// adapted from http://bl.ocks.org/1747543
function force_layout() {
  var width = 500,
      height = 400,
      padding = 3;

  var circle = svg.selectAll("circle");

  var nodes = circle
      .each(function(d) {
        var my = d3.select(this);
        d.radius = parseFloat(my.attr("r"));
        d.color = my.attr("fill");
        d.x = parseFloat(my.attr("cx")) + Math.random();
        d.y = parseFloat(my.attr("cy")) + Math.random();
      })
      .data();

  var force = d3.layout.force()
      .nodes(nodes)
      .size([width, height])
      .gravity(.02)
      .charge(0)
      .on("tick", tick)
      .start();

  circle.call(force.drag);

  function tick(e) {
    circle
        .each(cluster(10 * e.alpha * e.alpha))
        .each(collide(.5))
        .attr("cx", function(d) { return d.x; })
        .attr("cy", function(d) { return d.y; });
  }

  // Move d to be adjacent to the cluster node.
  function cluster(alpha) {
    var max = {};

    // Find the largest node for each cluster.
    nodes.forEach(function(d) {
      if (!(d.color in max) || (d.radius > max[d.color].radius)) {
        max[d.color] = d;
      }
    });

    return function(d) {
      var node = max[d.color],
          l,
          r,
          x,
          y,
          i = -1;

      if (node == d) return;

      x = d.x - node.x;
      y = d.y - node.y;
      l = Math.sqrt(x * x + y * y);
      r = d.radius + node.radius;
      if (l != r) {
        l = (l - r) / l * alpha;
        d.x -= x *= l;
        d.y -= y *= l;
        node.x += x;
        node.y += y;
      }
    };
  }

  // Resolves collisions between d and all other circles.
  function collide(alpha) {
    var quadtree = d3.geom.quadtree(nodes);
    return function(d) {
      var r = d.radius + padding,
          nx1 = d.x - r,
          nx2 = d.x + r,
          ny1 = d.y - r,
          ny2 = d.y + r;
      quadtree.visit(function(quad, x1, y1, x2, y2) {
        if (quad.point && (quad.point !== d)) {
          var x = d.x - quad.point.x,
              y = d.y - quad.point.y,
              l = Math.sqrt(x * x + y * y),
              r = d.radius + quad.point.radius + (d.color !== quad.point.color) * padding;
          if (l < r) {
            l = (l - r) / l * alpha;
            d.x -= x *= l;
            d.y -= y *= l;
            quad.point.x += x;
            quad.point.y += y;
          }
        }
        return x1 > nx2
            || x2 < nx1
            || y1 > ny2
            || y2 < ny1;
      });
    };
  }
};