collin/mahjong

2317d367e8f3d4971d4fb5809b0bd7e6ec2c9c6d / 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, because there is one game. It deals
11	you in at the bottom edge with the computer on the other three, on this device,
12	without a word said to any server. Everything people call a mode is that table
13	with different hands changing hands, one at a time, mid-game:
14
15	- somebody sits down at this screen and **taps a hand the computer has been
16	playing** — the tiles turn over and are theirs;
17	- somebody scans a seat's QR and takes that hand onto their phone;
18	- somebody opens the invite link from three cities away and takes one from
19	their own laptop, with the table turned so their seat is the near edge.
20
21	Each of those swaps one seat and redeals nothing, and each undoes: a hand goes
22	back to the computer with a tap, and one that loses its device gets given back
23	to the computer on its own so the table never stops. See
24	[one table](#one-table), [computer players](#computer-players) and
25	[playing online](#playing-online).
26
27	```
28	npm install
29	npm run dev # then open the browser full-screen (F11)
30	npm test # rules engine, the bots, and 200 randomly-played hands
31	```
32
33	## Screen layout
34
35	```
36	┌──── 對家 (rotated 180°) ────┐
37	上家 (rotated 90°) │ wall, and the discard pool │ 下家 (rotated −90°)
38	└──── 自家 (upright, near you) ┘
39	```
40
41	Seat 0 is the bottom edge, and play runs 0 → 1 (right) → 2 (top) → 3 (left),
42	which is the normal counter-clockwise 東南西北 order.
43
44	Each strip shows, from the player's edge inwards: nameplate (seat wind, 莊
45	marker, 聽 badge, chip count) → concealed hand → action buttons → melds and
46	flowers. Discards are not in the strips: they are thrown into the middle and
47	stay there for the hand, the way they would be on a table. (A phone has no
48	middle to throw into, so the compact layout keeps a discard row per seat — see
49	[on a phone](#on-a-phone).)
50
51	## Playing
52
53	- Your hand is face down at a shared screen, and a tap turns it over. It
54	covers itself again when the hand is dealt and when your turn ends — the two
55	moments the tiles stop being needed, and without them a hand turned over once
56	stays turned over all evening, which is the same as never having covered it.
57	蓋牌 beside 理牌 puts it back sooner. While it is covered the only button the
58	strip has is 看牌: every other one names a tile — 打出五萬, 吃 with these two
59	— and would say through the card what is under it. The 看牌 of whoever the
60	table is waiting on wears a gold ring, since whose go it is was never a
61	secret. Nothing is covered where the screen is playing one hand, which is a
62	screen that belongs to one player — three computers and you, or your own
63	tiles on your own device.
64	- The tile you just drew is held apart from the sorted hand with a gold ring
65	and a 摸 label, instead of being sorted invisibly into it. It merges into the
66	hand once you discard. 補花 replacements and kong replacements are marked the
67	same way.
68	- The wall is drawn in the centre as a square of four staggered sides, laid
69	out like a `#` the way it is built on a real table — every corner covered by
70	one wall carrying past the end of the next. All 144 tiles, at the size of the
71	tiles in your hand, because they are the same tiles. The gold stack is the
72	break point you draw from, and where that is depends on what the dealer threw:
73	stacks shrink to a single tile and then vanish as they are used, so the gap
74	opens wherever the dice opened the wall and grows from there. The 16-tile 底牌
75	tail at the other end is where kong and flower replacements come from, so the
76	square is eaten from both directions at once. See
77	[擲骰](#擲骰--where-the-wall-is-broken).
78	- Discard — drag a tile out of your row and it comes up out of your hand and
79	follows your finger anywhere on the table; let go and it goes in, from where
80	you let go and at the speed you let go at. Dragging along the row is still
81	arranging your hand — which of the two you meant is decided once, from whether
82	the movement is mostly along the row or out of it, and then it sticks. Let go
83	back over your own hand and the tile goes back. Tapping a tile twice, or the
84	打出 button, still works and lobs it in for you.
85	- The discard pool — every tile thrown lands in the middle and stays there
86	until the hand ends, with real weight: it skids, turns, knocks the tiles
87	already there out of its way, and comes to rest against them. Nothing is ever
88	stacked on anything — tiles are solved as rectangles, so two lying at an angle
89	lean on each other rather than overlapping. The tile still to be claimed is
90	the lit one.
91	- Over the wall or across the table — which of the two ways a tile goes in is
92	not a setting, it is whatever the wall allows. At the start of a hand the
93	square is a solid barrier and a discard has to be lobbed over it, so the
94	flick's speed decides how far across the pool it lands. As the wall is eaten
95	away, gaps open in front of one seat and then another, and from then on that
96	seat can slide a tile in instead — flat across the cloth at exactly the speed
97	it was flicked. Which it will be is a ray cast at the stacks actually standing
98	there, so a gap off to one side counts, and a stack worn down to a single tile
99	is low enough to go over. A seat whose wall is still up says 牆未開 on its
100	nameplate, so a lob is never a mystery.
101	- Throw it badly and it ends up in somebody's tiles — the middle stops
102	exactly where their hand starts, so a tile that comes off the pool arrives in
103	their lap, and one that falls short lands out by their seat. Either way their
104	row takes the knock and rocks back, and with 報牌 on they tell you about it —
105	喂，小心點 if you are lucky, 是在哈囉 or 你牌品很差欸 if you are not, and
106	never the same line twice running. How hard it got there only decides how
107	loudly. Never your own edge: you cannot be barged by your own discard.
108	- Arranging — drag any tile in your hand to reorder it; 理牌 sorts it back
109	into suit order. Hands are never auto-sorted after the deal, so an arrangement
110	you set up survives draws, claims and kongs.
111	- Rules — the `?` on your nameplate opens a bilingual rules panel anchored to
112	your own edge of the table and rotated to face you.
113	- Claiming — after a discard, every seat that can claim gets 胡 / 槓 / 碰 /
114	吃 / 過 buttons on their own strip, and they resolve by priority (胡 > 槓/碰 >
115	吃; ties go to the player nearest the discarder in turn order).
116	The table never blocks on a claim. The next player gets a 摸牌 button and
117	may draw whenever they like, which shuts the window on anyone who hasn't
118	called — exactly like shouting 碰 before the next player picks up. A claim
119	already declared still stands, so calling in time always wins the tile. If
120	everyone answers first, play advances on its own without the extra tap.
121	The seat that draws next gets no separate 過 button, because for them the two
122	are the same move: picking up is how you decline. Their button says 摸牌
123	either way; what they would be giving up by pressing it is the 碰 or the 吃
124	sitting next to it on the bar, so it does not have to be spelled out on the
125	tile as well. Wanting to decline but leave the window open for everyone else
126	is just waiting, which is what not pressing anything already does. (On a
127	phone the button still reads 過．摸牌 — a phone's bar is a row with room to
128	write on, and the hand it belongs to is not on the same screen as the table.)
129	- On your turn — 自摸, 暗槓 and 加槓 appear automatically when legal.
130	加槓 offers everyone else a 搶槓 chance.
131	- Where the buttons go — on the same row as your tiles, at the right-hand end
132	of it, where the tile you are about to throw already is. The bar is out of the
133	strip's flow, so a button arriving moves nothing: 打出 appearing when you
134	lift a tile used to shove the melds and flowers above it up the strip. Every
135	button on the bar is given one height for the same reason. A bar with more on
136	it than the room beside the hand stacks up the strip in tile-wide lines, most
137	urgent call at the bottom, rather than running off the end of it. And the room
138	for the first line is in the tile size itself: a strip is sized to hold sixteen
139	tiles, the slot it keeps for the tile it is about to draw, and one button —
140	which is why the tiles down the sides of a short window are the size they are.
141	- What the buttons say — the call is written down the tile the way the
142	character on a tile's face is, one on top of the next in the middle of the
143	face, with the English along the bottom edge. Two characters stacked are the
144	size a tile's own character is; two side by side had to be shrunk to fit
145	across it. A call too long for the height (電腦代打) takes a second column to
146	the left of the first, which is where the next column of vertical Chinese
147	goes. 打出 names no tile: the one it would throw is lifted out of the row
148	beside it wearing a gold ring, so saying 五萬 on the button as well only
149	crowds the face.
150	- Whose go it is — a screen with four people round it is the one place a
151	game can go quiet without anybody noticing: three of them are watching the
152	table and the fourth is looking at their phone. So the table says it loudly.
153	The strip of the seat being waited on lights from its inner edge, their name
154	goes gold at the far end of it, and seven seconds of gold drain along the
155	edge that faces the middle — from both ends towards the centre, because two
156	of the four strips are upside down from wherever you are sitting. **Nothing
157	happens when it reaches zero.** No tile is thrown for you and no seat is
158	skipped; the line the bar leaves behind breathes, and the table goes on
159	waiting. It is a nudge, not a rule. Said on a covered hand too — whose turn
160	it is was never one of the things the card is hiding — and only where the
161	screen is playing more than one hand, since a screen playing one has nobody
162	to tell. A phone at the table gets the same seven seconds
163	across its top, a gold rim edge to edge (red for a claim window), and one
164	short buzz as the game comes round to it.
165	- On the keys — ← and → walk the selection along your hand and space throws
166	the tile that is lifted. How long you hold space is how hard it goes: the
167	tile draws back out of the hand while the key is down and leaves when you let
168	go, so a tap slides it in and a long hold sends it across the square. Escape
169	puts it back. The wall still decides whether it can be slid or has to be
170	lobbed, exactly as with a finger.
171	- When a hand ends — a big arrow drops onto the winner's edge of the table.
172	It lives inside their strip, so it turns with the seat and lands on the right
173	person wherever they are sitting, and 一炮多響 lights up every seat that
174	called. A 流局 has no winner to point at, so it turns the hands over instead:
175	who was 聽牌 and the tiles they were waiting on, the same reveal as at a real
176	table.
177	- The gear — beside the `?` on every nameplate, and it opens that player's
178	own copy of the table's controls: undo, sound, full screen, 設定. Anchored in
179	their strip and turned to face their edge, like the rules panel, so what you
180	open reads the right way up to you and to nobody else. These used to be a bar
181	in the middle of the table, which is the one place that belongs to everybody
182	— the pile lands there and it is upside down to two of the four.
183	- Undo — 復原 in that panel takes back the last action, naming what it will
184	undo (`復原玩家 2 打五萬`). It is in all four panels rather than only the
185	seat that made the mistake, because whoever spots the mis-tap should be able
186	to reach it. Twenty actions deep, cleared when the next hand is dealt.
187	Snapshots live outside the saved state, so an undo does not survive a
188	refresh: what you can take back is what happened while everyone was still
189	watching it happen.
190	- Sound — on by default, muted from the 🔊 button behind any seat's gear, or
191	from settings. A dry clack as a tile goes down, and a distinct
192	two-note chime when a claim window opens, which is how a slow player notices
193	their 碰 is available before the next player draws it shut. Everything is
194	synthesised with a few oscillators (`src/game/sound.ts`) rather than sampled,
195	so there is nothing to load and the cue lands on the same frame as the tile.
196	Browsers refuse to start audio before the page has been clicked, so the first
197	sound anyone hears is the deal — triggered by the button that starts it.
198	- 報牌 (voice) — a second switch under sound calls the game out loud: 碰,
199	吃, 槓, 胡了, 自摸, and the name of every tile as it is discarded — 三條,
200	五萬, 東風. A flower says 補花 and then which one. A new call cuts off one
201	still being spoken; at table pace the newest is the only one that matters.
202	See [the voice pack](#the-voice-pack) for where the audio comes from.
203	- 方位音 (sound by seat) — a third switch, on by default. Every noise comes
204	out of the edge its seat is sitting at: 上家 to your left, 下家 to your right,
205	and 對家 across the table, where it is quieter and has the top taken off it —
206	a metre of air and three people's arms do not carry 12 kHz. So you can tell
207	whose 碰 that was without looking up, which is the whole point of hearing it.
208	Tiles out in the middle are placed properly rather than by edge: the physics
209	knows to the pixel where a tile landed, and that is where the knock comes
210	from. A device holding one hand turns the table so that hand is at the
211	bottom, and the sound turns with it. See [audio, by seat](#audio-by-seat).
212	- Settings — 設定 from the lobby, or from the game-over panel: player names,
213	底 / 台 / starting chips, the house-rule switches below, and sound. Kept in
214	`localStorage` separately from the save, so they carry over to the next game.
215	Stakes are locked once a game is under way — they'd otherwise rewrite chips
216	already won.
217
218	## One table
219
220	There were three buttons on this lobby — 單人對局, 四人同桌, 線上對戰 — and
221	they were never three games. Every party table played its unclaimed seats
222	hotseat-style at the screen already; every hotseat table would have taken a
223	phone if it had had a room to put it in; every online table filled its empty
224	chairs with the same computer players the solo game is made of. They differed
225	in who was holding which hand, and that is not something a lobby should have to
226	know before the tiles are dealt.
227
228	So there is one button, and the question it used to ask is answered
229	continuously instead. A hand is played by one of three things — **the computer,
230	somebody at this screen, or a device of its own** — and any of them becomes any
231	other while the hand is in progress, with nothing redealt and nobody consulted
232	but the person doing it.
233
234	\| \| played by \| becomes a person here \| becomes a phone \|
235	\|---\|---\|---\|---\|
236	\| 電腦 \| the bots in `autoplay.ts` \| tap the strip — 坐下 \| its QR, from its gear \|
237	\| 這裡 \| this screen, face down under a tap \| — \| its QR, from its gear \|
238	\| 手機 \| whoever scanned in \| they leave → the computer \| — \|
239
240	Nothing touches the network until a phone is wanted. A table of people and
241	computers is four hands and an engine; there is nothing for a server to do, and
242	`src/net/` is never constructed. The room is opened by the first thing that
243	needs one — the 用手機拿牌 in a seat's gear — and when it opens it *publishes
244	the hand in progress* rather than dealing over the top of it
245	(`NetSession.publishTable`). The engine is the page's own either way:
246	`NetTable` is a wrapper that goes on around a live `Game`, not a second one, so
247	opening a room costs a `structuredClone` and changes nothing on screen except
248	that the QRs start working.
249
250	Each seat's QR is its own, and lives in that seat's gear beside the sound
251	and the undo, carrying `?room=…&seat=N`. Scanning it lands on that chair
252	without anybody having to say which one they are in: the hand leaves the shared
253	screen for good — face down there, not under a card that lifts — and the phone
254	gets it face up with the claim buttons and the throw. There is a QR for the
255	whole table in the middle of the felt as well, but only while the table is
256	still being sat down at; the moment the first tile is thrown it steps aside,
257	because from then on the middle is where the tiles go.
258
259	**A hand that loses its device is kept for fifteen seconds and then given to
260	the computer** (`EMPTY_SEAT_GRACE`). A phone locking itself looks exactly like
261	a phone leaving for good and only one of them means it, so the nameplate shows
262	⏳ and the table waits; when the wait is up the computer picks the tiles up and
263	play carries on. Nothing is lost either way — coming back takes the seat
264	straight off the computer again, and so does one tap on it at the table. The
265	same countdown covers the screen closing its lid: the room records which device
266	the table is being played at (`screen` in room state), and hands with neither a
267	device nor a table behind them go the same way.
268
269	The save follows the table. Whichever device is running the game writes
270	`localStorage` (`engine.persist = authority`; every mirror has it off, and no
271	business writing it), so 繼續對局 restores here and the room, if one is wanted
272	again, opens behind it exactly as it did the first time.
273
274	Covering is arithmetic, not a mode. A hand starts face down when this
275	device is playing more than one of them — which is the definition of a screen
276	several people are sitting round. One hand is one person, whether they are
277	playing three computers or holding their own tiles on their own phone, and
278	covering your own tiles from yourself is a tap in the way.
279
280	## Playing online
281
282	Every device that is not the table is on one room relay of our own:
283	`server/rooms.ts`,
284	a couple hundred lines of Node on `ws`. The relay knows nothing about mahjong:
285	rooms with codes, host election by join order, one bag of shared state only the
286	host may write, and events fanned out to everyone else. One client — the
287	host — runs the engine, publishes the whole `GameState` as room state after
288	every move, and plays intents the other devices send as events. The engine was
289	already a pure JSON state machine, so nothing in `src/game/` knows the network
290	exists — the same trick `autoplay.ts` plays, stretched over a wire. The client
291	end of the wire is `src/net/room.ts`; the rest of `src/net/` is the game
292	riding it.
293
294	- A device that holds one hand shows that hand. A phone shows just the
295	hand, face up, with the claim buttons — and the throw: flick a tile up off
296	the top of the phone and it sails in from your edge of the common screen, at
297	the speed and angle you let go of it (`NetThrow`, mapped into table
298	coordinates by `TablePool.throwFromNet`). Anything bigger shows the whole
299	table instead, turned so that seat is the bottom edge, with everyone else's
300	tiles face down. Same room, same seat map; the only thing deciding between
301	them is how much screen there is.
302	- A device that holds no hand yet gets the seat picker, and every seat is
303	offered — including the ones the computer is playing, which is the ordinary
304	way in, and the ones somebody already holds, which is how two people play one
305	hand off two devices. A seat's own QR names its chair and skips the picker.
306	- The device the table was opened on goes on showing the table: the felt,
307	the middle, the wall, and every hand no phone has taken, covered-with-a-tap
308	like a hotseat game. Hold the 📱 tag on a nameplate to show the table's QR
309	again for a player who lost theirs. With sound on, the phone in each player's
310	hand is what says their calls and the table goes quiet for that seat — see
311	[audio, by seat](#audio-by-seat).
312
313	Design notes, in the order they bit:
314
315	- Seats are claims on a shared map, `seats: {ids, name}[]` in room state,
316	arbitrated by the host. Your own hand's arrangement never goes over the wire
317	— the state carries a multiset and each device keeps its own order
318	(`src/net/handOrder.ts`), because a round-trip inside a drag gesture is lag
319	you can feel.
320	- The host can change. The relay re-elects — the earliest joiner still
321	connected — when the host leaves; every other client already mirrors the
322	full state, so the new host starts its engine from what it was just watching
323	and the game carries on.
324	- A hidden host must keep hosting. Browsers suspend requestAnimationFrame
325	and throttle timers in background tabs, so nothing in `src/net/` runs on a
326	loop: state writes flush on a microtask, everything inbound arrives over the
327	WebSocket (whose delivery is not throttled), and dead connections are the
328	server's ping loop's problem. A table screen somebody tabbed away from
329	keeps answering.
330	- Identity is a UUID in sessionStorage, chosen by the device and taken at
331	its word by the relay. The seat map is keyed on it, so a reload or a wifi
332	blip walks back into its own seat. No auth — this is a home server for one
333	table of friends.
334	- Fairness is social, not cryptographic. Room state carries the whole
335	game, wall and hands included — a friend with devtools open can cheat.
336	Moving the engine into the relay would fix that; the server is ours now, so
337	only the work stands in the way (TODO).
338
339	### Running it
340
341	```
342	npm run dev # vite, with the relay on /ws of the same origin
343	npm run build && npm run serve # production: dist/ + relay, one process
344	```
345
346	The relay always lives at `/ws` on the page's own origin. In development
347	`vite.config.ts` attaches it to vite's server, so `npm run dev` is the whole
348	stack; in production `server/index.ts` serves the built `dist/` and the relay
349	from one port (`PORT`, default 8080), listening on the LAN by default so
350	phones on the same wifi can scan straight in. Anything further away wants TLS
351	in front — a reverse proxy with a certificate — since phones only get camera
352	and fullscreen on https.
353
354	Rooms live in memory. A restart drops them; the next visitor on an old link
355	recreates the room empty, and a host mid-game republishes its state on the
356	next move. Joining a room writes `?room=` back into the address bar, so the
357	page URL — the host's included — is the invite link.
358
359	A QR pointing at `localhost` is a QR only the host's machine can scan, so the
360	server always gives the invite links a public face: `PUBLIC_URL` if set, else
361	it runs its own rsgrok tunnel (the house ngrok replacement) — spawned
362	the moment the server knows its port, the https URL read off the tunnel-up
363	line, no `:4040` inspection API (another agent may own that port). Every QR
364	and invite link wears that origin instead of the page's. If the tunnel dies,
365	or `rsgrok` is not on the PATH (`RSGROK_BIN` points elsewhere), links fall
366	back to the page's own address and the tunnel is retried on later joins.
367
368	The tunnel always asks for the same subdomain — `mahjong-table`, or whatever
369	`TUNNEL_NAME` says — so restarts land back on the URL people already have
370	instead of burning a fresh name each run. A name someone else already holds
371	kills that first attempt before it prints a URL; the retry then goes out
372	nameless and takes whatever it is given.
373
374	## On a phone
375
376	The table assumes four people sitting around a screen lying flat, and about
377	770px in both directions before the middle is worth looking at. A phone has
378	neither, so under `(max-height: 620px), (max-width: 820px)` it switches to the
379	compact layout: nothing is rotated, everything reads upright for the one person
380	holding it, the other three seats shrink to cards showing what is public about
381	them, and the depth that frees up goes to your hand, which is allowed to wrap.
382
383	There is no centre square there — the middle collapses to a one-line bar showing
384	what the wall square was telling you anyway — so there is nowhere to throw a tile
385	to. The compact layout therefore keeps a discard row per seat, and discarding
386	is tap-twice or 打出, exactly as it was. The pool and the throw are a full-size
387	feature.
388
389	The breakpoint is written down once, in `COMPACT_QUERY` in `src/ui/compact.ts`,
390	which sets a `compact` class on `<html>` for the CSS to key off.
391
392	## Computer players
393
394	Single player seats you at the bottom and gives seats 1-3 to the computer.
395	Their tiles go face down and their 聽 / 過水 badges come off, since both would
396	give the hand away; everything public — the pool, melds, flowers, chips — stays
397	exactly as it is, and everyone turns their hand over when the hand ends. They
398	throw their tiles in like anyone else, and under the same rule: a computer seat
399	whose wall is open slides them across, and one still walled in lobs them over.
400
401	The engine does not know the bots exist. `AutoPlay` (`src/game/autoplay.ts`)
402	watches the same state the screen does, and when the seat being waited on
403	belongs to the computer it calls the identical method the button would have
404	called — `discard`, `respond`, `declareConcealedKong`. So scoring, sound,
405	saving, 過水 and 包牌 all work without a special case, and a bot cannot make a
406	move a person could not. It never acts for you and never closes your claim
407	window: if the table is waiting on you, it waits.
408
409	How they play (`src/game/bot.ts`) is one idea applied everywhere. A hand is
410	judged at its resting size — the (5 − melds) × 3 + 1 tiles you hold between a
411	discard and your next draw — by its 向聽 first and by its 進張 count second.
412
413	- Discard — try every distinct tile, keep the one that leaves the best
414	resting hand. Ties go to whatever is hardest to build on: a lone honour with
415	most of its copies already gone, before a lone 五萬 that still has neighbours
416	to meet.
417	- 碰 / 吃 — only when the hand that comes out the other side is strictly
418	closer to home. Melding costs concealment and flexibility, so a claim that
419	merely holds the 向聽 steady is declined — which is why the bots pass on
420	pungs that would narrow a two-sided wait to a single tile.
421	- 槓 — judged more kindly: it pays 台 and fetches a replacement, so standing
422	still is good enough.
423	- 胡 — always taken.
424
425	What they watch you do (`src/game/danger.ts`) is the other half. Efficiency
426	alone throws whatever is fastest and pays for it; these bots read the table
427	first, off the face-up table only — discard rows, exposed melds, the 過水 locks
428	the nameplates already show. Two separate questions:
429
430	- How close does each seat look? Turns taken (a hand is usually settled
431	inside ten goes each, long before the wall looks finished, so counting the
432	wall is the wrong clock), melds exposed, and what they have been throwing
433	lately. Nobody discards 五條 out of a hand that still needs shaping — it comes
434	out once the shape is done, so late middle tiles are the tell. A 過水 lock is
435	a confession: to be locked out of a tile you had to have been able to win on
436	it.
437	- How likely is this tile the one they want? An honour can only be caught by
438	a pair or a triplet; a 五萬 sits in the middle of every run through it. A tile
439	they discarded themselves was not their tile when it went down — not proof
440	now, but the best evidence there is. A tile with no copies left unseen cannot
441	be a pair or triplet wait at all. Two melds in one suit means stay out of that
442	suit. And two dragon pungs down means the third dragon is not going anywhere
443	near the table, because 包牌 bills the feeder for the entire hand.
444
445	Whether any of that changes the discard depends on the bot's own hand. At 聽牌
446	it pushes — nothing short of 包牌 is worth breaking a ready hand for. Two or
447	three away with somebody live across the table and it will give up a whole 向聽
448	step to throw something safe, which is what folding is.
449
450	The estimate is checked against ground truth in the tests rather than assumed:
451	over the tiles bots actually considered, the ones the model rated below 0.2 deal
452	in 0% of the time and the ones above 0.8 deal in 10.5%, cleanly monotonic in
453	between. Switching the read on cuts deal-ins by about 13% over a few hundred
454	hands, takes hands from ~30 discards to ~38, and moves the draw rate from
455	almost nothing to about one hand in ten — which is what a table where people
456	stop feeding each other actually looks like.
457
458	What they know is still only what they can see. `unseenFor` counts the four
459	copies of each tile and subtracts the bot's own hand plus every discard and
460	exposed meld on the table. Neither module ever reads an opponent's concealed
461	tiles or looks at the wall.
462
463	There is no difficulty setting, and the reading stops at the discard: claims are
464	still judged purely on speed, and a bot will take a 碰 that walks it into
465	trouble.
466
467	向聽 lives in `src/game/shanten.ts`, apart from `hu.ts`, because the two want
468	different things: the win check has to be exact, and this one has to be fast —
469	it runs a few hundred times for every discard a bot considers. The tests hold
470	them against each other over random hands, since they work in completely
471	different ways and any disagreement is a bug in the fast one.
472
473	Undo behaves differently against the computer: rewinding into the middle of its
474	turn would only hand the move straight back to it, so one press goes back to
475	the last point you had a decision to make, computer replies and all.
476
477	## Saving
478
479	The game state is plain data, so a save is just its JSON in `localStorage`,
480	rewritten after every move. Close the lid, refresh, or run the battery flat and
481	nothing is lost — the lobby offers 繼續對局 Resume above 開始, showing the
482	round, hand number, chip counts and when it was saved. Which seats the computer
483	holds is part of the state, so a save comes back with the same hands in the
484	same places, and everybody who was on a phone scans back in exactly as they did
485	the first time — a room is opened around a resumed table the same way it is
486	opened around a fresh one, when somebody wants one. It is written by whichever
487	device is running the game (`engine.persist = authority`), so the mirrors on
488	everyone else's screens never fight it for the slot. A
489	save is validated before it is offered (four players, 144 tiles accounted for),
490	so a truncated or hand-edited one is ignored rather than loaded into a broken
491	table. `VERSION` in `save.ts` retires old saves if the state shape changes.
492
493	## Rules implemented
494
495	- 144 tiles (four of each suit/honour, eight flowers), 16-tile hands, dealer
496	draws the 17th.
497	- 補花 at the deal and on every drawn flower, replacements from the back of the
498	wall.
499	- 吃 only from 上家; 碰/槓/胡 from anyone. 明槓, 暗槓, 加槓, 搶槓, 槓上開花.
500	- 流局 when 16 tiles remain (`Rules.wallReserve`); the dealer keeps the deal on a
501	draw or on a dealer win (連莊), otherwise the deal passes and the round wind
502	advances every four passes. A full 四圈 game is 16 dealer passes.
503
504	### House rules (switches in `Rules`, all on the settings screen)
505
506	- 過水 `sacredDiscard` (on) — pass on a tile you could have won with and it
507	is dead to you until your own next draw; the seat shows a 過水 badge listing
508	what it is locked out of, so the missing 胡 button is never a mystery. A draw
509	from either end of the wall lifts it. `sacredClearedByClaim` (off) decides
510	whether taking a 吃 / 碰 / 槓 lifts it too — tables genuinely differ.
511	- 一炮多響 `multipleWinners` (off) — one discard pays out to every seat that
512	calls on it, each settled separately against the discarder. With it off the
513	tile goes to the caller nearest the discarder. Either way the table now waits
514	for other seats that can win before settling, so the nearest seat wins the
515	tile rather than the quickest hand — and the 摸牌 button still closes the
516	window on anyone dithering.
517	- 包牌 `liability` (on) — feeding the pung that completes a visible 大三元
518	or 大四喜 makes the feeder answer for the whole hand, in place of all three
519	payers. Only the seat that fed the last of those sets is on the hook, and
520	only if it came off a discard: a hand that assembled them itself, or closed
521	the set with a 暗槓, has nobody to blame.
522
523	### 台 scoring (`src/game/tai.ts`)
524
525	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 ·
526	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 ·
527	小四喜 8 · 大四喜 16 · 碰碰胡 4 · 混一色 4 · 清一色 8 · 字一色 16 ·
528	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 ·
529	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)
530
531	Ambiguous hands are decomposed every legal way and scored at the best reading.
532
533	Payment (`Game.settle`): one unit is `底 + 台 × 台值`
534	(`DEFAULT_RULES` = 底 3, 台 1, 100 chips each).
535	放槍一家付 — the discarder alone pays one unit; on 自摸 all three pay one unit
536	each. 拉莊 is billed to the dealer alone when the dealer is a payer, and added
537	to the whole hand when the dealer wins.
538
539	House rules vary a lot; the tai table and payments are plain data/functions in
540	`tai.ts` and `types.ts` if yours differ.
541
542	## Audio, by seat
543
544	A sound at a mahjong table comes from somewhere. You know whose 碰 that was
545	because it came from your left, and you know a tile has been thrown hard at the
546	far corner without watching it go. Both of those are cheap to keep, and both
547	are gone the moment everything comes out of the middle of one screen.
548
549	Every cue is a seat's. `SoundCue` carries a `seat` alongside the kind and
550	the tile — the seat that threw, called, or won — and leaves it off the things
551	that belong to the table rather than to anybody: the deal, a 流局, an undo. The
552	engine says whose only because a sound has to come from somewhere; what that
553	means in the room is entirely the UI's business, as it always was.
554
555	A seat becomes a place. `App.tsx` turns a seat into a *position round this
556	screen* — 0 the near edge, 1 the right, 2 across, 3 the left — because that is
557	what the ear wants and the seat number is not: a device holding one hand turns
558	the table so that hand is at the bottom, and the sound has to turn with it.
559	`game/sound.ts` gives each position a bus of its own: a pan, a gain, and a
560	lowpass that opens all the way for the near edge and closes to 5 kHz for the
561	far one. Distance is two things at once — quieter, and with the top taken off
562	— because between you and 對家 there is a metre of air, a wall of tiles, and
563	three people's arms, none of which carry treble. Tiles out in the middle skip
564	the edges entirely and are placed continuously, panned by where on the felt
565	they actually landed; the physics knows that to the pixel. Buses are built once
566	per spot and quantised, so a pool full of skidding tiles reuses a handful
567	rather than building one per contact. Old Safari has no `StereoPannerNode`; it
568	plays where it always did.
569
570	**Where there is a table and phones round it, the words come out of the
571	player.** The split the sound should take is the one the room already has: the
572	table makes the noise, because the tiles are on the table, and the phone in
573	somebody's hand says their calls, because the calls are theirs. So the table's
574	screen drops the voice for a seat while keeping the clack, and the phone
575	holding that seat says 碰 and names its own discards and makes no table noise
576	at all.
577
578	Only a phone claims that job, and that is the whole of how the split knows
579	where it applies. A phone is small enough to be in a hand and a hand is in the
580	room; a friend playing from their own laptop three cities away is not, and
581	never takes the words away from anybody's table. It is a proxy rather than a
582	fact — software cannot know who is sitting where — but it is the one property
583	that actually correlates with it, and it fails in the harmless direction:
584	the worst a wrong guess costs is a call said twice in two rooms.
585
586	That only works if the phone really is going to say it, so it is asked rather
587	than assumed. `sound.voiceLive()` is the honest answer — sound on, 報牌 on, the
588	audio hardware awake, the pack decoded — and every device reports it to the
589	host whenever it changes (`EV_VOICE`). The host works out, seat by seat, which
590	hands have a device that will speak for them and publishes that as `voices` in
591	room state; a nameplate wearing 🔊 next to its 📱 is a seat the table has gone
592	quiet for on purpose. Mute the phone, put it to sleep, hand the seat back —
593	the flag drops and the table takes the calls back mid-hand. The device the
594	table is being played at never counts, even when it holds seats: it is the
595	table, and a table that fell silent on the grounds that it was about to speak
596	would say nothing at all.
597
598	A browser will not start audio before the page has been touched, and a phone
599	that arrived by QR may never touch anything that asks for it by name — so the
600	first touch anywhere unlocks it, which is also the moment it can start
601	claiming the job.
602
603	## The voice pack
604
605	A mahjong table only ever says about fifty things — 42 tile names and a handful
606	of calls — so the whole vocabulary is rendered ahead of time into
607	`public/voice/` (~330 kB of mp3) rather than left to whatever speech synthesis
608	the browser happens to have. On Linux that is usually espeak-ng, which is
609	intelligible but sounds like a modem; and on a machine with no Chinese voice at
610	all the feature would silently do nothing. Shipping the audio makes playback
611	instant, identical everywhere, and lets a call be cut off mid-word when the
612	next one lands, all through the same Web Audio graph as the chimes.
613
614	`scripts/voice.mjs` is the authoring step, not part of `npm run build`. It needs
615	[piper](https://github.com/OHF-Voice/piper1-gpl) and ffmpeg on PATH:
616
617	```
618	node scripts/voice.mjs # → public/voice/*.mp3 + manifest.json
619	PIPER_MODEL=/path/to/voice.onnx node scripts/voice.mjs # a different voice
620	```
621
622	The wording lives in that script, and some of it is deliberately not the bare
623	tile character: 東 alone is a direction where 東風 is the tile, and the dragons
624	are 紅中 / 發財 / 白板 the way they are actually called — which also gives the
625	phonemiser enough context to get the tone right, since 中 on its own is as
626	likely to come out zhòng.
627
628	The clips here were rendered with piper's `zh_CN-huayan-medium`. If you
629	redistribute this app, check that voice's model card in
630	[piper-voices](https://huggingface.co/rhasspy/piper-voices) for the terms
631	attached to it, the same way you would the tile art below — or re-render the
632	pack with a voice whose terms suit you, which is a single command.
633
634	## Tile art
635
636	`public/tiles/tiles.svg` is the postmodern tileset from
637	[gnome-mahjongg](https://gitlab.gnome.org/GNOME/gnome-mahjongg), extracted from
638	the installed binary's GResource — a 43×2 sprite sheet (second row is the
639	highlighted variant, used for a lifted tile). `public/tiles/back.png` is the
640	blank tile from its smooth theme, tinted jade, since a solitaire game has no
641	face-down art of its own.
642
643	How a tile is drawn on the canvas — the outline round it and the shadow under
644	it — lives in `src/ui/tileArt.ts`, and `npm run dev` serves `/tiles.html`, a
645	page of every face at every angle over the felt with a slider on each of those
646	numbers and the finished block ready to paste back. None of it can be judged
647	from the values: it is 246 white against a dark green at 35 pixels across, and
648	the only way to know is to look at it and move something. (`tiles.html` is not
649	one of the build's entry points, so it does not ship.)
650
651	The sheet is not a picture of a tile's face. It is a picture of a whole tile,
652	drawn in an oblique projection, with its own sides painted down the left and
653	along the bottom and its own light coming from the top right — a 16px bevel on a
654	128px cell. That is fine while every tile stands the same way up, and wrong the
655	moment they are scattered: rotating the sprite rotates a pre-rendered solid, so
656	a pool of discards ends up with a different sun on each tile.
657
658	So the art is cropped back to the flat face, and the solid is worked out
659	instead. A tile is a box lying on cloth: take the four corners of the rotated
660	rectangle, offset them by the projection (`depthAngle`, down and to the left,
661	the same direction the art used), and fill the two or three quads whose edges
662	the base falls away from — each shaded from the direction that edge faces *on
663	screen*. Turn the tile and the same physical side comes round to face a
664	different way and its shading changes with it, which is the whole point. No 3D
665	engine, no z-buffer: one projection vector and a dot product per edge.
666
667	Each cell of the sheet is cut out into its own canvas the first time it is
668	wanted. `drawImage` with a source rectangle is allowed to sample past that
669	rectangle when it resamples, and the row under every face is the highlighted
670	variant, whose border is bright blue — so a tile at any angle other than a right
671	one wore a blue fringe along one edge. Square on it never showed, which is why
672	it looked like a rotation bug rather than a spritesheet one.
673
674	A tile used to be drawn as two passes — a slab of ivory offset down the screen
675	with the face on top, so a sliver of the tile's own side showed along the bottom
676	edge. At the size a discard is actually drawn that never read as thickness; it
677	read as a second, paler shape stuck to the bottom of every tile. Shading it
678	better did not help, because the problem was that it was there. What gives a
679	tile its weight now is the shadow and the outline, both of which are true at
680	any size.
681
682	That art is GPL-2.0-or-later. Fine for playing at home; if you ever
683	distribute this app, either honour the GPL or swap the two files for art of your
684	own — `SPRITE_COL` in `tiles.ts` is the only mapping that would need updating.
685
686	Known gaps and house rules that aren't implemented yet are listed in
687	[TODO.md](TODO.md).
688
689	## Layout
690
691	```
692	public/tiles/ sprite sheet + tile back
693	src/game/tiles.ts tile codes, wall, shuffle, sprite mapping
694	src/game/hu.ts hand decomposition, 聽 detection, wait shapes
695	src/game/shanten.ts 向聽 / 進張 counting, for the computer players
696	src/game/bot.ts what a computer player discards and claims (pure)
697	src/game/danger.ts reading the table: who looks ready, which tiles are hot
698	src/game/autoplay.ts when it does it, and how long it appears to think
699	src/game/tai.ts 台 scoring
700	src/game/engine.ts state machine: deal, turns, claim resolution, settlement
701	src/game/ctl.ts TableCtl — the surface the UI drives, local or networked
702	src/game/wall.ts what is left of the square, and what still blocks a throw
703	src/net/room.ts the wire itself: join, reconnect, state, events
704	src/net/session.ts the room: host duties, seats, intents, migration
705	src/net/table.ts TableCtl over the wire: mirror + intents + hand overlay
706	src/net/protocol.ts what crosses the wire, in one place
707	src/net/handOrder.ts your own arrangement, kept on your own device
708	src/ui/Controller.tsx a hand held on a phone, plus the seat picker for any device
709	server/rooms.ts the relay: rooms, host election, state fan-out
710	server/index.ts production: dist/ and the relay from one port
711	src/table/physics.ts the discard pool: rectangles with weight, pure and testable
712	src/table/geometry.ts the table measured — colliders, launch points, throw gate
713	src/table/pool.ts game state ⇄ tiles in the middle, by diffing the discards
714	src/ui/ TileView, Hand (drag and throw), Seat, Center, Pool, Help
715	```
716
717	## The tiles in the middle
718
719	The pool is a small rigid-body solver (`src/table/physics.ts`) that knows nothing
720	about mahjong, the DOM, or the clock: fixed 1/120s steps, no randomness of its
721	own, and everything in table pixels. Given the same throws it produces the same
722	pile every time, which is what lets a refresh mid-hand come back to the pile you
723	had rather than a freshly scattered one — positions are derived from the hand
724	number and each tile's place in its thrower's discards, never saved.
725
726	The pile is handled directly. Press on a discard and it comes up out of the
727	pile, carried over the top of the others rather than ploughing through them;
728	let go slowly and it is set down there, flick and it goes off across the felt at
729	the speed it left your fingers — the same throw a tile gets out of a hand, made
730	from the middle of the table instead. Pressing on bare felt does nothing, and
731	the press passes through to whatever was really under it: the tiles are drawn on
732	a canvas with no elements to hit, so the hit test is against the physics and the
733	listener is a capturing one on the table itself.
734
735	A flick is the whole of a throw: the direction and the speed it leaves at are
736	the hand's, and nothing between the fingers and the felt changes either. The one
737	thing the table decides is height — with a standing wall in front of the
738	throw a flat tile would only skid into it, so it is lofted over instead, exactly
739	as far as it was thrown and simply in the air for the first part of the way. A
740	tile that carries past the square and out into the open stays there. (It used to
741	be re-aimed at the middle of the square when the flick's line missed it, which
742	put the tile on a line nobody had thrown and read, fairly, as a bounce off
743	nothing.)
744
745	What stops a tile is other tiles. There is no fence: the standing stacks of the
746	wall, the sets people have laid down and the sixteen each of them is holding are
747	all measured as colliders, so the pile is held in by the ring of tiles round it
748	exactly as it is on a real table, and spills through whatever gaps the wall has
749	been eaten into. The table's own edge is a backstop at the screen edge, there
750	only so a tile that got past all of that isn't lost.
751
752	A tile that only ever slid would look like a puck. What makes a pool of discards
753	look like one is that every tile in it has come to rest at some angle nobody
754	chose, and that is a thing to earn rather than to scatter in. A tile turns
755	because of where it was hit.
756
757	So a contact happens somewhere in particular. The separating axis test says which
758	way to push and by how much; `contactOf` says where, and the arm from there to
759	the tile's middle is what a turn is made of. A tile shouldered square in the back
760	is pushed along and does not turn at all. The same shove, moved along until it is
761	catching one end of the tile, turns it instead, and which way round follows from
762	which end. What touches is a footprint and not a point: a tile lying square to a
763	wall meets it along the whole of one side and is pushed through its own middle
764	however far along the wall it is, and the same tile at forty-five degrees meets
765	it on one corner and swings on round it. Everything a tile can hit is another
766	tile, so the standing stacks, the laid-down sets and the table's own straight
767	edge are all the same contact with the same friction across the face of it.
768
769	Two things keep that honest. The first is that a tile is not free to pivot. It is
770	lying flat on cloth, and the cloth under the whole of its face resists the turn
771	for as long as the contact lasts, so only a tenth of the moment a knock would
772	give a free rectangle actually reaches it (`FELT_HOLD`). Without that, a tile off
773	the corner of a stack comes away spinning like a top. The second is that the felt
774	is one patch of cloth doing one thing: what it spends stopping the slide it has
775	not got left for the turn, so the two share a single friction budget rather than
776	running as two brakes side by side. What that buys is the ending. A tile
777	flicked hard with a little turn on it used to stop turning early and skate on
778	looking dead, and one barely pushed with a lot of turn on it stopped dead and
779	went on spinning where it lay. They now run out together.
780
781	The turn a tile arrives with is the flick's own. `ui/flick.ts` reads a release
782	the same way wherever it was made, out of a hand or off the pile in the middle,
783	and the spin it reports is the angle the stroke swung through between its first
784	half and its second, over the time it took. That is a rate, in rad/s, which is
785	what a wrist actually does: a straight flick turns nothing however hard it is
786	thrown, and the same curve made twice as fast puts twice the turn on the tile.
787
788	It is drawn on a canvas that spans the whole table, not just the centre.
789	That is deliberate: `.slot` and `.seat` both clip their own contents, which is
790	why a tile can't be animated out of a strip as an element. On the canvas there is
791	nothing to clip it, so a tile lifted out of a hand crosses the table in one
792	piece.
793
794	The square is the whole wall: eighteen stacks of two a side, four sides, 144
795	tiles, the way it is built on a table. It used to be rebuilt to what was left
796	after a hand had been dealt — barely half of it — because eighteen full-size
797	stacks a side wants about 560px and no ordinary window has that between the top
798	and bottom strips. That bought a square at the price of it not being the wall:
799	a side ran out of stacks before it reached its corner, so the four of them never
800	met.
801
802	Nothing gives now, and in particular the tile does not. Every tile on the table
803	is one size — `--tile-w` in styles.css, in a hand, in the wall, and lying in the
804	middle — because they are the same tiles, and a tile that changed size between
805	the wall and your hand read as a different, smaller set sitting in the middle.
806	The square is built from that tile and is however big that comes to:
807	`wallSquare()` in `game/wall.ts` takes the tile and hands back the lengths, and
808	`WallRing` measures one rendered cell to find out what CSS made it. The wall
809	being bigger than the room between the strips is a fact about a 144-tile wall,
810	not a thing to solve by shrinking it.
811
812	The four walls are placed by CSS alone: each is pinned to its own edge of the
813	opening with `left`/`top`, runs its whole length from the corner it is built
814	from, and so carries one wall-depth past the far corner and over the end of the
815	next wall along. Every corner of the square is covered by exactly one wall and
816	no two overlap — that is the pinwheel. Nothing is rotated; a square wants no
817	rotation to describe it.
818
819	The overhang is exactly one wall-depth, which is what it takes to close a right
820	angle: two bands of thickness `t` meeting at an interior angle ψ overlap by
821	`t / tan(ψ/2)`, and at ninety degrees that is `t`. So the whole square measures a
822	wall's length plus one depth across, and leaves that length less one depth in
823	the middle — which is the opening, and which is what `.wall-ring` itself is.
824
825	The CSS only places what it is told: `.wall-side` is a row of stacks, laid across
826	on the two flat walls and down on the two upright ones, and `--wall-len` from
827	`WallRing` is the only length it is given. Every place in the wall is rendered
828	whether or not there is still a stack standing on it, because with the break
829	anywhere but a corner a wall is eaten from its middle — a row that closed the
830	gap up would drag the whole tail of the wall along the table behind it.
831
832	Where the four walls stand is found at the table itself rather than on a page
833	of sliders: take hold of a wall in a real game and push it. It goes where the
834	finger goes, and lets go saying where it landed — `[wall] top pushed to 0.5,-4.9
835	— top 0.5,-4.9 · right 0,0 · …` in the console, in tiles, which is the form
836	`WALL_PLACED` in `game/wall.ts` is written in, so what a push finds pastes
837	straight back in. Nothing about the hand moves: the wall is the same wall in the
838	same order and the next draw is still the next draw. There used to be a
839	`/wall.html` for this; the table is the honest place to do it, so that page is
840	gone.
841
842	## 擲骰 — where the wall is broken
843
844	The dealer throws three dice to open a hand, and the total does two jobs. It
845	counts round the seats — the dealer being one, and the count going the way the
846	turn goes — to pick whose wall is opened. Then the same number counts stacks in
847	from the right-hand end of that wall, and the break is behind them: the first
848	tile drawn is the next one along, and the drawing runs away to the left from
849	there, on round the square.
850
851	The square makes that cheap to say. Every wall is laid down from the corner its
852	own player's right hand falls on — that is what the pinwheel is — so the run of
853	72 places already starts at each seat's right, and the break is simply so many
854	places into the side the dice picked. `breakAt()` is those two lines.
855
856	Nothing about the tiles turns on it: the wall was shuffled before it was built,
857	so which stack is drawn first is decided either way. What it moves is where in
858	the middle of the table the gap opens and who has to reach furthest for the next
859	draw — which is the whole of what it does at a table too. The throw is made from
860	the same seeded rng that shuffled, so a seed replays a hand's gap as well as its
861	tiles.
862
863	Where the wall opened is thrown for once and then lived with, which is right at
864	a table and awkward to look at: half the question about the square is what it
865	looks like opened at each of the seventy-two places it can be opened at.
866	`game.reroll(1, 1, 1)` through `game.reroll(6, 6, 6)` from the console walks the
867	break all the way round, and `game.reroll()` throws afresh. It moves the gap and
868	nothing else: the tiles were shuffled before the wall was built, so nobody's
869	hand changes and the draw order is the draw order.
870	Nothing about the square is calculated twice. Its size lives entirely in CSS
871	(`--ws`, `--wd`), so `geometry.ts` measures it instead of restating that
872	arithmetic: `WallRing` already renders every stack as a real element tagged with
873	its index, and the colliders are those elements' bounding rects. That one
874	mapping is what makes the wall a physical object — the thing a thrown tile
875	bounces off, and the thing the throw gate casts its ray at.
876
877	### Seeing the colliders
878
879	Because every boundary is measured off the DOM rather than written down, the
880	only honest way to check a bounce is to draw the boxes in the coordinate space
881	the tiles are drawn in. Add `?colliders` to the address, or call `colliders()`
882	from the console while a hand is running:
883
884	\| drawn \| is \|
885	\| --- \| --- \|
886	\| 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 \|
887	\| 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 \|
888	\| dashed rectangle \| the wall square's opening — where a throw is aimed. It stops nothing, which is why the pile spills out of it \|
889	\| 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 \|
890	\| box hugging each pool tile \| the rotated rectangle it really collides with \|
891	\| 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 \|
892
893	\| 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 \|
894	\| 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 \|
895
896	Colliders mode is also a throwing sandbox: a flick throws for real but is not a
897	discard, so the tile stays in your hand, the turn never passes and the computer
898	players hold still. Throw the same tile at the same corner as many times as you
899	like. Sixty stay on the table before the oldest is swept off; `pool.clearLoose()`
900	sweeps them all. Tapping a tile to discard does nothing in this mode — throwing
901	is the thing being tested.
902
903	Add `pause=1` (`?colliders&pause=1`, or `pause()` from the console) to **stop
904	dead on contact**. The physics halts on the fixed step that resolved it and
905	drops the rest of the frame, so what's on screen is where the tile touched
906	rather than where it had got to by the end of the frame. Space carries on, →
907	takes one step, ↑ takes ten; the console says what it stopped on.
908
909	`colliders(false)` turns it off again. It repaints on toggle, so a settled table
910	doesn't have to be poked first. In a dev build `pool` is the live pool: the
911	overlay draws `pool.world.walls`, the physics' own copy rather than a fresh
912	measurement, so if what's drawn ever disagrees with the DOM it is the world that
913	went stale — which `pool.measuredAt` will show, since the stamp carries every
914	hand's shape.
915
916	## What is left of the wall
917
918	Four walls of eighteen full-size stacks want about 560px a side. No ordinary
919	window has that between the top and bottom strips, and the tile is not allowed
920	to shrink to make it fit — a tile in the wall is the same tile you play with, so
921	drawing it smaller was a visible lie.
922
923	What saves it is that nobody ever sees a whole wall: four hands come off the
924	front before the table is on screen, so barely half of it is left by then. So
925	only the stacks still standing are drawn, and they are laid out around a ring
926	cut to the middle rather than to eighteen — `ringLayout` in `game/wall.ts`
927	measures how many stacks a side can take (`WallRing` renders one hidden cell and
928	asks CSS how big it came out) and shares what is left over the four sides in
929	proportion. Every seat keeps a wall in front of it instead of the remainder
930	piling into one corner, and the order runs all the way round, so the break point
931	is still at the head of it and the 底牌 tail still at the end.
932
933	Shuffling the wall along as it is eaten costs nothing to look at, because every
934	stack shows the same green back — and it is what people do to a half-eaten wall
935	anyway.
936
937	`pool.ts` never listens for events. It diffs the discards against what it is
938	already drawing, keyed by `seat:index` — and those keys never shift, because
939	discards are only ever pushed and a claim only ever pops the one on top. So a
940	key that appeared is a tile to throw in and a key that went is a tile to take
941	off, which makes undo, 下一局, a claim and resuming a saved game all the same
942	code path. The engine has no idea any of it exists, and the save format did not
943	change.