collin/mahjong

0ab65f87c2895780d606c237f0b3b3a255897fd0 / README.md

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