index.html

## index.html
<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html;charset=utf-8"/>
    <title>Force Layouts - Collision</title>
    <script type="text/javascript" src="http://mbostock.github.com/d3/d3.js?1.26.0"></script>
    <script type="text/javascript" src="http://mbostock.github.com/d3/d3.geom.js?1.26.0"></script>
    <script type="text/javascript" src="http://mbostock.github.com/d3/d3.layout.js?1.26.0"></script>
    <style type="text/css">

circle {
  stroke: #000;
  stroke-opacity: .5;
}

    </style>
  </head>
  <body>
    <div id="body">
      <div id="chart"></div>
      <div id="header">collision detection</div>
    </div>
    <script type="text/javascript">

var w = 1280,
    h = 800;

var nodes = d3.range(200).map(function() { return {radius: Math.random() * 12 + 4}; }),
    color = d3.scale.category10();

var force = d3.layout.force()
    .gravity(0.05)
    .charge(function(d, i) { return i ? 0 : -2000; })
    .nodes(nodes)
    .size([w, h]);

var root = nodes[0];
root.radius = 0;
root.fixed = true;

force.start();

var svg = d3.select("#chart").append("svg:svg")
    .attr("width", w)
    .attr("height", h);

svg.append("svg:rect")
    .attr("width", w)
    .attr("height", h);

svg.selectAll("circle")
    .data(nodes.slice(1))
  .enter().append("svg:circle")
    .attr("r", function(d) { return d.radius - 2; })
    .style("fill", function(d, i) { return color(i % 3); });

force.on("tick", function(e) {
  var q = d3.geom.quadtree(nodes),
      i = 0,
      n = nodes.length;

  while (++i < n) {
    q.visit(collide(nodes[i]));
  }

  svg.selectAll("circle")
      .attr("cx", function(d) { return d.x; })
      .attr("cy", function(d) { return d.y; });
});

svg.on("mousemove", function() {
  var p1 = d3.svg.mouse(this);
  root.px = p1[0];
  root.py = p1[1];
  force.resume();
});

function collide(node) {
  var r = node.radius + 16,
      nx1 = node.x - r,
      nx2 = node.x + r,
      ny1 = node.y - r,
      ny2 = node.y + r;
  return function(quad, x1, y1, x2, y2) {
    if (quad.point && (quad.point !== node)) {
      var x = node.x - quad.point.x,
          y = node.y - quad.point.y,
          l = Math.sqrt(x * x + y * y),
          r = node.radius + quad.point.radius;
      if (l < r) {
        l = (l - r) / l * .5;
        node.x -= x *= l;
        node.y -= y *= l;
        quad.point.x += x;
        quad.point.y += y;
      }
      return true;
    }
  };
}

    </script>
  </body>
</html>