The Grain of Before

function simulate(input) {
  var W=24,H=18,N=W*H;
  var gx=new Array(N).fill(0),gy=new Array(N).fill(0),load=new Array(N).fill(0),base=[];
  var frames=[],last=null,transitions=0,forgotten=0,hash=2166136261;
  var seed=input.seed;
  for(var k=0;k<seed.length;k++) hash=Math.imul(hash^seed.charCodeAt(k),16777619)>>>0;
  function bounded(v,a,b){return Math.max(a,Math.min(b,v));}
  var p=input.parameters||{};
  var retention=typeof p.retention==='number'?bounded(p.retention,0,1):0.82;
  var learning=typeof p.learning==='number'?bounded(p.learning,0,2):0.9;
  for(var i=0;i<N;i++) {
    hash=(Math.imul(hash,1664525)+1013904223)>>>0;
    var bx=(i%W)/(W-1),by=Math.floor(i/W)/(H-1);
    base.push(75+16*Math.sin(bx*9+by*5)+12*(hash/4294967296-0.5));
  }
  function energy(){var s=0;for(var j=0;j<N;j++)s+=Math.sqrt(gx[j]*gx[j]+gy[j]*gy[j]);return s/N;}
  function snapshot(index,label){
    var values=[];
    for(var j=0;j<N;j++){
      var g=Math.sqrt(gx[j]*gx[j]+gy[j]*gy[j]);
      var height=base[j]+65*g/(1+g)+760*(1-Math.exp(-load[j]/760));
      values.push(Math.round(bounded(height,0,1000)));
    }
    frames.push({event_index:index,label:label,visual:{kind:'field',width:W,height:H,values:values,palette:['#081923','#235166','#52AFA8','#E1D8A0','#FFF3DB'],description:'Height and color encode resting substrate plus residual grain seam and current deformation. A press creates a central ridge and a stronger displaced ridge along remembered grain.'}});
  }
  snapshot(0,'Untaught surface');
  for(var e=0;e<input.events.length;e++){
    var ev=input.events[e],x=ev.x/1000,y=ev.y/1000,s=ev.strength/1000;
    if(ev.kind==='release'){
      load.fill(0);last=null;
    }else if(ev.kind==='wait'){
      var before=energy(),decay=Math.pow(retention,1+3*s);
      for(var j=0;j<N;j++){gx[j]*=decay;gy[j]*=decay;load[j]*=0.45;}
      forgotten+=before-energy();
    }else{
      if(last!==null){
        var dx=x-last.x,dy=y-last.y,len=Math.sqrt(dx*dx+dy*dy);
        if(len>0.000001){
          var ux=dx/len,uy=dy/len,amount=learning*Math.sqrt(s*last.s);
          for(var j=0;j<N;j++){
            var px=(j%W)/(W-1),py=Math.floor(j/W)/(H-1);
            var t=bounded(((px-last.x)*dx+(py-last.y)*dy)/(len*len),0,1);
            var qx=px-last.x-t*dx,qy=py-last.y-t*dy;
            var a=amount*Math.exp(-(qx*qx+qy*qy)/(2*0.105*0.105));
            gx[j]+=a*ux;gy[j]+=a*uy;
          }
          transitions++;
        }
      }
      for(var j=0;j<N;j++)load[j]*=0.35;
      for(var j=0;j<N;j++){
        var px=(j%W)/(W-1),py=Math.floor(j/W)/(H-1);
        var rx=px-x,ry=py-y;
        var deposit=560*s*Math.exp(-(rx*rx+ry*ry)/(2*0.095*0.095));
        var g=Math.sqrt(gx[j]*gx[j]+gy[j]*gy[j]);
        var split=g/(0.28+g),shift=0.19*split;
        var tx=px+(g>0?shift*gx[j]/g:0),ty=py+(g>0?shift*gy[j]/g:0);
        load[j]+=deposit*(1-0.72*split);
        var fx=bounded(tx,0,1)*(W-1),fy=bounded(ty,0,1)*(H-1);
        var ix=Math.floor(fx),iy=Math.floor(fy),ax=fx-ix,ay=fy-iy;
        var ix1=Math.min(ix+1,W-1),iy1=Math.min(iy+1,H-1),m=deposit*0.72*split;
        load[iy*W+ix]+=m*(1-ax)*(1-ay);
        load[iy*W+ix1]+=m*ax*(1-ay);
        load[iy1*W+ix]+=m*(1-ax)*ay;
        load[iy1*W+ix1]+=m*ax*ay;
      }
      last={x:x,y:y,s:s};
    }
    snapshot(e+1,ev.kind==='release'?'Lift: grain remains':ev.kind==='wait'?'Wait: grain fades':'Press: inherited direction');
  }
  var mass=0,cx=0,cy=0,mx=0,my=0;
  for(var j=0;j<N;j++){
    mass+=load[j];cx+=load[j]*(j%W)/(W-1);cy+=load[j]*Math.floor(j/W)/(H-1);mx+=gx[j];my+=gy[j];
  }
  function rounded(v){return Math.round(v*1000000)/1000000;}
  return {frames:frames,metrics:{directed_transitions:transitions,mean_grain_magnitude:rounded(energy()),mean_grain_x:rounded(mx/N),mean_grain_y:rounded(my/N),forgotten_grain:rounded(forgotten),deformation_mass:rounded(mass),deformation_centroid_x:rounded(mass?1000*cx/mass:500),deformation_centroid_y:rounded(mass?1000*cy/mass:500)}};
}