Sistem fluida piksel — permukaan berpegas, gelembung, air sel, benda apung

// 17-fluid.js — tiga model fluida murah untuk piksel: (1) PX.Surface = permukaan 1D berpegas, (2) PX.Bubbles = gelembung dari ventilasi, (3) PX.CAWater = air 2D berbasis sel (automata) + pelengkap: PX.Foam (busa), PX.Fizz (gelembung mikro).

// (1) Permukaan: tiap kolom = pegas (Hooke) + penyebaran ke tetangga. h>0 = naik. Langkah tetap 1/60 dtk agar stabil.
PX.Surface = class {
  constructor(n, base, o = {}) {
    this.n = n; this.base = base; this.h = new Float32Array(n); this.v = new Float32Array(n); this.dl = new Float32Array(n); this.dr = new Float32Array(n);
    this.k = o.k ?? 0.03; this.damp = o.damp ?? 0.03; this.spread = o.spread ?? 0.22; this.acc = 0;
  }
  splash(x, f) { const i = PX.clamp(Math.round(x), 0, this.n - 1); this.v[i] += f; if (i > 0) this.v[i - 1] += f * 0.5; if (i < this.n - 1) this.v[i + 1] += f * 0.5; }   // f<0 = ditekan ke bawah
  y(x) { x = PX.clamp(x, 0, this.n - 1); const i = Math.floor(x), j = Math.min(this.n - 1, i + 1), f = x - i; return this.base - (this.h[i] * (1 - f) + this.h[j] * f); }
  update(dt) { this.acc += dt; while (this.acc >= 1 / 60) { this.acc -= 1 / 60; this.step(); } }
  step() {
    const { n, h, v, dl, dr, k, damp, spread } = this;
    for (let i = 0; i < n; i++) { v[i] += -k * h[i] - damp * v[i]; h[i] += v[i]; }                    // pegas ke tinggi 0 + redaman
    for (let pass = 0; pass < 4; pass++) {                                                            // sebar: tetangga ditarik ke selisih tinggi
      for (let i = 0; i < n; i++) { if (i > 0) { dl[i] = spread * (h[i] - h[i - 1]); v[i - 1] += dl[i]; } if (i < n - 1) { dr[i] = spread * (h[i] - h[i + 1]); v[i + 1] += dr[i]; } }
      for (let i = 0; i < n; i++) { if (i > 0) h[i - 1] += dl[i]; if (i < n - 1) h[i + 1] += dr[i]; }
    }
  }
};

// (2) Gelembung (dari sistem Sprite Fusion, ditulis ulang): ventilasi -> naik, goyang, tumbuh, menyatu (volume kekal), pecah di permukaan.
PX.Bubbles = class {
  constructor({ x0, x1, top, ground, surfY, vents, density = 1, seed = 1 }) {
    Object.assign(this, { x0, x1, top, ground, D: density, surfY: surfY ?? (() => top), t: 0, list: [], pops: [] });
    const R = (this.R = PX.rng(seed)); this.rg = (a, b) => a + (b - a) * R();
    this.vents = (vents ?? [0.17, 0.5, 0.83].map((f) => x0 + f * (x1 - x0))).map((x) => ({ x, acc: R(), burst: this.rg(0.5, 2.5), seed: this.rg(0, 100) }));
    for (const v of this.vents) for (let y = ground(v.x) - 4; y > top; y -= this.rg(9, 23) / density) {      // isi awal sepanjang kolom supaya adegan tidak kosong
      this.spawn(v, y, 0); const b = this.list[this.list.length - 1]; b.r *= 1 + (1 - (y - top) / (ground(v.x) - top)) * 0.4; b.vy = -(20 + 13 * b.r);
    }
  }
  spawn(v, y, big) {
    const R = this.R, rg = this.rg, r = big ? rg(1.5, 3.2) * big : R() < 0.7 ? rg(0.5, 1.4) : rg(1.4, 2.6);
    this.list.push({ x0: v.x + rg(-2, 2), x: v.x, y, r, vy: 0, ph: rg(0, 6.283), wob: 0, near: R() < 0.35 });
  }
  add(x, y, r) { this.list.push({ x0: x, x, y, r, vy: -(20 + 13 * r), ph: this.rg(0, 6.283), wob: 0, near: false }); }   // gelembung dari sumber lain (benda jatuh)
  update(dt) {
    const L = this.list, rg = this.rg, n1 = (x, s) => PX.vnoise(x, s * 3.7) * 2 - 1; this.t += dt; const t = this.t;
    for (const v of this.vents) {
      v.acc += dt * 5 * this.D * (0.7 + 0.3 * n1(t * 0.8 + v.seed, 3));                                           // laju spawn berfluktuasi pelan
      for (; v.acc > 1; v.acc--) this.spawn(v, this.ground(v.x) - 2, 0);
      if ((v.burst -= dt) < 0) { v.burst = rg(1.4, 3.2); const n = 5 + Math.floor(this.R() * 6); for (let i = 0; i < n; i++) this.spawn(v, this.ground(v.x) - 2 - i * 2.5, rg(0.6, 1)); }   // semburan
    }
    for (let i = L.length - 1; i >= 0; i--) {
      const b = L[i];
      b.vy += (-(20 + 13 * b.r) - b.vy) * Math.min(1, dt * 3);                                                     // kecepatan naik ∝ ukuran, mendekati bertahap
      b.y += b.vy * dt; b.ph += dt * (4 + 2 / b.r);
      b.x = b.x0 + Math.sin(b.ph) * (0.5 + 0.8 * b.r); b.x0 += n1(t * 0.4 + i * 0.13, 5) * 3 * dt;                 // goyang kiri-kanan + hanyut
      b.r = Math.min(5, b.r + dt * 0.12); b.wob *= 1 - Math.min(1, dt * 4);                                         // tumbuh (tekanan turun)
      if (b.y < this.surfY(b.x) + 1) { this.pops.push({ x: b.x, y: this.surfY(b.x), r: b.r, t: 0 }); L.splice(i, 1); }
    }
    for (let i = 0; i < L.length; i++) {                                                                           // menyatu: volume kekal r = ∛(ra³+rb³)
      const a = L[i]; if (a.r < 1) continue;
      for (let j = i + 1; j < L.length; j++) { const b = L[j]; if (b.r < 1 || Math.hypot(a.x - b.x, a.y - b.y) > (a.r + b.r) * 0.75) continue; a.r = Math.cbrt(a.r ** 3 + b.r ** 3); a.x0 = (a.x0 + b.x0) / 2; a.wob = 1; L.splice(j--, 1); }
    }
    for (let i = this.pops.length - 1; i >= 0; i--) if ((this.pops[i].t += dt) > 0.2) this.pops.splice(i, 1);
  }
  draw(buf, lit, layer) {
    if (layer !== 'front') { for (const b of this.list) if (!b.near) this.bubble(buf, lit, b); for (const p of this.pops) this.pop(buf, p); }
    if (layer !== 'back') for (const b of this.list) if (b.near) this.bubble(buf, lit, b);
  }
  bubble(buf, lit, b) {                                              // ukuran menentukan detail: <0.9 = 1px, <1.5 = 2×2 empat warna, lainnya = cincin + highlight
    const { HI, LIT, RIM, SHADE } = PX.WATER, FILL = PX.mix(RIM, SHADE, 0.5), cx = b.x, cy = b.y, r = b.r;
    if (r < 0.9) { buf.px(Math.floor(cx), Math.floor(cy), r < 0.7 ? RIM : LIT); return; }
    if (r < 1.5) { const px = Math.floor(cx), py = Math.floor(cy); buf.px(px, py, HI); buf.px(px + 1, py, LIT); buf.px(px, py + 1, RIM); buf.px(px + 1, py + 1, SHADE); lit.glow(cx + 0.5, cy + 0.5, 3, LIT, 0.12); return; }
    const sq = 0.1 * Math.sin(b.ph * 2) * Math.min(1, (r - 1.5) / 2) + b.wob * 0.25, rx = r * (1 + sq), ry = r * (1 - sq);   // bentuk bergetar (gepeng bergantian) + kejut saat menyatu
    for (let y = Math.floor(cy - ry - 1); y <= Math.ceil(cy + ry + 1); y++) for (let x = Math.floor(cx - rx - 1); x <= Math.ceil(cx + rx + 1); x++) {
      const dx = (x + 0.5 - cx) / rx, dy = (y + 0.5 - cy) / ry, d = Math.sqrt(dx * dx + dy * dy) * r; if (d > r + 0.5) continue;
      const l = (-dx - dy) / Math.SQRT2 / Math.max(0.01, d / r);                                                    // + = menghadap cahaya kiri-atas
      if (d > r - 0.6) buf.px(x, y, l > 0.45 ? LIT : l < -0.55 ? SHADE : RIM); else if (PX.bayer(x, y) < 0.12) buf.px(x, y, FILL);   // tepi 1px; isi hanya 12% dither
    }
    const hx = Math.round(cx - rx * 0.45), hy = Math.round(cy - ry * 0.45);
    buf.px(hx, hy, HI); if (r >= 2.5) { buf.px(hx + 1, hy, LIT); buf.px(hx, hy + 1, LIT); } if (r >= 2) buf.px(Math.round(cx + rx * 0.5), Math.round(cy + ry * 0.5), LIT);
    lit.glow(cx, cy, r * 2.4 + 2, RIM, 0.14);
  }
  pop(buf, p) {                                                      // pecah: setengah elips percikan melebar + 2 tetes naik, memudar via Bayer
    const k = p.t / 0.2, r = p.r + 1 + k * 3, dot = (x, y, c) => { const px = Math.floor(x), py = Math.floor(y); if (1 - k > PX.bayer(px, py)) buf.px(px, py, c); };
    for (let a = 0; a < 6.283; a += 0.5 / r) if (Math.sin(a) < 0.2) dot(p.x + Math.cos(a) * r, p.y + Math.sin(a) * r * 0.5, k < 0.5 ? PX.WATER.HI : PX.WATER.LIT);
    if (p.r > 1.5) { dot(p.x - 2, p.y - 2 - k * 4, PX.WATER.LIT); dot(p.x + 2, p.y - 3 - k * 5, PX.WATER.LIT); }
  }
};

// (3) Air sel (cellular automata). g: 0 kosong, 1 air, 2 tembok. Pindai bawah->atas, arah kiri/kanan bergantian tiap langkah agar tidak miring sebelah.
// fall[i] = berapa langkah sel itu jatuh berturut-turut. Saat berhenti (mendarat) dicatat ke ev = {x, y, v, water} -> sumber percikan, gelombang, gelembung.
PX.CAWater = class {
  constructor(w, h, seed = 3) { this.w = w; this.h = h; this.g = new Uint8Array(w * h); this.m = new Uint8Array(w * h); this.fall = new Uint8Array(w * h); this.ev = []; this.lv = []; this.lvRate = 6; this.stamp = 0; this.dir = 0; this.flow = 4; this.R = PX.rng(seed); }
  step() {
    const { w, h, g, m, fall: fl, ev } = this; if (++this.stamp > 250) { this.stamp = 1; m.fill(0); } const st = this.stamp; this.dir ^= 1;
    for (let y = h - 2; y >= 0; y--) for (let k = 0; k < w; k++) {
      const x = this.dir ? k : w - 1 - k, i = y * w + x; if (g[i] !== 1 || m[i] === st) continue;       // m: sel yang baru dipindah langkah ini jangan diproses lagi
      const b = i + w, d = this.R() < 0.5 ? -1 : 1; let to = -1;
      if (g[b] === 0) to = b;                                                                          // 1) jatuh lurus
      else {
        for (let q = 0; q < 2 && to < 0; q++) { const s = q ? -d : d; if (x + s >= 0 && x + s < w && g[b + s] === 0 && g[i + s] === 0) to = b + s; }   // 2) diagonal bawah
        for (let q = 0; q < 2 && to < 0; q++) { const s = q ? -d : d; let j = 0; while (j < this.flow && x + s * (j + 1) >= 0 && x + s * (j + 1) < w && g[i + s * (j + 1)] === 0) j++; if (j > 0) to = i + s * j; }   // 3) mengalir ke samping
      }
      if (to !== b) { if (fl[i] >= 3 && ev.length < 48) ev.push({ x, y, v: fl[i], water: g[b] === 1 }); fl[i] = 0; }   // tidak lagi jatuh lurus = mendarat
      if (to >= 0) { g[to] = 1; g[i] = 0; m[to] = st; fl[to] = to === b ? Math.min(255, fl[i] + 1) : 0; fl[i] = 0; }
    }
    if (this.lvRate > 0) this.level();
  }
  // Bejana berhubungan: tiap badan air (sel air terhubung 4 arah) disamakan tingginya. Muka tertinggi di badan itu menyumbang sel,
  // muka terendah menerimanya (sel diletakkan tepat di atasnya), selama selisih >= 2 baris. Maks lvRate sel per badan per langkah = debit penyamaan.
  // Sel yang masih jatuh (fall > 0) dan sel tanpa topangan bukan muka air, jadi aliran keran tidak ikut "disedot". Posisi pindah dicatat di lv (x asal, x tujuan) untuk gelombang.
  level() {
    const { w, h, g, fall: fl, R } = this, n = w * h, lab = this.lab || (this.lab = new Int32Array(n)), stk = this.stk || (this.stk = new Int32Array(n)); lab.fill(0); let c = 0;
    for (let s0 = 0; s0 < n; s0++) {
      if (g[s0] !== 1 || lab[s0]) continue;
      let sp = 0, hy = 1e9, ly = -1, S = [], K = []; stk[sp++] = s0; lab[s0] = ++c;
      while (sp) {
        const i = stk[--sp], x = i % w, y = (i / w) | 0;
        if (y > 0 && y < h - 1 && g[i - w] === 0 && g[i + w] !== 0 && fl[i] === 0) {
          if (y < hy) { hy = y; S = []; } if (y === hy) S.push(i);
          if (y > ly) { ly = y; K = []; } if (y === ly) K.push(i);
        }
        if (x > 0 && g[i - 1] === 1 && !lab[i - 1]) { lab[i - 1] = c; stk[sp++] = i - 1; }
        if (x < w - 1 && g[i + 1] === 1 && !lab[i + 1]) { lab[i + 1] = c; stk[sp++] = i + 1; }
        if (y > 0 && g[i - w] === 1 && !lab[i - w]) { lab[i - w] = c; stk[sp++] = i - w; }
        if (y < h - 1 && g[i + w] === 1 && !lab[i + w]) { lab[i + w] = c; stk[sp++] = i + w; }
      }
      if (ly - hy < 2) continue;
      for (let m = Math.min(this.lvRate, (ly - hy) >> 1); m > 0 && S.length && K.length; m--) {
        const a = S.splice((R() * S.length) | 0, 1)[0], b = K.splice((R() * K.length) | 0, 1)[0] - w;
        g[a] = 0; g[b] = 1; fl[a] = fl[b] = 0; if (this.lv.length < 256) this.lv.push(a % w, b % w);
      }
    }
  }
};

// (4) Busa permukaan: intensitas per kolom, meluruh pelan; digambar sebagai piksel putih berambang Bayer tepat di muka air.
PX.Foam = class {
  constructor(n) { this.f = new Float32Array(n); }
  add(x, a, s = 3) { for (let dx = -s; dx <= s; dx++) { const i = Math.round(x) + dx; if (i >= 0 && i < this.f.length) this.f[i] = Math.min(1, this.f[i] + a * (1 - Math.abs(dx) / (s + 1))); } }
  update(dt) { for (let i = 0; i < this.f.length; i++) this.f[i] = Math.max(0, this.f[i] - dt * 0.35); }
  draw(buf, surfY) {                                                   // surfY(x) < 0 = kolom tanpa muka air
    for (let x = 0; x < this.f.length; x++) { const a = this.f[x]; if (a < 0.04) continue; const y = Math.round(surfY(x)); if (y < 0) continue;
      if (a > PX.bayer(x, y)) buf.px(x, y, PX.WATER.HI); if (a * 0.8 > PX.bayer(x + 1, y - 1)) buf.px(x, y - 1, PX.WATER.LIT); }
  }
};

// (5) Gelembung mikro di dalam air (titik-titik): tiap gelembung naik dengan goyang kecil, hilang bila keluar dari air, pecah (onPop) bila mencapai muka air.
// inWater(x, y) -> bool menentukan di mana gelembung boleh ada, jadi cocok untuk air sel berbentuk bebas.
PX.Fizz = class {
  constructor(inWater, onPop, seed = 7) { Object.assign(this, { inW: inWater, onPop, list: [], max: 320, R: PX.rng(seed) }); }
  burst(x, y, n, spread = 3, down = 0) {                              // down > 0: awalnya terseret turun oleh air yang jatuh, lalu naik
    const R = this.R, rg = (a, b) => a + (b - a) * R();
    for (let i = 0; i < n && this.list.length < this.max; i++) this.list.push({ x: x + rg(-spread, spread), y: y + rg(0, spread), r: rg(0.4, 1.3), vy: rg(-4, down), ph: rg(0, 6.283) });
  }
  update(dt) {
    const L = this.list;
    for (let i = L.length - 1; i >= 0; i--) {
      const b = L[i]; b.ph += dt * (6 + 2 / b.r); b.vy += (-(10 + 14 * b.r) - b.vy) * Math.min(1, 2.5 * dt); b.y += b.vy * dt; b.x += Math.sin(b.ph) * 6 * dt;
      const ix = Math.round(b.x), iy = Math.round(b.y);
      if (!this.inW(ix, iy)) { if (b.vy < 0 && this.inW(ix, iy + 1) && this.onPop) this.onPop(ix, iy); L.splice(i, 1); }
    }
  }
  draw(buf, lit) {
    const P = PX.WATER;
    for (const b of this.list) {
      const x = Math.round(b.x), y = Math.round(b.y);
      if (b.r < 0.9) buf.px(x, y, Math.sin(b.ph * 0.7) > 0.6 ? P.HI : P.RIM);                    // kecil = 1 piksel, berkedip
      else { buf.px(x, y, P.HI); buf.px(x + 1, y, P.LIT); buf.px(x, y + 1, P.RIM); buf.px(x + 1, y + 1, P.SHADE); lit.glow(x + 0.5, y + 0.5, 3, P.LIT, 0.12); }
    }
  }
};

// Demo fluida: permukaan berpegas + benda berapung/tenggelam (Archimedes sederhana) + percikan + gelembung. Klik = jatuhkan batu.
PX.demos.fluid = (host) => {
  const W = 160, H = 90, TOP = 30, FLOOR = 82, cv = PX.canvas(host, W, H, 800), c = cv.getContext('2d'), buf = new PX.Buf(W, H), lit = new PX.Light(W, H), p = PX.pointer(cv);
  const wat = new PX.Water(0, W, TOP, FLOOR), sc = PX.poolScene(W, H, TOP, () => FLOOR), surf = new PX.Surface(W, TOP), R = PX.rng(5), rg = (a, b) => a + (b - a) * R();
  const objs = [], drops = []; let t = 0, rho = 1.6, caus = true;
  const sy = (x) => surf.y(x) + Math.sin(x * 0.11 + t * 1.6) * 0.4 + Math.sin(x * 0.043 - t * 0.9) * 0.5;      // permukaan = simulasi + ombak halus
  const bub = new PX.Bubbles({ x0: 0, x1: W, top: TOP, ground: () => FLOOR, surfY: sy, density: 0.7, seed: 2 });
  const foam = new PX.Foam(W), ui = PX.ui(host);
  ui.range('Massa jenis benda', 0.4, 2.5, 0.1, rho, (v) => (rho = v)); ui.check('Caustic', true, (v) => (caus = v));
  ui.button('Hujan batu', () => { for (let i = 0; i < 6; i++) drop(rg(10, W - 10), -i * 12); });
  const drop = (x, y) => { objs.push({ x, y, vx: rg(-8, 8), vy: 0, r: rg(2, 4), rho, wet: false }); if (objs.length > 14) objs.shift(); };
  const spray = (x, y, speed, r) => { const n = Math.min(14, Math.floor(speed * r * 0.05)); for (let i = 0; i < n; i++) drops.push({ x: x + rg(-r, r), y, vx: rg(-30, 30), vy: -(40 + rg(0.2, 0.7) * speed) }); };
  PX.loop(host, (dt) => {
    t += dt; for (const k of p.clicks.splice(0)) drop(k.x, 2);
    for (const o of objs) {
      const s = sy(o.x), sub = PX.clamp((o.y + o.r - s) / (2 * o.r), 0, 1);                                  // fraksi tercelup 0..1
      o.vy += 150 * (1 - sub / o.rho) * dt;                                                                    // gravitasi - gaya apung (∝ tercelup / massa jenis)
      o.vy -= o.vy * 2.2 * sub * dt; o.vx -= o.vx * 1.5 * sub * dt; o.x += o.vx * dt; o.y += o.vy * dt;        // hambatan hanya saat tercelup
      if (o.y + o.r > FLOOR) { o.y = FLOOR - o.r; o.vy *= -0.2; o.vx *= 0.5; }
      if (!o.wet && sub > 0) { o.wet = true; surf.splash(o.x, -Math.min(3, o.vy * o.r * 0.004)); spray(o.x, s, Math.abs(o.vy), o.r); foam.add(o.x, Math.min(1, Math.abs(o.vy) * o.r * 0.004), 3); for (let n = Math.min(16, Math.floor(Math.abs(o.vy) * o.r * 0.06)); n > 0; n--) bub.add(o.x + rg(-o.r, o.r), s + rg(3, 3 + 2 * o.r), rg(0.4, 0.8)); }   // masuk air: tekan permukaan + percikan
      if (sub === 0) o.wet = false;
      if (sub > 0.5 && Math.hypot(o.vx, o.vy) > 30 && R() < dt * 12) bub.add(o.x + rg(-1, 1), o.y - o.r * 0.5, rg(0.6, 1.4));
      if (sub > 0.3 && Math.hypot(o.vx, o.vy) > 15 && R() < dt * 30) bub.add(o.x + rg(-o.r, o.r), o.y + rg(-o.r, o.r), rg(0.4, 0.8));   // titik-titik gelembung mikro   // jejak gelembung
    }
    for (let i = drops.length - 1; i >= 0; i--) { const d = drops[i]; d.vy += 220 * dt; d.x += d.vx * dt; d.y += d.vy * dt; if (d.vy > 0 && d.y >= sy(d.x)) { surf.splash(d.x, -0.25); foam.add(d.x, 0.15, 1); if (R() < 0.35) bub.add(d.x, sy(d.x) + rg(2, 5), rg(0.4, 0.7)); drops.splice(i, 1); } }   // tetes jatuh -> riak kecil
    surf.update(dt); bub.update(dt); foam.update(dt);
    lit.clear(); wat.compose(buf, sc.sky, sc.under, sy);
    for (const o of objs) { buf.disc(o.x, o.y, o.r, [58, 52, 78]); buf.disc(o.x - 0.5, o.y - 0.5, o.r - 1, [110, 106, 128]); buf.px(o.x - o.r * 0.4, o.y - o.r * 0.4, [207, 205, 210]); }
    wat.tint(buf, sy); wat.snow(buf, t); bub.draw(buf, lit, 'back'); wat.surface(buf, lit, t, sy); foam.draw(buf, sy); bub.draw(buf, lit, 'front');
    for (const d of drops) { buf.px(d.x, d.y, PX.WATER.HI); buf.px(d.x - d.vx * 0.02, d.y - d.vy * 0.02, PX.WATER.LIT); }
    if (caus) wat.caustics(lit, t);
    buf.flush(c); lit.flush(c);
  });
};

// Demo air sel hibrida: badan air = sel (mengalir + penyamaan tinggi / bejana berhubungan), muka air = pegas 1D (gelombang), tumbukan -> percikan + busa + gelembung mikro. Alat 'aduk' = mengobok air.
PX.demos.waterca = (host) => {
  const W = 128, H = 72, FL = H - 2, cv = PX.canvas(host, W, H, 768, false, 'Seret di atas kanvas untuk memakai alat'), c = cv.getContext('2d'), buf = new PX.Buf(W, H), p = PX.pointer(cv);
  const sim = new PX.CAWater(W, H), g = sim.g, lit = new PX.Light(W, H), wat = new PX.Water(0, W, 0, FL), sc = PX.poolScene(W, H, H * 0.45, () => FL), P = PX.WATER;
  const surf = new PX.Surface(W, 0), foam = new PX.Foam(W), R = PX.rng(11), rg = (a, b) => a + (b - a) * R();
  const STONE = ['#2b2a3a', '#4a4a5e', '#6e6f80'].map(PX.hex), TA = [0.1, 0.18, 0.26, 0.34];
  const top = new Int16Array(W).fill(-1), y0 = new Int16Array(W).fill(-1), dp = new Uint16Array(W), drops = [], cool = new Float32Array((W >> 2) + 1);     // top = muka air efektif (-1 = kolom tanpa muka air), y0 = muka sel mentah
  const inW = (x, y) => x >= 0 && y >= 0 && x < W && y < H && g[y * W + x] === 1 && !(top[x] >= 0 && y < top[x]);
  const spray = (x, y, v, n) => { for (let i = 0; i < n && drops.length < 400; i++) drops.push({ x: x + rg(-1.5, 1.5), y: y - 1, vx: rg(-30, 30), vy: -(30 + rg(0.3, 1) * Math.min(70, v * 3)) }); };
  const fizz = new PX.Fizz(inW, (x) => { surf.splash(x, -0.05); foam.add(x, 0.1, 1); if (R() < 0.4) spray(x, top[x] >= 0 ? top[x] : 0, 4, 1); });
  let tool = 'water', faucet = true, caus = true, t = 0, px0 = 0, py0 = 0, fx = 40, fq = 4;
  const box = (x0, x1, y0, y1) => { for (let y = y0; y <= y1; y++) for (let x = x0; x <= x1; x++) g[y * W + x] = 2; };
  const build = (layout) => { g.fill(0); sim.fall.fill(0); drops.length = 0; fizz.list.length = 0; surf.h.fill(0); surf.v.fill(0); foam.f.fill(0); fx = 40;
    box(0, W - 1, H - 2, H - 1); box(0, 0, 0, H - 1); box(W - 1, W - 1, 0, H - 1);
    if (layout === 'vessels') {                                                                                // 5 bejana beda lebar, tersambung saluran di bawah (sekat berhenti di y=66); A penuh, sisanya kosong
      box(97, 97, 0, H - 1); box(W - 1, W - 1, 0, H - 1); box(7, 8, 4, 66); box(29, 30, 26, 66); box(43, 44, 18, 66); box(75, 76, 34, 66); fx = 14;
      for (let y = 6; y < FL; y++) for (let x = 1; x <= 6; x++) g[y * W + x] = 1;
    } else { box(24, 71, 44, 45); box(90, 91, 30, 45); } };
  const ui = PX.ui(host);
  ui.select('Alat', [{ label: 'tuang air', value: 'water' }, { label: 'aduk', value: 'stir' }, { label: 'tembok', value: 'wall' }, { label: 'hapus', value: 'erase' }], tool, (v) => (tool = v));
  ui.range('Aliran samping', 1, 8, 1, sim.flow, (v) => (sim.flow = v)); ui.range('Penyamaan tinggi', 0, 12, 1, sim.lvRate, (v) => (sim.lvRate = v)); ui.range('Debit keran', 1, 4, 1, fq, (v) => (fq = v));
  const fc = ui.check('Keran', true, (v) => (faucet = v)); ui.check('Caustic', true, (v) => (caus = v));
  ui.button('Reset', () => build()); ui.button('Bejana berhubungan', () => { build('vessels'); faucet = fc.checked = false; });
  build();
  const impacts = () => {                                                                                       // sel jatuh mendarat -> gelombang, busa, gelembung terseret, percikan
    for (const e of sim.ev) {
      const sp = Math.min(40, e.v), k = e.x >> 2; if (t - cool[k] < 0.12) continue; cool[k] = t;                    // jeda per ~4 kolom: aliran utuh tidak memicu semburan tiap sel
      if (e.water) { surf.splash(e.x, -Math.min(2.2, sp * 0.06)); foam.add(e.x, Math.min(1, sp * 0.04), 3); fizz.burst(e.x, e.y + 2, Math.min(6, 1 + ((sp * 0.15) | 0)), 3, 24); }
      spray(e.x, e.y, sp, Math.min(e.water ? 5 : 3, 1 + ((sp * 0.12) | 0)));
    }
    sim.ev.length = 0;
  };
  const surface = () => {                                                                                       // muka air per kolom: sel teratas yang ditopang, dihaluskan, lalu digeser gelombang pegas
    for (let x = 0; x < W; x++) { y0[x] = top[x] = -1; let y = 0; while (y < H && g[y * W + x] === 0) y++; if (y < H - 1 && g[y * W + x] === 1 && g[(y + 1) * W + x] !== 0) y0[x] = y; }
    for (let x = 0; x < W; x++) {
      const a = y0[x]; if (a < 0) continue;
      const l = x > 0 && y0[x - 1] >= 0 ? y0[x - 1] : a, r = x < W - 1 && y0[x + 1] >= 0 ? y0[x + 1] : a;
      let ty = Math.round(PX.clamp(PX.clamp((l + 2 * a + r) / 4, a - 1, a + 1) - PX.clamp(surf.h[x] * 1.3, -3, 3), a - 3, a + 3));
      while (ty > a && g[ty * W + x] !== 1) ty--; while (ty < a && g[ty * W + x] !== 0) ty++;                      // lembah hanya memotong sel air, puncak hanya mengisi sel kosong
      top[x] = ty;
    }
  };
  PX.loop(host, (dt) => {
    t += dt; const mx = p.x | 0, my = p.y | 0, sp = Math.hypot(mx - px0, my - py0);
    if (p.down && p.in) for (let dy = -2; dy <= 2; dy++) for (let dx = -2; dx <= 2; dx++) {
      const x = mx + dx, y = my + dy, i = y * W + x; if (x < 1 || y < 0 || x >= W - 1 || y >= H - 2 || dx * dx + dy * dy > 5) continue;
      if (tool === 'water' && g[i] === 0) g[i] = 1; else if (tool === 'wall' && g[i] !== 2) g[i] = 2; else if (tool === 'erase') g[i] = 0;
    }
    if (p.down && p.in && tool === 'stir' && sp > 0.3) {                                                           // kobok: geser sel air acak searah seretan + gelembung + riak + percikan
      const sx = Math.sign(mx - px0), sy = Math.sign(my - py0);
      for (let k = 0; k < 14; k++) {
        const x = mx + ((R() * 7 - 3.5) | 0), y = my + ((R() * 7 - 3.5) | 0); if (x < 1 || x >= W - 1 || y < 0 || y >= H - 2 || g[y * W + x] !== 1) continue;
        const nx = x + (R() < 0.6 ? sx : ((R() * 3 - 1) | 0)), ny = y + (R() < 0.6 ? sy : ((R() * 3 - 1) | 0));
        if (nx > 0 && nx < W - 1 && ny >= 0 && ny < H - 2 && g[ny * W + nx] === 0) { g[ny * W + nx] = 1; g[y * W + x] = 0; sim.fall[ny * W + nx] = 0; }
      }
      fizz.burst(mx, my, Math.min(3, 1 + ((sp * 0.6) | 0)), 3, 6);
      const ts = top[mx]; if (ts >= 0 && Math.abs(my - ts) < 7) { surf.splash(mx, -Math.min(1.2, sp * 0.1)); foam.add(mx, Math.min(0.6, sp * 0.05), 3); if (R() < Math.min(1, sp * 0.1)) spray(mx, ts, sp * 2, 1); }
    }
    px0 = mx; py0 = my;
    for (let k = 0; k < 2; k++) { if (faucet) for (let x = fx; x < fx + fq; x++) if (g[2 * W + x] === 0) { g[2 * W + x] = 1; sim.fall[2 * W + x] = 0; } sim.step(); }   // keran menyuntik tiap langkah -> aliran utuh
    for (let k = 0; k < sim.lv.length; k += 2) { surf.splash(sim.lv[k], -0.04); surf.splash(sim.lv[k + 1], 0.04); }      // penyamaan tinggi: muka asal turun, muka tujuan naik -> ombak
    sim.lv.length = 0;
    for (let x = 0; x < W; x++) surf.v[x] = PX.clamp(surf.v[x], -1.5, 1.5);
    impacts(); surf.update(dt); surface(); fizz.update(dt); foam.update(dt);
    for (let i = drops.length - 1; i >= 0; i--) {                                                                 // tetes: gravitasi; jatuh ke air = riak + busa + kadang gelembung
      const d = drops[i]; d.vy += 220 * dt; d.x += d.vx * dt; d.y += d.vy * dt; const ix = Math.round(d.x), iy = Math.round(d.y);
      if (ix < 0 || ix >= W || iy >= H) { drops.splice(i, 1); continue; }
      if (iy >= 0) { const v = g[iy * W + ix]; if (v === 2) drops.splice(i, 1); else if (v === 1 && d.vy > 0 && inW(ix, iy)) { surf.splash(ix, -0.12); foam.add(ix, 0.12, 1); if (R() < 0.3) fizz.burst(ix, iy + 1, 1 + ((R() * 2) | 0), 1.5, 14); drops.splice(i, 1); } }
    }
    dp.fill(0); lit.clear();
    for (let y = 0; y < H; y++) for (let x = 0; x < W; x++) {
      const o = y * W + x, v = g[o], tx = top[x], open = tx >= 0;
      if (v === 2) { buf.px(x, y, STONE[PX.tone(0.25 + 0.6 * PX.hash(x >> 1, y >> 1, 3), x, y, 3, 0.25)]); dp[x] = 0; continue; }
      const w = open ? (v === 1 && y >= tx) || (v === 0 && y >= tx && y < y0[x]) : v === 1;
      if (!w) { buf.d[o] = sc.sky.d[o]; dp[x] = 0; continue; }
      const d = y > 0 && g[o - W] === 2 ? 4 : dp[x]; dp[x] = d + 1;                                              // d = kedalaman dari muka; di bawah tembok dianggap dalam (tanpa kilau muka)
      buf.d[o] = sc.under.d[o];
      if (!open && y < H - 1 && g[o + W] === 0) { buf.px(x, y, (x + y) & 1 ? P.LIT : P.RIM); continue; }       // sel jatuh bebas (aliran/tetes) = terang
      buf.px(x, y, P.TINT, TA[PX.tone(1 - Math.min(1, d / 14), x, y, 4, 0.25)]);
      if (d === 0) { const glint = Math.sin(x * 0.37 - t * 2.3) * Math.sin(x * 0.13 + t * 1.1) > 0.55; buf.px(x, y, glint ? P.HI : P.LIT); if ((x & 3) === 0) lit.glow(x, y + 2, 7, P.LIT, glint ? 0.2 : 0.09); }
      else if (d === 1) buf.px(x, y, P.RIM);
      else if (d < 7 && PX.bayer(x, y) < 0.5 - d * 0.07) buf.px(x, y, P.SHADE);
    }
    foam.draw(buf, (x) => top[x]); fizz.draw(buf, lit);
    for (const d of drops) { buf.px(d.x, d.y, P.HI); buf.px(d.x - d.vx * 0.02, d.y - d.vy * 0.02, P.LIT); }
    if (caus) wat.caustics(lit, t, (x, y) => y < H && inW(x, y));
    buf.flush(c); lit.flush(c);
  });
};