collin/mahjong

c1de3f155bd40675a20193a465ff3683cf1015df / README.md

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