to the pixel. Buses are built once
568	per spot and quantised, so a pool full of skidding tiles reuses a handful
569	rather than building one per contact. Old Safari has no `StereoPannerNode`; it
570	plays where it always did.
571
572	**Where there is a table and phones round it, the words come out of the
573	player.** The split the sound should take is the one the room already has: the
574	table makes the noise, because the tiles are on the table, and the phone in
575	somebody's hand says their calls, because the calls are theirs. So the table's
576	screen drops the voice for a seat while keeping the clack, and the phone
577	holding that seat says 碰 and names its own discards and makes no table noise
578	at all.
579
580	Only a phone claims that job, and that is the whole of how the split knows
581	where it applies. A phone is small enough to be in a hand and a hand is in the
582	room; a friend playing from their own laptop three cities away is not, and
583	never takes the words away from anybody's table. It is a proxy rather than a
584	fact — software cannot know who is sitting where — but it is the one property
585	that actually correlates with it, and it fails in the harmless direction:
586	the worst a wrong guess costs is a call said twice in two rooms.
587
588	That only works if the phone really is going to say it, so it is asked rather
589	than assumed. `sound.voiceLive()` is the honest answer — sound on, 報牌 on, the
590	audio hardware awake, the pack decoded — and every device reports it to the
591	host whenever it changes (`EV_VOICE`). The host works out, seat by seat, which
592	hands have a device that will speak for them and publishes that as `voices` in
593	room state; a nameplate wearing 🔊 next to its 📱 is a seat the table has gone
594	quiet for on purpose. Mute the phone, put it to sleep, hand the seat back —
595	the flag drops and the table takes the calls back mid-hand. The device the
596	table is being played at never counts, even when it holds seats: it is the
597	table, and a table that fell silent on the grounds that it was about to speak
598	would say nothing at all.
599
600	A browser will not start audio before the page has been touched, and a phone
601	that arrived by QR may never touch anything that asks for it by name — so the
602	first touch anywhere unlocks it, which is also the moment it can start
603	claiming the job.
604
605	## The voice pack
606
607	A mahjong table only ever says about fifty things — 42 tile names and a handful
608	of calls — so the whole vocabulary is rendered ahead of time into
609	`public/voice/` (~330 kB of mp3) rather than left to whatever speech synthesis
610	the browser happens to have. On Linux that is usually espeak-ng, which is
611	intelligible but sounds like a modem; and on a machine with no Chinese voice at
612	all the feature would silently do nothing. Shipping the audio makes playback
613	instant, identical everywhere, and lets a call be cut off mid-word when the
614	next one lands, all through the same Web Audio graph as the chimes.
615
616	`scripts/voice.mjs` is the authoring step, not part of `npm run build`. It needs
617	[piper](https://github.com/OHF-Voice/piper1-gpl) and ffmpeg on PATH:
618
619	```
620	node scripts/voice.mjs # → public/voice/*.mp3 + manifest.json
621	PIPER_MODEL=/path/to/voice.onnx node scripts/voice.mjs # a different voice
622	```
623
624	The wording lives in that script, and some of it is deliberately not the bare
625	tile character: 東 alone is a direction where 東風 is the tile, and the dragons
626	are 紅中 / 發財 / 白板 the way they are actually called — which also gives the
627	phonemiser enough context to get the tone right, since 中 on its own is as
628	likely to come out zhòng.
629
630	The clips here were rendered with piper's `zh_CN-huayan-medium`. If you
631	redistribute this app, check that voice's model card in
632	[piper-voices](https://huggingface.co/rhasspy/piper-voices) for the terms
633	attached to it, the same way you would the tile art below — or re-render the
634	pack with a voice whose terms suit you, which is a single command.
635
636	## Tile art
637
638	`public/tiles/tiles.svg` is the postmodern tileset from
639	[gnome-mahjongg](https://gitlab.gnome.org/GNOME/gnome-mahjongg), extracted from
640	the installed binary's GResource — a 43×2 sprite sheet (second row is the
641	highlighted variant, used for a lifted tile). `public/tiles/back.png` is the
642	blank tile from its smooth theme, tinted jade, since a solitaire game has no
643	face-down art of its own.
644
645	How a tile is drawn on the canvas — the outline round it and the shadow under
646	it — lives in `src/ui/tileArt.ts`, and `npm run dev` serves `/tiles.html`, a
647	page of every face at every angle over the felt with a slider on each of those
648	numbers and the finished block ready to paste back. None of it can be judged
649	from the values: it is 246 white against a dark green at 35 pixels across, and
650	the only way to know is to look at it and move something. (`tiles.html` is not
651	one of the build's entry points, so it does not ship.)
652
653	The sheet is not a picture of a tile's face. It is a picture of a whole tile,
654	drawn in an oblique projection, with its own sides painted down the left and
655	along the bottom and its own light coming from the top right — a 16px bevel on a
656	128px cell. That is fine while every tile stands the same way up, and wrong the
657	moment they are scattered: rotating the sprite rotates a pre-rendered solid, so
658	a pool of discards ends up with a different sun on each tile.
659
660	So the art is cropped back to the flat face, and the solid is worked out
661	instead. A tile is a box lying on cloth: take the four corners of the rotated
662	rectangle, offset them by the projection (`depthAngle`, down and to the left,
663	the same direction the art used), and fill the two or three quads whose edges
664	the base falls away from — each shaded from the direction that edge faces *on
665	screen*. Turn the tile and the same physical side comes round to face a
666	different way and its shading changes with it, which is the whole point. No 3D
667	engine, no z-buffer: one projection vector and a dot product per edge.
668
669	Each cell of the sheet is cut out into its own canvas the first time it is
670	wanted. `drawImage` with a source rectangle is allowed to sample past that
671	rectangle when it resamples, and the row under every face is the highlighted
672	variant, whose border is bright blue — so a tile at any angle other than a right
673	one wore a blue fringe along one edge. Square on it never showed, which is why
674	it looked like a rotation bug rather than a spritesheet one.
675
676	A tile used to be drawn as two passes — a slab of ivory offset down the screen
677	with the face on top, so a sliver of the tile's own side showed along the bottom
678	edge. At the size a discard is actually drawn that never read as thickness; it
679	read as a second, paler shape stuck to the bottom of every tile. Shading it
680	better did not help, because the problem was that it was there. What gives a
681	tile its weight now is the shadow and the outline, both of which are true at
682	any size.
683
684	That art is GPL-2.0-or-later. Fine for playing at home; if you ever
685	distribute this app, either honour the GPL or swap the two files for art of your
686	own — `SPRITE_COL` in `tiles.ts` is the only mapping that would need updating.
687
688	Known gaps and house rules that aren't implemented yet are listed in
689	[TODO.md](TODO.md).
690
691	## Layout
692
693	```
694	public/tiles/ sprite sheet + tile back
695	src/game/tiles.ts tile codes, wall, shuffle, sprite mapping
696	src/game/hu.ts hand decomposition, 聽 detection, wait shapes
697	src/game/shanten.ts 向聽 / 進張 counting, for the computer players
698	src/game/bot.ts what a computer player discards and claims (pure)
699	src/game/danger.ts reading the table: who looks ready, which tiles are hot
700	src/game/autoplay.ts when it does it, and how long it appears to think
701	src/game/tai.ts 台 scoring
702	src/game/engine.ts state machine: deal, turns, claim resolution, settlement
703	src/game/ctl.ts TableCtl — the surface the UI drives, local or networked
704	src/game/wall.ts what is left of the square, and what still blocks a throw
705	src/net/room.ts the wire itself: join, reconnect, state, events
706	src/net/session.ts the room: host duties, seats, intents, migration
707	src/net/table.ts TableCtl over the wire: mirror + intents + hand overlay
708	src/net/protocol.ts what crosses the wire, in one place
709	src/net/handOrder.ts your own arrangement, kept on your own device
710	src/ui/Controller.tsx a hand held on a phone, plus the seat picker for any device
711	server/rooms.ts the relay: rooms, host election, state fan-out
712	server/index.ts production: dist/ and the relay from one port
713	src/table/physics.ts the discard pool: rectangles with weight, pure and testable
714	src/table/geometry.ts the table measured — colliders, launch points, throw gate
715	src/table/pool.ts game state ⇄ tiles in the middle, by diffing the discards
716	src/ui/ TileView, Hand (drag and throw), Seat, Center, Pool, Help
717	```
718
719	## The tiles in the middle
720
721	The pool is a small rigid-body solver (`src/table/physics.ts`) that knows nothing
722	about mahjong, the DOM, or the clock: fixed 1/120s steps, no randomness of its
723	own, and everything in table pixels. Given the same throws it produces the same
724	pile every time, which is what lets a refresh mid-hand come back to the pile you
725	had rather than a freshly scattered one — positions are derived from the hand
726	number and each tile's place in its thrower's discards, never saved.
727
728	The pile is handled directly. Press on a discard and it comes up out of the
729	pile, carried over the top of the others rather than ploughing through them;
730	let go slowly and it is set down there, flick and it goes off across the felt at
731	the speed it left your fingers — the same throw a tile gets out of a hand, made
732	from the middle of the table instead. Pressing on bare felt does nothing, and
733	the press passes through to whatever was really under it: the tiles are drawn on
734	a canvas with no elements to hit, so the hit test is against the physics and the
735	listener is a capturing one on the table itself.
736
737	A flick is the whole of a throw: the direction and the speed it leaves at are
738	the hand's, and nothing between the fingers and the felt changes either. The one
739	thing the table decides is height — with a standing wall in front of the
740	throw a flat tile would only skid into it, so it is lofted over instead, exactly
741	as far as it was thrown and simply in the air for the first part of the way. A
742	tile that carries past the square and out into the open stays there. (It used to
743	be re-aimed at the middle of the square when the flick's line missed it, which
744	put the tile on a line nobody had thrown and read, fairly, as a bounce off
745	nothing.)
746
747	What stops a tile is other tiles. There is no fence: the standing stacks of the
748	wall, the sets people have laid down and the sixteen each of them is holding are
749	all measured as colliders, so the pile is held in by the ring of tiles round it
750	exactly as it is on a real table, and spills through whatever gaps the wall has
751	been eaten into. The table's own edge is a backstop at the screen edge, there
752	only so a tile that got past all of that isn't lost.
753
754	A tile that only ever slid would look like a puck. What makes a pool of discards
755	look like one is that every tile in it has come to rest at some angle nobody
756	chose, and that is a thing to earn rather than to scatter in. A tile turns
757	because of where it was hit.
758
759	So a contact happens somewhere in particular. The separating axis test says which
760	way to push and by how much; `contactOf` says where, and the arm from there to
761	the tile's middle is what a turn is made of. A tile shouldered square in the back
762	is pushed along and does not turn at all. The same shove, moved along until it is
763	catching one end of the tile, turns it instead, and which way round follows from
764	which end. What touches is a footprint and not a point: a tile lying square to a
765	wall meets it along the whole of one side and is pushed through its own middle
766	however far along the wall it is, and the same tile at forty-five degrees meets
767	it on one corner and swings on round it. Everything a tile can hit is another
768	tile, so the standing stacks, the laid-down sets and the table's own straight
769	edge are all the same contact with the same friction across the face of it.
770
771	Two things keep that honest. The first is that a tile is not free to pivot. It is
772	lying flat on cloth, and the cloth under the whole of its face resists the turn
773	for as long as the contact lasts, so only a tenth of the moment a knock would
774	give a free rectangle actually reaches it (`FELT_HOLD`). Without that, a tile off
775	the corner of a stack comes away spinning like a top. The second is that the felt
776	is one patch of cloth doing one thing: what it spends stopping the slide it has
777	not got left for the turn, so the two share a single friction budget rather than
778	running as two brakes side by side. What that buys is the ending. A tile
779	flicked hard with a little turn on it used to stop turning early and skate on
780	looking dead, and one barely pushed with a lot of turn on it stopped dead and
781	went on spinning where it lay. They now run out together.
782
783	The turn a tile arrives with is the flick's own. `ui/flick.ts` reads a release
784	the same way wherever it was made, out of a hand or off the pile in the middle,
785	and the spin it reports is the angle the stroke swung through between its first
786	half and its second, over the time it took. That is a rate, in rad/s, which is
787	what a wrist actually does: a straight flick turns nothing however hard it is
788	thrown, and the same curve made twice as fast puts twice the turn on the tile.
789
790	It is drawn on a canvas that spans the whole table, not just the centre.
791	That is deliberate: `.slot` and `.seat` both clip their own contents, which is
792	why a tile can't be animated out of a strip as an element. On the canvas there is
793	nothing to clip it, so a tile lifted out of a hand crosses the table in one
794	piece.
795
796	The square is the whole wall: eighteen stacks of two a side, four sides, 144
797	tiles, the way it is built on a table. It used to be rebuilt to what was left
798	after a hand had been dealt — barely half of it — because eighteen full-size
799	stacks a side wants about 560px and no ordinary window has that between the top
800	and bottom strips. That bought a square at the price of it not being the wall:
801	a side ran out of stacks before it reached its corner, so the four of them never
802	met.
803
804	Nothing gives now, and in particular the tile does not. Every tile on the table
805	is one size — `--tile-w` in styles.css, in a hand, in the wall, and lying in the
806	middle — because they are the same tiles, and a tile that changed size between
807	the wall and your hand read as a different, smaller set sitting in the middle.
808	The square is built from that tile and is however big that comes to:
809	`wallSquare()` in `game/wall.ts` takes the tile and hands back the lengths, and
810	`WallRing` measures one rendered cell to find out what CSS made it. The wall
811	being bigger than the room between the strips is a fact about a 144-tile wall,
812	not a thing to solve by shrinking it.
813
814	The four walls are placed by CSS alone: each is pinned to its own edge of the
815	opening with `left`/`top`, runs its whole length from the corner it is built
816	from, and so carries one wall-depth past the far corner and over the end of the
817	next wall along. Every corner of the square is covered by exactly one wall and
818	no two overlap — that is the pinwheel. Nothing is rotated; a square wants no
819	rotation to describe it.
820
821	The overhang is exactly one wall-depth, which is what it takes to close a right
822	angle: two bands of thickness `t` meeting at an interior angle ψ overlap by
823	`t / tan(ψ/2)`, and at ninety degrees that is `t`. So the whole square measures a
824	wall's length plus one depth across, and leaves that length less one depth in
825	the middle — which is the opening, and which is what `.wall-ring` itself is.
826
827	The CSS only places what it is told: `.wall-side` is a row of stacks, laid across
828	on the two flat walls and down on the two upright ones, and `--wall-len` from
829	`WallRing` is the only length it is given. Every place in the wall is rendered
830	whether or not there is still a stack standing on it, because with the break
831	anywhere but a corner a wall is eaten from its middle — a row that closed the
832	gap up would drag the whole tail of the wall along the table behind it.
833
834	Where the four walls stand is found at the table itself rather than on a page
835	of sliders: take hold of a wall in a real game and push it. It goes where the
836	finger goes, and lets go saying where it landed — `[wall] top pushed to 0.5,-4.9
837	— top 0.5,-4.9 · right 0,0 · …` in the console, in tiles, which is the form
838	`WALL_PLACED` in `game/wall.ts` is written in, so what a push finds pastes
839	straight back in. Nothing about the hand moves: the wall is the same wall in the
840	same order and the next draw is still the next draw. There used to be a
841	`/wall.html` for this; the table is the honest place to do it, so that page is
842	gone.
843
844	## 擲骰 — where the wall is broken
845
846	The dealer throws three dice to open a hand, and the total does two jobs. It
847	counts round the seats — the dealer being one, and the count going the way the
848	turn goes — to pick whose wall is opened. Then the same number counts stacks in
849	from the right-hand end of that wall, and the break is behind them: the first
850	tile drawn is the next one along, and the drawing runs away to the left from
851	there, on round the square.
852
853	The square makes that cheap to say. Every wall is laid down from the corner its
854	own player's right hand falls on — that is what the pinwheel is — so the run of
855	72 places already starts at each seat's right, and the break is simply so many
856	places into the side the dice picked. `breakAt()` is those two lines.
857
858	Nothing about the tiles turns on it: the wall was shuffled before it was built,
859	so which stack is drawn first is decided either way. What it moves is where in
860	the middle of the table the gap opens and who has to reach furthest for the next
861	draw — which is the whole of what it does at a table too. The throw is made from
862	the same seeded rng that shuffled, so a seed replays a hand's gap as well as its
863	tiles.
864
865	Where the wall opened is thrown for once and then lived with, which is right at
866	a table and awkward to look at: half the question about the square is what it
867	looks like opened at each of the seventy-two places it can be opened at.
868	`game.reroll(1, 1, 1)` through `game.reroll(6, 6, 6)` from the console walks the
869	break all the way round, and `game.reroll()` throws afresh. It moves the gap and
870	nothing else: the tiles were shuffled before the wall was built, so nobody's
871	hand changes and the draw order is the draw order.
872	Nothing about the square is calculated twice. Its size lives entirely in CSS
873	(`--ws`, `--wd`), so `geometry.ts` measures it instead of restating that
874	arithmetic: `WallRing` already renders every stack as a real element tagged with
875	its index, and the colliders are those elements' bounding rects. That one
876	mapping is what makes the wall a physical object — the thing a thrown tile
877	bounces off, and the thing the throw gate casts its ray at.
878
879	### Seeing the colliders
880
881	Because every boundary is measured off the DOM rather than written down, the
882	only honest way to check a bounce is to draw the boxes in the coordinate space
883	the tiles are drawn in. Add `?colliders` to the address, or call `colliders()`
884	from the console while a hand is running:
885
886	\| drawn \| is \|
887	\| --- \| --- \|
888	\| solid rectangle at the screen edge \| the table itself. Nothing meets this in play — it is only there so a tile that got past everybody's tiles is still on the table afterwards \|
889	\| solid box on every tile \| a collider. Every tile on the table is one: the stacks still standing, the sets people have laid down, and the sixteen each of them is holding. Half stacks aren't drawn because they aren't colliders — a tile goes over them \|
890	\| dashed rectangle \| the wall square's opening — where a throw is aimed. It stops nothing, which is why the pile spills out of it \|
891	\| dotted rectangle, inset \| where the moving tile's own centre may be against the table's edge, which is its half width in from it \|
892	\| box hugging each pool tile \| the rotated rectangle it really collides with \|
893	\| dotted box round the moving tile \| the upright box the straight edges use instead. It comes apart from the rotated one as the tile turns \|
894
895	\| blue rays from a throw \| faint: the flick, as the tile was let go of. Bright: the line it was actually sent along, labelled `slide` or `lob`. These should lie on top of each other — the flick is the throw, and a tile leaving on a line nobody threw reads exactly like a bounce off something invisible \|
896	\| yellow cross \| where contact was actually made, fading over a second or so. One of these sitting in open felt with no box under it is a bounce off something that isn't a collider at all — a different bug from a box in the wrong place \|
897
898	Colliders mode is also a throwing sandbox: a flick throws for real but is not a
899	discard, so the tile stays in your hand, the turn never passes and the computer
900	players hold still. Throw the same tile at the same corner as many times as you
901	like. Sixty stay on the table before the oldest is swept off; `pool.clearLoose()`
902	sweeps them all. Tapping a tile to discard does nothing in this mode — throwing
903	is the thing being tested.
904
905	Add `pause=1` (`?colliders&pause=1`, or `pause()` from the console) to **stop
906	dead on contact**. The physics halts on the fixed step that resolved it and
907	drops the rest of the frame, so what's on screen is where the tile touched
908	rather than where it had got to by the end of the frame. Space carries on, →
909	takes one step, ↑ takes ten; the console says what it stopped on.
910
911	`colliders(false)` turns it off again. It repaints on toggle, so a settled table
912	doesn't have to be poked first. In a dev build `pool` is the live pool: the
913	overlay draws `pool.world.walls`, the physics' own copy rather than a fresh
914	measurement, so if what's drawn ever disagrees with the DOM it is the world that
915	went stale — which `pool.measuredAt` will show, since the stamp carries every
916	hand's shape.
917
918	## What is left of the wall
919
920	Four walls of eighteen full-size stacks want about 560px a side. No ordinary
921	window has that between the top and bottom strips, and the tile is not allowed
922	to shrink to make it fit — a tile in the wall is the same tile you play with, so
923	drawing it smaller was a visible lie.
924
925	What saves it is that nobody ever sees a whole wall: four hands come off the
926	front before the table is on screen, so barely half of it is left by then. So
927	only the stacks still standing are drawn, and they are laid out around a ring
928	cut to the middle rather than to eighteen — `ringLayout` in `game/wall.ts`
929	measures how many stacks a side can take (`WallRing` renders one hidden cell and
930	asks CSS how big it came out) and shares what is left over the four sides in
931	proportion. Every seat keeps a wall in front of it instead of the remainder
932	piling into one corner, and the order runs all the way round, so the break point
933	is still at the head of it and the 底牌 tail still at the end.
934
935	Shuffling the wall along as it is eaten costs nothing to look at, because every
936	stack shows the same green back — and it is what people do to a half-eaten wall
937	anyway.
938
939	`pool.ts` never listens for events. It diffs the discards against what it is
940	already drawing, keyed by `seat:index` — and those keys never shift, because
941	discards are only ever pushed and a claim only ever pops the one on top. So a
942	key that appeared is a tile to throw in and a key that went is a tile to take
943	off, which makes undo, 下一局, a claim and resuming a saved game all the same
944	code path. The engine has no idea any of it exists, and the save format did not
945	change.