collin/mahjong

0d844b664c9743d56a156daefdfe49f0e86fb760 / src / table / physics.ts

1	import type { Tile } from '../game/tiles';
2	import type { SeatId } from '../game/types';
3
4	/**
5	* The tiles in the middle of the table, as things with weight.
6	*
7	* Pure and deterministic: no DOM, no clock, no randomness. Where the world's
8	* measurements come from is `geometry.ts`; what gets thrown into it and when is
9	* `pool.ts`; drawing it is `ui/Pool.tsx`. Anything random — the scatter on a
10	* placed tile, where a rebuilt pile's tiles sit — is decided by the caller and
11	* arrives as a spawn impulse, which is what lets the same hand come back to the
12	* same pile after a refresh.
13	*
14	* Tiles are rectangles and they are solved as rectangles: two of them can lean
15	* against each other at any angle but never share the same ground. Everything is
16	* in CSS pixels, table-local, and seconds.
17	*/
18
19	/** One fixed step. Small enough that a hard flick can't tunnel a wall. */
20	export const FIXED_DT = 1 / 120;
21	/**
22	* Most catch-up a single frame will simulate. A tab that was in the background
23	* for a minute comes back to a settled pile, not to a spin through 7200 steps.
24	*/
25	const MAX_SUBSTEPS = 8;
26
27	/** Falls onto the felt in about half a second from a normal throw. */
28	const GRAVITY = 2600;
29	/** Tiles are dense and land flat; they do not bounce much. */
30	const Z_RESTITUTION = 0.34;
31	/** Below this the tile is flat on the felt: sliding, and in the way of others. */
32	const ON_FELT = 8;
33	/**
34	* How high a standing stack of two reaches. A tile still above this sails over
35	* the wall — which is exactly what a placed tile does, and what a thrown one
36	* must not need to do.
37	*/
38	export const WALL_HEIGHT = 30;
39
40	/**
41	* Felt friction: an exponential part that kills speed, and a constant part that
42	* actually brings it to a stop rather than to an asymptote.
43	*
44	* Deliberately light. A tile thrown hard should cross the square, come off the
45	* far side and barge through what is already lying there before it settles —
46	* heavier friction ate the throw in the first few inches and every discard
47	* ended up looking the same however it was let go of.
48	*/
49	const FELT_DAMP = 2.6;
50	const FELT_STOP = 70;
51	const SPIN_DAMP = 3.4;
52	const SPIN_STOP = 0.9;
53
54	/**
55	* Roughly how much longer a tile keeps travelling after it lands, as a time.
56	* Aiming a throw means solving for where it comes to rest, not where it first
57	* touches down, so `pool.ts` adds this to the flight time.
58	*/
59	export const SLIDE_TIME = 1 / FELT_DAMP;
60
61	/** Off the inside of the wall square, and off another tile. Bouncy enough that
62	* a hard throw ricochets instead of sticking where it first lands. */
63	const BOUND_RESTITUTION = 0.55;
64	const BOUND_FRICTION = 0.9;
65	const TILE_RESTITUTION = 0.42;
66	/**
67	* Tile on tile, across the face of the contact. Everything a tile can hit is
68	* another tile: the standing stacks, the sets laid down, the sixteen in
69	* somebody's hand. So there is one number for all of it, and it is what turns
70	* a glancing blow into a turn rather than a slide.
71	*/
72	const TILE_FRICTION = 0.32;
73	/**
74	* How much of a knock's moment the tile actually keeps.
75	*
76	* A free rectangle struck off its middle takes the whole of it, and would come
77	* off the corner of a stack spinning like a top. A tile does not, and the
78	* reason is not in the solver: it is lying flat on cloth, and the cloth under
79	* the whole of its face resists the turn for as long as the contact lasts. The
80	* felt is modelled as a drag on the centre, not as a torque on the face, so
81	* that resistance has nowhere else to go and is put back here, as the fraction
82	* of the moment that survives the felt. Only the angular half of a contact is
83	* scaled: a head-on hit is unaffected, because there is no moment in one.
84	*/
85	const FELT_HOLD = 0.1;
86
87	/** Asleep after this many consecutive steps of going nowhere. */
88	const SLEEP_SPEED = 7;
89	const SLEEP_SPIN = 0.2;
90	const SLEEP_STEPS = 10;
91	/**
92	* Overlap small enough to live with. Without this a crowded pool never sleeps:
93	* a tile squeezed between its neighbours and the pool's edge gets pushed out of
94	* contact and shoved back into it on every step, for ever. A fifth of a pixel is
95	* not something you can see, and it is what lets the pile go quiet.
96	*/
97	const CONTACT_SLOP = 0.2;
98	/**
99	* A body moving this slowly for this long is put to sleep whatever it thinks it
100	* is doing. The backstop that guarantees the pile always settles — counted from
101	* when it last woke, not from when it was thrown.
102	*/
103	const AWAKE_LIMIT = 600;
104
105	/** Pulls a tile that landed short of the pool in towards the rest of them. */
106	const ENTRY_PULL = 700;
107	/**
108	* How long a tile gets to find its own way into the pool before it is simply
109	* put there. A discard is game state made visible — it has to be in the middle
110	* where it can be seen and counted — so a tile that has got itself wedged
111	* against a standing stack, or is bouncing between two of them, cannot be left
112	* out there. Two seconds is far longer than any real throw takes to settle.
113	*/
114	const ENTRY_GRACE = 240;
115
116	/**
117	* How hard a tile that merely arrived counts as having arrived: enough for
118	* the seat it came in past to notice, deliberately under the threshold the
119	* renderer plays a clack at. Nothing was hit, so nothing should be heard.
120	*/
121	const ARRIVED = 0.1;
122
123	/** How fast a tile drawn at hand size shrinks to the size it is in the pool. */
124	const SCALE_EASE = 6;
125
126	export const TAU = Math.PI * 2;
127	/**
128	* How quickly whatever is left of a tumble is taken up once the tile is down.
129	* A thrown tile turns over in the air a whole number of times, so it should be
130	* face up as it lands — but a bounce or a shove can leave it part-way round, and
131	* a mahjong tile on a table is never resting on its face. This takes up the
132	* remainder, and `sleep` guarantees the rest.
133	*/
134	const FLIP_SETTLE = 14;
135
136	export interface Rect {
137	x: number;
138	y: number;
139	w: number;
140	h: number;
141	}
142
143	/** Anything the collision routines can treat as a rotated rectangle. */
144	interface Box {
145	x: number;
146	y: number;
147	w: number;
148	h: number;
149	angle: number;
150	}
151
152	export interface Body {
153	tile: Tile;
154	/** Who threw it — for the fly-out when it is claimed, not for any marking. */
155	seat: SeatId;
156	/** Centre on the felt plane. */
157	x: number;
158	y: number;
159	vx: number;
160	vy: number;
161	/** Height above the felt, and its rate. Only ever >= 0. */
162	z: number;
163	vz: number;
164	angle: number;
165	spin: number;
166	/**
167	* Turning over in the air, about an axis lying in the table — what a thrown
168	* tile does, as opposed to `angle`, which is it turning flat on the cloth.
169	* Zero, or any whole number of turns, is face up.
170	*/
171	flip: number;
172	flipSpin: number;
173	/** Which way that axis points: a tile tumbles across its line of travel. */
174	flipAxis: number;
175	w: number;
176	h: number;
177	/**
178	* Render-only: what to multiply the drawn size by, easing to 1. A tile is
179	* drawn big in a hand so it can be read, and small in the pool because that is
180	* the size it is in the wall, so a thrown one has to settle from one to the
181	* other. Eased here rather than in the renderer so it does not run at a
182	* different speed on a different frame rate.
183	*/
184	scale: number;
185	/**
186	* Whether it has ever been inside the pool. Until it has, it is free to fly
187	* over the wall from a seat's edge; once it has, the pool keeps it.
188	*/
189	entered: boolean;
190	/**
191	* Which side of the table it is still on its way in over. Every tile is
192	* thrown from outside, so every tile has one until it lands; it is what the
193	* table reports when the tile finally gets in, since by then there is no
194	* longer an edge to read off its position.
195	*/
196	over?: Edge;
197	/**
198	* Whether it has got clear of the hand it was thrown from. A tile leaves from
199	* inside its own row — `throwFrom` is the middle of the last cell — so on
200	* the frame it is let go it is overlapping its thrower's own tiles, and
201	* without this every throw would ricochet off the hand that made it. Your own
202	* tiles are not solid to you until you are out of them.
203	*/
204	leftSeat: boolean;
205	/**
206	* In somebody's fingers. Its position is set from outside — a finger is not
207	* a force — so the solver leaves it entirely alone: it does not fall, drift,
208	* settle or take a shove, and it is held high enough to pass over everything
209	* else while it is being carried.
210	*/
211	carried: boolean;
212	resting: boolean;
213	/** Consecutive slow steps, counting towards sleep. */
214	still: number;
215	/** Steps lived, for the grace period on getting into the pool. */
216	age: number;
217	/** Steps since it last woke, for the backstop on settling. */
218	awake: number;
219	}
220
221	/**
222	* Which edge of the middle a tile ran into. The middle stops where the four
223	* players' tiles start, so an edge is somebody's hand: a tile that reaches one
224	* hard has been thrown into their tiles, and they have something to say about
225	* it. Seats are the caller's business; this only says which way it went off.
226	*/
227	export type Edge = 'left' \| 'right' \| 'top' \| 'bottom';
228
229	/** A collision loud enough to hear. `strength` is 0..1. */
230	export interface Impact {
231	x: number;
232	y: number;
233	strength: number;
234	/** Set when it was the edge of the table it hit, not a wall or another tile. */
235	edge?: Edge;
236	/** Whose tiles it ran into, where what it hit belonged to somebody. */
237	into?: SeatId;
238	/** Whose discard it was — you cannot be barged by your own tile. */
239	seat?: SeatId;
240	}
241
242	/**
243	* Something solid on the table. Everything that stops a tile is another tile:
244	* the stacks still standing in the wall, the sets people have laid down, and
245	* the sixteen everyone is holding. `seat` is whose it is, where it is somebody's
246	* — a tile thrown into a hand is a thing that hand's owner answers.
247	*/
248	export interface Barrier {
249	rect: Rect;
250	seat?: SeatId;
251	}
252
253	export interface World {
254	/** Where tiles come to rest — inside the wall square. */
255	bounds: Rect;
256	/** Stacks of two still standing. Half and spent stacks are not obstacles. */
257	walls: Barrier[];
258	bodies: Body[];
259	/** Left-over time from the last frame, so steps stay a fixed size. */
260	carry: number;
261	/**
262	* Overlaps pushed apart in the last step. The pile is not settled while this
263	* is non-zero, however still everything looks — two tiles asleep inside each
264	* other would otherwise stay that way for the rest of the hand, because a
265	* pile that has stopped moving stops being stepped.
266	*/
267	contacts: number;
268	/** Steps spent untangling a pile that is otherwise asleep, so it cannot spin. */
269	relaxed: number;
270	}
271
272	export interface Spawn {
273	tile: Tile;
274	seat: SeatId;
275	x: number;
276	y: number;
277	vx: number;
278	vy: number;
279	z: number;
280	vz: number;
281	angle: number;
282	spin: number;
283	/** Whole turns to make in the air. It lands face up whatever this is. */
284	flipTurns?: number;
285	/** Seconds of flight those turns are spread over. */
286	flipOver?: number;
287	flipAxis?: number;
288	w: number;
289	h: number;
290	/** Drawn this much bigger to start with, easing to its pool size. */
291	scale?: number;
292	/** Already settled — a pile being rebuilt after a reload or an undo. */
293	atRest?: boolean;
294	}
295
296	export function createWorld(bounds: Rect, walls: Barrier[] = []): World {
297	return { bounds, walls, bodies: [], carry: 0, contacts: 0, relaxed: 0 };
298	}
299
300	export function addBody(world: World, s: Spawn): Body {
301	const body: Body = {
302	tile: s.tile,
303	seat: s.seat,
304	x: s.x,
305	y: s.y,
306	vx: s.atRest ? 0 : s.vx,
307	vy: s.atRest ? 0 : s.vy,
308	z: s.atRest ? 0 : s.z,
309	vz: s.atRest ? 0 : s.vz,
310	angle: s.angle,
311	spin: s.atRest ? 0 : s.spin,
312	flip: 0,
313	// Spread the turns over the flight, so the tile comes down as it comes back
314	// round to face up rather than being snapped there on landing.
315	flipSpin:
316	s.atRest \|\| !s.flipTurns \|\| !s.flipOver ? 0 : (TAU * s.flipTurns) / s.flipOver,
317	flipAxis: s.flipAxis ?? 0,
318	w: s.w,
319	h: s.h,
320	scale: s.atRest ? 1 : s.scale ?? 1,
321	entered: !!s.atRest,
322	// A rebuilt pile is already lying in the middle, not in anybody's hand.
323	leftSeat: !!s.atRest,
324	carried: false,
325	resting: !!s.atRest,
326	still: s.atRest ? SLEEP_STEPS : 0,
327	age: 0,
328	awake: 0,
329	};
330	world.bodies.push(body);
331	if (s.atRest) {
332	// A tile dropped straight into a settled pile still has to make room. This
333	// is a first shove only — the relaxing below finishes the job, which matters
334	// for a whole pile rebuilt at once when a saved game is picked up.
335	separate(world, body);
336	world.contacts++;
337	}
338	return body;
339	}
340
341	// ---- rotated rectangles ---------------------------------------------------
342
343	/** Distance from a box's centre to its edge, measured along `ax`. */
344	function reach(b: Box, ax: { x: number; y: number }): number {
345	const c = Math.cos(b.angle);
346	const s = Math.sin(b.angle);
347	return (
348	Math.abs(c * ax.x + s * ax.y) * (b.w / 2) + Math.abs(-s * ax.x + c * ax.y) * (b.h / 2)
349	);
350	}
351
352	/** The rotated rectangle's own bounding box, for the pool's straight edges. */
353	function aabbHalf(b: Box) {
354	const c = Math.abs(Math.cos(b.angle));
355	const s = Math.abs(Math.sin(b.angle));
356	return { x: (b.w / 2) * c + (b.h / 2) * s, y: (b.w / 2) * s + (b.h / 2) * c };
357	}
358
359	/** Enough to skip a pair without doing the real work. */
360	const circumradius = (b: Box) => Math.hypot(b.w, b.h) / 2;
361
362	/**
363	* Separating axis test between two rectangles at any angle. Returns the
364	* shallowest way out — the direction to push `b` away from `a`, and by how much
365	* — or null if they are clear of each other.
366	*
367	* Four axes is all a pair of rectangles needs: each box's two edge normals.
368	*/
369	function overlapOf(a: Box, b: Box): { nx: number; ny: number; depth: number } \| null {
370	const ca = Math.cos(a.angle);
371	const sa = Math.sin(a.angle);
372	const cb = Math.cos(b.angle);
373	const sb = Math.sin(b.angle);
374	const axes = [
375	{ x: ca, y: sa },
376	{ x: -sa, y: ca },
377	{ x: cb, y: sb },
378	{ x: -sb, y: cb },
379	];
380
381	const dx = b.x - a.x;
382	const dy = b.y - a.y;
383	let depth = Infinity;
384	let nx = 0;
385	let ny = 0;
386
387	for (const ax of axes) {
388	const gap = reach(a, ax) + reach(b, ax) - Math.abs(dx * ax.x + dy * ax.y);
389	if (gap <= 0) return null;
390	if (gap < depth) {
391	depth = gap;
392	// Always pointing from a towards b, so callers need not guess.
393	const sign = dx * ax.x + dy * ax.y < 0 ? -1 : 1;
394	nx = ax.x * sign;
395	ny = ax.y * sign;
396	}
397	}
398	return { nx, ny, depth };
399	}
400
401	/**
402	* How hard a tile is to turn, as 1 / I.
403	*
404	* A rectangle of uniform weight about its own middle: I = m(w² + h²) / 12.
405	* Every tile on the table weighs the same as every other, so mass is 1
406	* throughout and this is the whole of it. That also means an impulse and a
407	* change of speed are the same number, so the two halves of a contact can be
408	* written side by side.
409	*
410	* Scaled by `FELT_HOLD`, because a tile is turning against cloth and not in
411	* mid-air. See there.
412	*/
413	function invInertia(b: Box): number {
414	return (12 * FELT_HOLD) / (b.w * b.w + b.h * b.h);
415	}
416
417	/**
418	* How far out from the middle the felt has hold of a tile, as one radius.
419	*
420	* A tile spinning on the cloth is rubbed over the whole of its face, not at a
421	* point, and what that comes to is an average distance from the middle. A
422	* quarter of the diagonal is close enough to the true mean over a rectangle,
423	* and it is the number that puts a turn and a slide into the same units: a
424	* spin of ω rubs the felt at ω × this many pixels a second, which is a speed,
425	* and can be weighed against how fast the tile is travelling. See `damp`.
426	*/
427	const gripRadius = (b: Box): number => Math.hypot(b.w, b.h) / 4;
428
429	/**
430	* Where a tile is pressing on the thing it has run into.
431	*
432	* The separating axis test says which way to push and by how much, but not
433	* where, and where is the whole of whether a knock turns a tile or only shoves
434	* it. A push through the middle is a shove; the same push at one end is a turn,
435	* and the difference is the arm between the two.
436	*
437	* What touches is a footprint, not a point. A tile lying square to a wall
438	* meets it along the whole of one side and is pushed through its own middle,
439	* however far along the wall it happens to be. Turn the same tile forty-five
440	* degrees and it meets the wall on a single corner, and that corner is the only
441	* thing pushing back. `flat` is which of those it is, as \|sin 2θ\|, because a
442	* rectangle comes square to a face every quarter turn; the footprint shrinks
443	* from the whole side to the one corner as it goes.
444	*
445	* `a` is what was hit, where that has ends: only the part of the footprint the
446	* two of them share counts, so a tile that has caught the end of a stack is
447	* pushed at the end of itself. Left out for the table's own edge, which is a
448	* straight line with no ends to catch.
449	*/
450	function contactOf(b: Box, nx: number, ny: number, a?: Box): { x: number; y: number } {
451	const tx = -ny;
452	const ty = nx;
453	const flat = Math.abs(Math.sin(2 * b.angle));
454	const half = reach(b, { x: tx, y: ty }) * (1 - flat);
455
456	// The corner that arrived first: all there is to lean on when the tile is
457	// not square to what it hit, and where the footprint collapses to.
458	const c = Math.cos(b.angle);
459	const s = Math.sin(b.angle);
460	const u = (c * nx + s * ny <= 0 ? 1 : -1) * (b.w / 2);
461	const v = (-s * nx + c * ny <= 0 ? 1 : -1) * (b.h / 2);
462	const corner = (b.x + c * u - s * v) * tx + (b.y + s * u + c * v) * ty;
463	const mid = b.x * tx + b.y * ty + (corner - (b.x * tx + b.y * ty)) * flat;
464
465	let lo = mid - half;
466	let hi = mid + half;
467	let across: number;
468	if (a) {
469	const ca = a.x * tx + a.y * ty;
470	const ra = reach(a, { x: tx, y: ty });
471	lo = Math.max(lo, ca - ra);
472	hi = Math.min(hi, ca + ra);
473	if (lo > hi) lo = hi = (lo + hi) / 2;
474	across = a.x * nx + a.y * ny + reach(a, { x: nx, y: ny });
475	} else {
476	across = b.x * nx + b.y * ny - reach(b, { x: nx, y: ny });
477	}
478	const along = (lo + hi) / 2;
479	return { x: tx * along + nx * across, y: ty * along + ny * across };
480	}
481
482	/** A standing stack, as something the tests above can chew on. */
483	const boxOf = (r: Rect, angle = 0): Box => ({
484	x: r.x + r.w / 2,
485	y: r.y + r.h / 2,
486	w: r.w,
487	h: r.h,
488	angle,
489	});
490
491	// ---- stepping -------------------------------------------------------------
492
493	/**
494	* Where a tile's centre may be for the whole of it to be inside `r`.
495	*
496	* One place rather than three, because the three have to agree: the edge holds
497	* a tile at these limits, and the moment it starts holding it is the moment the
498	* tile reaches them. Deciding that on the centre instead would mean the tile is
499	* declared in while it is still half out, and the first thing the edge did
500	* would be to shove it the rest of the way — half a tile, in one frame, at a
501	* line that is nowhere near the one you can see.
502	*
503	* A pool narrower than a tile would otherwise fight itself, so it centres.
504	*/
505	function limitsOf(r: Rect, b: Box) {
506	const half = aabbHalf(b);
507	return {
508	minX: r.w <= half.x * 2 ? r.x + r.w / 2 : r.x + half.x,
509	maxX: r.w <= half.x * 2 ? r.x + r.w / 2 : r.x + r.w - half.x,
510	minY: r.h <= half.y * 2 ? r.y + r.h / 2 : r.y + half.y,
511	maxY: r.h <= half.y * 2 ? r.y + r.h / 2 : r.y + r.h - half.y,
512	};
513	}
514
515	/** Whether the whole tile has made it in — the edge's own test for `entered`. */
516	function wholly(r: Rect, b: Body): boolean {
517	const l = limitsOf(r, b);
518	return b.x >= l.minX && b.x <= l.maxX && b.y >= l.minY && b.y <= l.maxY;
519	}
520
521	/**
522	* Whose side of the table a tile has ended up off, if it is off any of them.
523	* The furthest it is out, so a tile that overshot a corner belongs to whichever
524	* of the two it went past by more.
525	*/
526	function outsideEdge(r: Rect, b: Body): Edge \| undefined {
527	const out: [Edge, number][] = [
528	['left', r.x - b.x],
529	['right', b.x - (r.x + r.w)],
530	['top', r.y - b.y],
531	['bottom', b.y - (r.y + r.h)],
532	];
533	let worst: [Edge, number] = out[0];
534	for (const one of out) if (one[1] > worst[1]) worst = one;
535	return worst[1] > 0 ? worst[0] : undefined;
536	}
537
538	/**
539	* Run the world forward. `elapsed` is real seconds since the last call; it is
540	* consumed in fixed steps so the result does not depend on the frame rate.
541	* Returns the impacts worth a sound.
542	*/
543	export function advance(world: World, elapsed: number, stopOnImpact = false): Impact[] {
544	world.carry += Math.max(0, elapsed);
545	let steps = Math.floor(world.carry / FIXED_DT);
546	if (steps > MAX_SUBSTEPS) {
547	// Drop the backlog rather than working through it.
548	world.carry = 0;
549	steps = MAX_SUBSTEPS;
550	} else {
551	world.carry -= steps * FIXED_DT;
552	}
553
554	const impacts: Impact[] = [];
555	for (let i = 0; i < steps; i++) {
556	step(world, impacts);
557	// Freeze on the step that resolved the contact, and drop what is left of
558	// the frame — the debugger wants the tile where it touched, not where it
559	// would have been fifteen milliseconds later. See ui/colliders.ts.
560	if (stopOnImpact && impacts.length) {
561	world.carry = 0;
562	break;
563	}
564	}
565	return impacts;
566	}
567
568	/** One fixed step. Exported for the tests, which drive it directly. */
569	export function step(world: World, impacts: Impact[] = []): Impact[] {
570	const dt = FIXED_DT;
571	world.contacts = 0;
572
573	for (const b of world.bodies) {
574	if (b.scale !== 1) {
575	b.scale += (1 - b.scale) * Math.min(1, SCALE_EASE * dt);
576	if (Math.abs(b.scale - 1) < 0.002) b.scale = 1;
577	}
578	// A tile in the fingers is driven, not simulated. Nothing below applies
579	// to it, and it stays awake so it is drawn where the finger put it.
580	if (b.carried) {
581	b.still = 0;
582	b.awake = 0;
583	continue;
584	}
585	if (b.resting) continue;
586	b.age++;
587	b.awake++;
588
589	// --- through the air ---
590	if (b.z > 0 \|\| b.vz !== 0) {
591	b.vz -= GRAVITY * dt;
592	b.z += b.vz * dt;
593	if (b.z <= 0) {
594	const landing = Math.abs(b.vz);
595	b.z = 0;
596	// Below a knock it stops dead rather than shivering out a bounce.
597	b.vz = landing > 40 ? landing * Z_RESTITUTION : 0;
598	const strength = Math.min(1, landing / 900);
599	if (strength > 0.12) impacts.push({ x: b.x, y: b.y, strength });
600	}
601	}
602
603	b.x += b.vx * dt;
604	b.y += b.vy * dt;
605	b.angle += b.spin * dt;
606
607	// Turning over. In the air it tumbles; once it is down, whatever is left of
608	// the turn is taken up so it ends flat on its back with its face showing.
609	if (b.flipSpin !== 0) {
610	if (b.z > 0) {
611	b.flip += b.flipSpin * dt;
612	// The axis it turns over about is a line drawn on the tile, so it goes
613	// round with the tile: a discard that is spinning as well as tumbling
614	// does not keep tumbling across the same line of the table.
615	b.flipAxis += b.spin * dt;
616	} else {
617	const target = Math.round(b.flip / TAU) * TAU;
618	b.flip += (target - b.flip) * Math.min(1, FLIP_SETTLE * dt);
619	if (Math.abs(target - b.flip) < 0.01) {
620	b.flip = target;
621	b.flipSpin = 0;
622	}
623	}
624	}
625
626	if (!b.entered) {
627	// Kept from step to step: on the step it finally gets in it is inside
628	// already, and there would be nothing left to read.
629	b.over = outsideEdge(world.bounds, b) ?? b.over;
630	// In means all of it in. Anything less and keepInBounds below would
631	// finish the job in a single frame, which is a collision with nothing.
632	if (wholly(world.bounds, b)) {
633	b.entered = true;
634	// It came in over somebody's side of the table. Said quietly — nothing
635	// was struck, so there is nothing to hear — but said, because a tile
636	// arriving past a seat is that seat's to complain about. Its own
637	// thrower's edge is every tile's way in, and the caller drops that one.
638	if (b.over) {
639	impacts.push({ x: b.x, y: b.y, strength: ARRIVED, edge: b.over, seat: b.seat });
640	}
641	b.over = undefined;
642	} else if (b.age > ENTRY_GRACE) {
643	// Out of time going the long way round. Put it in, gently — but this is
644	// still a tile that ended up off the table on somebody's side.
645	if (b.over) {
646	impacts.push({ x: b.x, y: b.y, strength: ARRIVED, edge: b.over, seat: b.seat });
647	}
648	b.entered = true;
649	b.over = undefined;
650	b.z = 0;
651	b.vz = 0;
652	clampInto(world.bounds, b);
653	}
654	}
655
656	// --- on the felt ---
657	// Strictly z <= 0, not ON_FELT: friction is between the tile and the cloth,
658	// so a tile still in the air keeps the speed it was thrown with. ON_FELT is
659	// only about being low enough to be in another tile's way.
660	if (b.z <= 0) {
661	damp(b, dt);
662	// A tile that fell short skids in rather than sitting outside the pool.
663	if (!b.entered) pullIn(world, b, dt);
664	}
665
666	// A tile still above the wall is clear of it.
667	if (b.z < WALL_HEIGHT) {
668	let inOwnHand = false;
669	for (const wall of world.walls) {
670	if (wall.seat === b.seat && !b.leftSeat) {
671	// Still inside the row it was thrown from: note it, but pass through.
672	const over = overlapOf(boxOf(wall.rect), b);
673	if (over && over.depth > CONTACT_SLOP) inOwnHand = true;
674	continue;
675	}
676	hitWall(b, wall, impacts);
677	}
678	if (!inOwnHand) b.leftSeat = true;
679	}
680
681	// The pool's edge is the inside face of the wall, and it is only as tall as
682	// the wall — a tile lobbed high is over the top of it, not stopped by it.
683	if (b.entered && b.z < WALL_HEIGHT) keepInBounds(world, b, impacts);
684	}
685
686	collideTiles(world, impacts);
687	// Being shoved by a neighbour can put a tile back over the edge, and a tile
688	// that then fell asleep out there would stay out there. Only awake ones need
689	// it: resolvePair never moves a tile that has settled.
690	// Resting tiles included: they get pushed about by the untangling above, and
691	// a tile shoved over the edge must not be left there just because it is asleep.
692	for (const b of world.bodies) {
693	if (b.entered && b.z < WALL_HEIGHT) clampInto(world.bounds, b);
694	}
695	// Deciding to sleep comes last, so it judges where the step actually left
696	// things — before, a contact resolved after the decision would undo it and a
697	// crowded pool would stay awake for ever.
698	for (const b of world.bodies) if (!b.resting) trySleep(b);
699
700	// A pile that has stopped moving but is still untangling itself keeps being
701	// stepped, up to a point. Without the limit a configuration that cannot quite
702	// be resolved would keep the loop running for the rest of the hand.
703	world.relaxed = world.contacts > 0 && world.bodies.every((b) => b.resting) ? world.relaxed + 1 : 0;
704	return impacts;
705	}
706
707	/**
708	* The table has been re-measured — the window resized, or the wall was eaten
709	* further back. The tiles keep where they are, but a pool that shrank must not
710	* leave any of them stranded outside it, and the ones it moves have to wake up
711	* so they can settle again.
712	*/
713	export function reshape(world: World, bounds: Rect, walls: Barrier[]) {
714	world.bounds = bounds;
715	world.walls = walls;
716	for (const b of world.bodies) {
717	if (!b.entered) continue;
718	const x = b.x;
719	const y = b.y;
720	clampInto(bounds, b);
721	if (b.x !== x \|\| b.y !== y) wake(b);
722	}
723
724	// A wall that has moved may be standing where a tile is lying. Anything it
725	// has come to overlap is woken, and the solver shoves it clear over the next
726	// few steps — which is a wall being pushed across a table taking the discards
727	// along in front of it, and is the whole reason you would push one.
728	//
729	// Only what is awake gets looked at each step, so without this a tile that
730	// had gone to sleep would sit inside the wall and stay there.
731	for (const b of world.bodies) {
732	if (b.z >= WALL_HEIGHT \|\| b.carried) continue;
733	for (const w of world.walls) {
734	// Your own tiles are not solid to you until you are clear of them.
735	if (w.seat === b.seat && !b.leftSeat) continue;
736	const hit = overlapOf(boxOf(w.rect), b);
737	if (hit && hit.depth > CONTACT_SLOP) {
738	wake(b);
739	break;
740	}
741	}
742	}
743	}
744
745	/**
746	* The felt, on a tile that is both skating and turning.
747	*
748	* There is one patch of cloth under the tile and it can only do one thing at a
749	* time: what it spends stopping the slide it does not have left to stop the
750	* turn, and the other way about. So the constant part of the friction, the part
751	* that actually brings a tile to a stop rather than to an asymptote, is shared
752	* out between the two in proportion to how much of the rubbing each accounts
753	* for, the turn counted at `gripRadius` so the two are in the same units.
754	*
755	* A tile doing only one of them is damped exactly as hard as it always was:
756	* all of `FELT_STOP` goes to a pure slide, all of `SPIN_STOP` to a pure spin.
757	* What is new is in between, and what it buys is not distance, which is a few
758	* per cent, but the ending. Two brakes running side by side stop at two
759	* different moments, so a tile flicked hard with a little turn on it stops
760	* turning early and skates the rest of the way looking dead, and one barely
761	* pushed with a lot of turn on it stops dead and goes on spinning where it
762	* lies. Sharing the one budget ends them together, which is what a tile
763	* flicked across a table does.
764	*/
765	function damp(b: Body, dt: number) {
766	const k = Math.exp(-FELT_DAMP * dt);
767	b.vx *= k;
768	b.vy *= k;
769	b.spin = Math.exp(-SPIN_DAMP dt);
770
771	const speed = Math.hypot(b.vx, b.vy);
772	const rim = Math.abs(b.spin) * gripRadius(b);
773	const rubbing = speed + rim;
774	if (rubbing <= 0) return;
775
776	if (speed > 0) {
777	const next = Math.max(0, speed - FELT_STOP * (speed / rubbing) * dt);
778	b.vx = (b.vx / speed) * next;
779	b.vy = (b.vy / speed) * next;
780	}
781	if (rim > 0) {
782	const spin = Math.abs(b.spin);
783	b.spin = Math.sign(b.spin) * Math.max(0, spin - SPIN_STOP * (rim / rubbing) * dt);
784	}
785	}
786
787	/** Nudges a tile that landed outside the pool towards the rest of them. */
788	function pullIn(world: World, b: Body, dt: number) {
789	const cx = world.bounds.x + world.bounds.w / 2;
790	const cy = world.bounds.y + world.bounds.h / 2;
791	const dx = cx - b.x;
792	const dy = cy - b.y;
793	const d = Math.hypot(dx, dy) \|\| 1;
794	b.vx += (dx / d) * ENTRY_PULL * dt;
795	b.vy += (dy / d) * ENTRY_PULL * dt;
796	}
797
798	/** Bounce off whatever is standing there, corner to corner if that is how it
799	* hits — a stack of the wall, a laid-down set, somebody's hand. */
800	function hitWall(b: Body, wall: Barrier, impacts: Impact[]) {
801	const box = boxOf(wall.rect);
802	const hit = overlapOf(box, b);
803	if (!hit \|\| hit.depth <= CONTACT_SLOP) return;
804
805	const speed = Math.hypot(b.vx, b.vy);
806	b.x += hit.nx * hit.depth;
807	b.y += hit.ny * hit.depth;
808
809	// A stack does not move, so the whole of the contact is the tile's. What is
810	// asked of it is the speed of the touching corner rather than of the
811	// middle: a tile that is already turning arrives at the wall faster on one
812	// side than the other, and that is what decides the bounce.
813	const p = contactOf(b, hit.nx, hit.ny, box);
814	const rx = p.x - b.x;
815	const ry = p.y - b.y;
816	const invI = invInertia(b);
817	const cvx = b.vx - b.spin * ry;
818	const cvy = b.vy + b.spin * rx;
819	const into = cvx * hit.nx + cvy * hit.ny;
820	if (into < 0) {
821	// How much of the knock the corner can take before the tile simply turns
822	// out of the way instead: a hit through the middle has none of this and
823	// rebounds whole, a hit on a corner spends most of itself on the turn.
824	const rn = rx * hit.ny - ry * hit.nx;
825	const j = (-(1 + BOUND_RESTITUTION) * into) / (1 + rn * rn * invI);
826	b.vx += hit.nx * j;
827	b.vy += hit.ny * j;
828	b.spin += rn * j * invI;
829
830	// And along the face. A tile skating past the corner of a stack is dragged
831	// round by it, which is the turn you actually see; how much is capped by
832	// how hard it went in, so a brush is a brush and not a stop.
833	const tx = -hit.ny;
834	const ty = hit.nx;
835	const rt = rx * ty - ry * tx;
836	const slip = cvx * tx + cvy * ty;
837	if (slip !== 0) {
838	const jt =
839	-Math.sign(slip) *
840	Math.min(Math.abs(slip) / (1 + rt * rt * invI), TILE_FRICTION * j);
841	b.vx += tx * jt;
842	b.vy += ty * jt;
843	b.spin += rt * jt * invI;
844	}
845	}
846	b.resting = false;
847	b.still = 0;
848	const strength = Math.min(1, speed / 1100);
849	if (wall.seat !== undefined) {
850	// Into somebody's tiles. Reported however gently it arrived — their row is
851	// theirs, and a discard rolling into it is an event whatever speed it got
852	// there at. Which of them are loud enough to hear is the caller's business.
853	if (speed > SLEEP_SPEED) {
854	impacts.push({ x: b.x, y: b.y, strength, into: wall.seat, seat: b.seat });
855	}
856	} else if (strength > 0.12) {
857	impacts.push({ x: b.x, y: b.y, strength });
858	}
859	}
860
861	/** The pool's edge. Once a tile is in, it stays in — this is the invariant the
862	* tests pin, since a discard that slid off the table would be lost. */
863	function keepInBounds(world: World, b: Body, impacts: Impact[]) {
864	const { minX, maxX, minY, maxY } = limitsOf(world.bounds, b);
865
866	let hit = 0;
867	let edge: Edge \| undefined;
868	// Which way the edge pushed back, kept so the slew below knows which corner of
869	// the tile it was that arrived.
870	let nx = 0;
871	let ny = 0;
872	if (b.x < minX) {
873	b.x = minX;
874	hit = Math.max(hit, Math.abs(b.vx));
875	edge = 'left';
876	nx = 1;
877	b.vx = Math.abs(b.vx) * BOUND_RESTITUTION;
878	b.vy *= BOUND_FRICTION;
879	} else if (b.x > maxX) {
880	b.x = maxX;
881	hit = Math.max(hit, Math.abs(b.vx));
882	edge = 'right';
883	nx = -1;
884	b.vx = -Math.abs(b.vx) * BOUND_RESTITUTION;
885	b.vy *= BOUND_FRICTION;
886	}
887	if (b.y < minY) {
888	b.y = minY;
889	if (Math.abs(b.vy) > hit) {
890	edge = 'top';
891	nx = 0;
892	ny = 1;
893	}
894	hit = Math.max(hit, Math.abs(b.vy));
895	b.vy = Math.abs(b.vy) * BOUND_RESTITUTION;
896	b.vx *= BOUND_FRICTION;
897	} else if (b.y > maxY) {
898	b.y = maxY;
899	if (Math.abs(b.vy) > hit) {
900	edge = 'bottom';
901	nx = 0;
902	ny = -1;
903	}
904	hit = Math.max(hit, Math.abs(b.vy));
905	b.vy = -Math.abs(b.vy) * BOUND_RESTITUTION;
906	b.vx *= BOUND_FRICTION;
907	}
908
909	// A tile does not meet a straight edge flat unless it happens to be lying
910	// square to it: what arrives is one corner, and the rest of the tile swings
911	// on round it. Only on a real knock: a tile held against the edge by its
912	// neighbours is clamped every step, and winding it up on each of those would
913	// leave the pool turning for ever.
914	if (hit > SLEEP_SPEED) {
915	const p = contactOf(b, nx, ny);
916	const invI = invInertia(b);
917	const rn = (p.x - b.x) * ny - (p.y - b.y) * nx;
918	b.spin += (rn * hit * (1 + BOUND_RESTITUTION) * invI) / (1 + rn * rn * invI);
919	}
920	// Only a real knock counts as being disturbed. A tile held against the edge
921	// by its neighbours is clamped every single step, and treating that as a knock
922	// would keep the whole pool awake for ever.
923	//
924	// Anything that does arrive under its own steam is reported, however gently:
925	// the edge is where somebody's tiles are, so a discard rolling into them is
926	// an event whatever speed it got there at. Which of them are loud enough to
927	// be worth a sound is the caller's business.
928	if (hit > SLEEP_SPEED) {
929	b.still = 0;
930	impacts.push({ x: b.x, y: b.y, strength: Math.min(1, hit / 1100), edge, seat: b.seat });
931	}
932	}
933
934	/**
935	* Tile against tile.
936	*
937	* Only tiles flat on the felt take part, so a thrown tile passes over the pile
938	* instead of shouldering through it, and a cheap circle test throws out the pairs
939	* that are nowhere near before the real rectangle test runs. Pairs where both
940	* tiles have settled are left alone entirely, which is nearly all of them.
941	*/
942	function collideTiles(world: World, impacts: Impact[]) {
943	const bodies = world.bodies;
944	for (let i = 0; i < bodies.length; i++) {
945	const a = bodies[i];
946	if (a.z > ON_FELT) continue;
947	for (let j = i + 1; j < bodies.length; j++) {
948	const b = bodies[j];
949	if (b.z > ON_FELT) continue;
950	if (Math.hypot(b.x - a.x, b.y - a.y) > circumradius(a) + circumradius(b)) continue;
951	// Two tiles that have both settled are still pushed apart if they are
952	// inside each other — they are just not woken up for it. Skipping the pair
953	// outright was how a pile could end up with tiles sharing ground and stay
954	// that way, since nothing steps a pile that has stopped moving.
955	resolvePair(a, b, impacts, world);
956	}
957	}
958	}
959
960	function resolvePair(a: Body, b: Body, impacts: Impact[], world: World) {
961	const hit = overlapOf(a, b);
962	if (!hit) return;
963	// Resting in contact is not a collision. See CONTACT_SLOP.
964	const depth = hit.depth - CONTACT_SLOP;
965	if (depth <= 0) return;
966	const { nx, ny } = hit;
967
968	world.contacts++;
969	// Push apart. A tile that has settled is the heavy one: a tile thrown into a
970	// pool shoulders in, rather than the pool scattering out of its way.
971	const aShare = a.resting === b.resting ? 0.5 : a.resting ? 0 : 1;
972	const bShare = 1 - aShare;
973	a.x -= nx * depth * aShare;
974	a.y -= ny * depth * aShare;
975	b.x += nx * depth * bShare;
976	b.y += ny * depth * bShare;
977
978	// Where they are actually touching, and how far that is off each middle. A
979	// tile shouldered square in the back is pushed along; the same tile caught on
980	// one corner is turned instead, and it is only this that tells them apart.
981	const p = contactOf(b, nx, ny, a);
982	const rax = p.x - a.x;
983	const ray = p.y - a.y;
984	const rbx = p.x - b.x;
985	const rby = p.y - b.y;
986	const invIa = invInertia(a);
987	const invIb = invInertia(b);
988
989	// Closing speed at the contact rather than between the middles, so a tile
990	// that is already turning arrives with whichever corner is coming round.
991	//
992	// Only a genuine approach speed counts, and only that wakes anything. Nudging
993	// two touching tiles apart is not a collision: a jammed corner of the pool is
994	// corrected by a fraction of a pixel every step for as long as it is jammed,
995	// and treating each of those as a shove would keep the pile awake for ever.
996	const rvx = b.vx - b.spin * rby - (a.vx - a.spin * ray);
997	const rvy = b.vy + b.spin * rbx - (a.vy + a.spin * rax);
998	const along = rvx * nx + rvy * ny;
999	if (along >= -SLEEP_SPEED) return;
1000
1001	// Two tiles of the same weight, so the shove splits in half, plus whatever
1002	// each of them can turn away with, which is the part that was a constant
1003	// before and is now the geometry of the hit.
1004	const ran = rax * ny - ray * nx;
1005	const rbn = rbx * ny - rby * nx;
1006	const jolt =
1007	(-along * (1 + TILE_RESTITUTION)) / (2 + ran * ran * invIa + rbn * rbn * invIb);
1008	a.vx -= nx * jolt;
1009	a.vy -= ny * jolt;
1010	a.spin -= ran * jolt * invIa;
1011	b.vx += nx * jolt;
1012	b.vy += ny * jolt;
1013	b.spin += rbn * jolt * invIb;
1014
1015	// Across the face of it. One tile skidding past another drags on it, and each
1016	// takes the turn that drag has the leverage for. The tiles in a pool ending
1017	// up at every angle to each other is this, and nothing else.
1018	const tx = -ny;
1019	const ty = nx;
1020	const slip = rvx * tx + rvy * ty;
1021	if (slip !== 0) {
1022	const rat = rax * ty - ray * tx;
1023	const rbt = rbx * ty - rby * tx;
1024	const jt =
1025	-Math.sign(slip) *
1026	Math.min(
1027	Math.abs(slip) / (2 + rat * rat * invIa + rbt * rbt * invIb),
1028	TILE_FRICTION * jolt,
1029	);
1030	a.vx -= tx * jt;
1031	a.vy -= ty * jt;
1032	a.spin -= rat * jt * invIa;
1033	b.vx += tx * jt;
1034	b.vy += ty * jt;
1035	b.spin += rbt * jt * invIb;
1036	}
1037
1038	const strength = Math.min(1, Math.abs(along) / 900);
1039	if (strength > 0.12) impacts.push({ x: b.x, y: b.y, strength });
1040
1041	// Whatever was asleep has been shoved, so it is awake now.
1042	wake(a);
1043	wake(b);
1044	}
1045
1046	function wake(b: Body) {
1047	if (b.resting) b.awake = 0;
1048	b.resting = false;
1049	b.still = 0;
1050	}
1051
1052	function sleep(b: Body) {
1053	b.vx = 0;
1054	b.vy = 0;
1055	b.spin = 0;
1056	b.z = 0;
1057	b.vz = 0;
1058	// Face up, always. A tile lying face down in the discards would be a tile
1059	// nobody can read, so this is a guarantee and not an easing.
1060	b.flip = Math.round(b.flip / TAU) * TAU;
1061	b.flipSpin = 0;
1062	b.resting = true;
1063	b.awake = 0;
1064	}
1065
1066	function trySleep(b: Body) {
1067	const speed = Math.hypot(b.vx, b.vy);
1068	const slow =
1069	b.entered &&
1070	b.z <= 0 &&
1071	b.vz === 0 &&
1072	b.flipSpin === 0 &&
1073	speed < SLEEP_SPEED &&
1074	Math.abs(b.spin) < SLEEP_SPIN;
1075	if (slow && ++b.still >= SLEEP_STEPS) return sleep(b);
1076	if (!slow) b.still = 0;
1077	// Backstop: shuffling about in a crowded pool for this long is settled enough.
1078	if (b.entered && b.awake > AWAKE_LIMIT && speed < SLEEP_SPEED * 4) sleep(b);
1079	}
1080
1081	/**
1082	* Make room for a tile placed straight into a settled pile, without running
1083	* time. Used when a pile is rebuilt — a reload, an undo — where the tiles have
1084	* to be somewhere sensible immediately and nothing should appear to move.
1085	*/
1086	function separate(world: World, body: Body, passes = 16) {
1087	// Which way to go when two tiles are exactly on top of each other. Stepping
1088	// by the golden angle fans successive tiles out in every direction, where a
1089	// fixed axis would march them all one way and pack them against a pool edge.
1090	const escape = world.bodies.indexOf(body) * 2.39996;
1091
1092	for (let p = 0; p < passes; p++) {
1093	let moved = false;
1094	for (const other of world.bodies) {
1095	if (other === body) continue;
1096	const hit = overlapOf(other, body);
1097	if (!hit \|\| hit.depth <= CONTACT_SLOP) continue;
1098	if (Math.hypot(body.x - other.x, body.y - other.y) < 0.0001) {
1099	body.x += Math.cos(escape + p) * hit.depth;
1100	body.y += Math.sin(escape + p) * hit.depth;
1101	} else {
1102	body.x += hit.nx * hit.depth;
1103	body.y += hit.ny * hit.depth;
1104	}
1105	moved = true;
1106	}
1107	clampInto(world.bounds, body);
1108	if (!moved) break;
1109	}
1110	}
1111
1112	function clampInto(r: Rect, b: Body) {
1113	const { minX, maxX, minY, maxY } = limitsOf(r, b);
1114	b.x = Math.min(maxX, Math.max(minX, b.x));
1115	b.y = Math.min(maxY, Math.max(minY, b.y));
1116	}
1117
1118	/**
1119	* How long a motionless pile is allowed to keep untangling itself before the
1120	* result is accepted as good enough. Anything still overlapping after this is
1121	* doing so by a fraction of a pixel, and the loop has better things to do than
1122	* keep running over it.
1123	*/
1124	const RELAX_LIMIT = 90;
1125
1126	/** Whether everything has settled — what the render loop idles on. */
1127	export const settled = (world: World) =>
1128	world.bodies.every((b) => b.resting && b.scale === 1) &&
1129	(world.contacts === 0 \|\| world.relaxed > RELAX_LIMIT);
1130
1131	/** How square-on the face is: 1 flat on its back, 0 edge-on, negative face down.
1132	* The renderer squashes the tile by this along its tumbling axis. */
1133	export const faceOn = (b: Body) => Math.cos(b.flip);
1134
1135	/**
1136	* A tile's own bounding box, upright. Not what it collides with — that is the
1137	* rotated rectangle — but what the felt's straight edges are worked out from,
1138	* so the overlay draws both and the difference is visible.
1139	*/
1140	export function aabbOf(b: Body): Rect {
1141	const half = aabbHalf(b);
1142	return { x: b.x - half.x, y: b.y - half.y, w: half.x * 2, h: half.y * 2 };
1143	}
1144
1145	/**
1146	* Where this tile's centre is allowed to be, which is the edge less its own
1147	* half. This is the line a bounce actually happens at, and it sits half a tile
1148	* inside the one you can see — which is why a tile looks like it turned around
1149	* early. For the overlay; the real work is `limitsOf`.
1150	*/
1151	export function centreLimits(r: Rect, b: Body): Rect {
1152	const l = limitsOf(r, b);
1153	return { x: l.minX, y: l.minY, w: l.maxX - l.minX, h: l.maxY - l.minY };
1154	}
1155
1156	/** How far outside the pool a tile is sitting. The tests' invariant. */
1157	export function outside(b: Body, pool: Rect): number {
1158	const half = aabbHalf(b);
1159	return Math.max(
1160	0,
1161	pool.x + half.x - b.x,
1162	b.x - (pool.x + pool.w - half.x),
1163	pool.y + half.y - b.y,
1164	b.y - (pool.y + pool.h - half.y),
1165	);
1166	}
1167
1168	/**
1169	* How high a tile is held while it is being dragged. Above `WALL_HEIGHT` so it
1170	* clears the wall, and above `ON_FELT` so it is out of the way of every tile it
1171	* is carried over — the pile does not part in front of a finger, the tile goes
1172	* over the top of it, which is what picking one up is.
1173	*/
1174	export const CARRY_Z = 46;
1175
1176	/** The topmost tile under a point, or null. What a finger has taken hold of.
1177	* Last first: later tiles were thrown later and lie on top. */
1178	export function tileAt(world: World, x: number, y: number): Body \| null {
1179	for (let i = world.bodies.length - 1; i >= 0; i--) {
1180	const b = world.bodies[i];
1181	const c = Math.cos(-b.angle);
1182	const s = Math.sin(-b.angle);
1183	const dx = x - b.x;
1184	const dy = y - b.y;
1185	// Into the tile's own frame, where the test is a rectangle again.
1186	if (Math.abs(dx * c - dy * s) <= b.w / 2 && Math.abs(dx * s + dy * c) <= b.h / 2) return b;
1187	}
1188	return null;
1189	}
1190
1191	/** Taken out of the pile and held above it. */
1192	export function lift(b: Body) {
1193	b.carried = true;
1194	b.resting = false;
1195	b.z = CARRY_Z;
1196	b.vx = 0;
1197	b.vy = 0;
1198	b.vz = 0;
1199	b.spin = 0;
1200	b.flipSpin = 0;
1201	// Whatever it comes down on, it comes down face up, as a tile does.
1202	b.flip = Math.round(b.flip / TAU) * TAU;
1203	}
1204
1205	/** Let go of it: dropped where it is, or sent off at whatever it was flicked. */
1206	export function drop(b: Body, vx: number, vy: number, spin: number) {
1207	b.carried = false;
1208	b.vx = vx;
1209	b.vy = vy;
1210	b.spin = spin;
1211	// It was being held up; from here it simply falls.
1212	b.vz = 0;
1213	b.still = 0;
1214	b.awake = 0;
1215	b.resting = false;
1216	// Thrown from inside the pile rather than from a hand, so it is already past
1217	// every seat's tiles and they are solid to it straight away.
1218	b.leftSeat = true;
1219	b.entered = true;
1220	}
1221
1222	/** Whether two tiles are sharing ground, which they must never do at rest. */
1223	export function overlapping(a: Body, b: Body): number {
1224	const hit = overlapOf(a, b);
1225	return hit ? hit.depth : 0;
1226	}
1227
1228	/** Scale a tile is drawn at, given how high it is. Sells the arc on a flat
1229	* canvas; the renderer offsets the shadow by the same height. */
1230	export const zScale = (z: number) => 1 + z * 0.0016;
1231
1232	/**
1233	* The upward kick that puts a tile back on the felt after `seconds` in the air.
1234	* How a throw is aimed: pick how long it should take, and the horizontal speed
1235	* follows from the distance.
1236	*/
1237	export const liftFor = (seconds: number) => (GRAVITY * seconds) / 2;
1238
1239	/** How high that kick gets — worth checking against WALL_HEIGHT. */
1240	export const peakOf = (vz: number) => (vz * vz) / (2 * GRAVITY);