The Grain Between Gestures

function simulate(input) {
  var W = 24, H = 18, N = W * H;
  var gx = new Array(N).fill(0), gy = new Array(N).fill(0);
  var load = new Array(N).fill(0), base = [], frames = [];
  var previous = null, links = 0, presses = 0, lost = 0;
  var p = input.parameters || {};
  function parameter(name, fallback, low, high) {
    var v = p[name];
    return typeof v === 'number' && Number.isFinite(v) ? Math.max(low, Math.min(high, v)) : fallback;
  }
  var retention = parameter('retention', 0.82, 0, 1);
  var reach = parameter('reach', 190, 0, 300);
  var hash = 2166136261;
  for (var s = 0; s < input.seed.length; s++) hash = Math.imul(hash ^ input.seed.charCodeAt(s), 16777619) >>> 0;
  var phase = (hash % 6283) / 1000;
  for (var i = 0; i < N; i++) {
    var bx = (i % W) / (W - 1), by = Math.floor(i / W) / (H - 1);
    base.push(125 + 20 * Math.sin(bx * 7 + phase) * Math.cos(by * 5 - phase));
  }
  function magnitude(i) { return Math.sqrt(gx[i] * gx[i] + gy[i] * gy[i]); }
  function snapshot(index, label) {
    var values = [];
    for (var i = 0; i < N; i++) {
      var x = (i % W) * 1000 / (W - 1), y = Math.floor(i / W) * 1000 / (H - 1);
      var m = magnitude(i), along = m > 0 ? ((x - 500) * gx[i] + (y - 500) * gy[i]) / m : 0;
      var grain = m / (1 + m);
      var seam = 125 * grain * (1 + 0.38 * Math.sin(along / 35));
      var rise = 600 * load[i] / (1 + load[i]);
      values.push(Math.round(base[i] + seam + rise));
    }
    frames.push({event_index:index,label:label,visual:{kind:'field',width:W,height:H,values:values,palette:['#081923','#225366','#64a79b','#e1ce88','#fff3d5'],description:'Height and color encode resting substrate, persistent grain corrugations, and temporary pressure relief. Corrugations retain the direction of earlier travel; new pressure bends along that grain.'}});
  }
  snapshot(0, 'Untaught surface');
  for (var e = 0; e < input.events.length; e++) {
    var ev = input.events[e], strength = ev.strength / 1000, label;
    if (ev.kind === 'press') {
      presses++;
      var cx = ev.x * (W - 1) / 1000, cy = ev.y * (H - 1) / 1000;
      var left = Math.min(W - 2, Math.floor(cx)), top = Math.min(H - 2, Math.floor(cy));
      var fx = cx - left, fy = cy - top, vx = 0, vy = 0;
      for (var dy = 0; dy <= 1; dy++) for (var dx = 0; dx <= 1; dx++) {
        var at = (top + dy) * W + left + dx;
        var weight = (dx ? fx : 1 - fx) * (dy ? fy : 1 - fy);
        vx += gx[at] * weight; vy += gy[at] * weight;
      }
      var vm = Math.sqrt(vx * vx + vy * vy), shift = reach / (1 + vm);
      for (var i = 0; i < N; i++) {
        var x = (i % W) * 1000 / (W - 1), y = Math.floor(i / W) * 1000 / (H - 1);
        var plume = 0;
        for (var k = 0; k < 5; k++) {
          var t = k / 4, rx = x - ev.x - t * vx * shift, ry = y - ev.y - t * vy * shift;
          var radius = 77 + 23 * t;
          plume += Math.exp(-(rx * rx + ry * ry) / (2 * radius * radius)) / 5;
        }
        load[i] = load[i] * 0.4 + 2.5 * strength * plume;
      }
      if (previous !== null) {
        var ax = ev.x - previous.x, ay = ev.y - previous.y;
        var length = Math.sqrt(ax * ax + ay * ay);
        if (length > 0 && strength > 0 && previous.strength > 0) {
          var ux = ax / length, uy = ay / length;
          var amount = 2.8 * Math.sqrt(strength * previous.strength / 1000);
          for (var i = 0; i < N; i++) {
            var x = (i % W) * 1000 / (W - 1), y = Math.floor(i / W) * 1000 / (H - 1);
            var t = Math.max(0, Math.min(1, ((x - previous.x) * ax + (y - previous.y) * ay) / (length * length)));
            var rx = x - previous.x - t * ax, ry = y - previous.y - t * ay;
            var ink = amount * Math.exp(-(rx * rx + ry * ry) / (2 * 67 * 67));
            gx[i] += ux * ink; gy[i] += uy * ink;
          }
          links++;
        }
      }
      previous = {x:ev.x,y:ev.y,strength:ev.strength};
      label = 'Press ' + presses + ': pressure reads grain, then teaches it';
    } else if (ev.kind === 'release') {
      load.fill(0); previous = null;
      label = 'Released: directional grain remains';
    } else {
      for (var i = 0; i < N; i++) {
        lost += magnitude(i) * (1 - retention);
        gx[i] *= retention; gy[i] *= retention; load[i] *= 0.25;
      }
      previous = null;
      label = 'Wait: grain erodes and the gesture chain ends';
    }
    snapshot(e + 1, label);
  }
  var sum = 0, sx = 0, sy = 0, totalLoad = 0, mx = 0, my = 0;
  for (var i = 0; i < N; i++) {
    sum += magnitude(i); sx += gx[i]; sy += gy[i];
    totalLoad += load[i];
    mx += load[i] * (i % W) * 1000 / (W - 1);
    my += load[i] * Math.floor(i / W) * 1000 / (H - 1);
  }
  return {frames:frames,metrics:{directed_links:links,mean_grain_milli:Math.round(1000 * sum / N),mean_grain_x_milli:Math.round(1000 * sx / N),mean_grain_y_milli:Math.round(1000 * sy / N),eroded_grain_milli:Math.round(1000 * lost / N),pressure_centroid_x:Math.round(totalLoad > 0 ? mx / totalLoad : 500),pressure_centroid_y:Math.round(totalLoad > 0 ? my / totalLoad : 500),pressure_present:totalLoad > 0 ? 1 : 0}};
}