D3 Examples - Finding adjacent voronoi cells

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<!DOCTYPE html>
<meta charset="utf-8">
<style>
​
.links {
  stroke: #000;
  stroke-opacity: 0.2;
}
​
.polygons {
  fill: none;
  stroke: #000;
}
​
​
​
.sites {
  fill: #000;
  stroke: #fff;
}
​
​
​
</style>
<svg width="960" height="500"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>
​
var svg = d3.select("svg").on("touchmove mousemove", moved),
    width = +svg.attr("width"),
    height = +svg.attr("height");
​
var sites = d3.range(30)
    .map(function(d) { return [Math.random() * width, Math.random() * height]; });
​
var voronoi = d3.voronoi()
    .extent([[-1, -1], [width + 1, height + 1]]);
​
var polygon = svg.append("g")
    .attr("class", "polygons")
  .selectAll("path")
  .data(voronoi.polygons(sites))
  .enter().append("path")
    .call(redrawPolygon);
​
var link = svg.append("g")
    .attr("class", "links")
  .selectAll("line")
  .data(voronoi.links(sites))
  .enter().append("line")
    .call(redrawLink);
​
var site = svg.append("g")
    .attr("class", "sites")
  .selectAll("circle")
  .data(sites)
  .enter().append("circle")
    .attr("r", 2.5)
    .call(redrawSite);
​
function moved() {
  sites[0] = d3.mouse(this);
  redraw();
}
  
​
​
function redraw() {
  
  //Aim is to find all cells which are adjacent to the current one
  //Then we can select them
  
  var diagram = voronoi(sites);
  var links = diagram.links()
  var polygons = diagram.polygons()
  polygons.forEach(function(p) {
    p.adjacent = false
  })
  
  var this_site = sites[0] //array[2]
  var this_polygon = polygons[0] //set of coordinates and data attribute
  var counter = 1
  links.forEach(function(l) {
    if (l.source == this_site || l.target == this_site) {
     
      //get corresponding polygons
      polygons.forEach(function(p) {
        if (p.data == l.target || p.data == l.source) {
          p["adjacent"] = true
        } 
         
      })
    }
    
  })
  
  
  //this site corresponds to polygon[0]
  //
  
  polygon = polygon.data(polygons).call(redrawPolygon);
  link = link.data(links), link.exit().remove();
  link = link.enter().append("line").merge(link).call(redrawLink);
  site = site.data(sites).call(redrawSite);
  
    
}
​
function redrawPolygon(polygon) {
  polygon
      .attr("d", function(d) { return d ? "M" + d.join("L") + "Z" : null; })
        .style("fill", function(d){
                        if (d.adjacent == 1) {
                return "#F1F"
              } else {
                return "#FF1"
              }
            })
}
​
function redrawLink(link) {
  link
      .attr("x1", function(d) { return d.source[0]; })
      .attr("y1", function(d) { return d.source[1]; })
      .attr("x2", function(d) { return d.target[0]; })
      .attr("y2", function(d) { return d.target[1]; });
}
​
function redrawSite(site) {
  site
      .attr("cx", function(d) { return d[0]; })
      .attr("cy", function(d) { return d[1]; });
}
​
</script>
​
    