var width = 960,
    height = 500;

  var radius = 30; /* radius of circles */
  var numCircles = 6; /* number of circles - you must update link source/target values to match changes in the number of circles */
  var d3LineLinear = d3.svg.line().interpolate("linear");
  var d3color = d3.interpolateRgb("#BAE4B3", "#006D2C"); /* color range for flow lines */

  //A LIST OF LINKS BETWEEN CIRCLES
  var links = [
    {source: 0, target: 5, strength: Math.round(Math.random()*10)},
    {source: 0, target: 2, strength: Math.round(Math.random()*10)},
    {source: 1, target: 3, strength: Math.round(Math.random()*10)},
    {source: 2, target: 4, strength: Math.round(Math.random()*10)},
    {source: 3, target: 5, strength: Math.round(Math.random()*10)},
    {source: 5, target: 0, strength: Math.round(Math.random()*10)},
    {source: 2, target: 0, strength: Math.round(Math.random()*10)},
    {source: 3, target: 1, strength: Math.round(Math.random()*10)}
  ];

  function createDefs(defs) {
    var dropShadowFilter = defs.append('svg:filter')
      .attr('id', 'dropShadow');
    dropShadowFilter.append('svg:feGaussianBlur')
      .attr('in', 'SourceAlpha')
      .attr('stdDeviation', 1);
    dropShadowFilter.append('svg:feOffset')
      .attr('dx', 0)
      .attr('dy', 1)
      .attr('result', 'offsetblur');
    var feMerge = dropShadowFilter.append('svg:feMerge');
      feMerge.append('svg:feMergeNode');
      feMerge.append('svg:feMergeNode')
      .attr('in', "SourceGraphic");
  }

  var drag = d3.behavior.drag()
  .origin(Object)
  .on("drag", function(){ dragmove(this); });

  //RANDOMLY GENERATE COORDINATES FOR CIRCLES
  var circles = d3.range(numCircles).map(function(i, d) { return [Math.round(50 + (i/numCircles)*(width - 50)), Math.round(30 + Math.random()*(height - 80))]; });

  //GLOBAL STRENGTH SCALE
  var strength_scale = d3.scale.linear()
    .range([2, 10]) /* thickness range for flow lines */
    .domain([0, d3.max(links, function(d) { return d.strength; })]);

  var color_scale = d3.scale.linear()
      .range([0, 1])
      .domain([0, d3.max(links, function(d) { return d.strength; })]);

  var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

  var g_lines = svg.append("g")
    .attr("class","lines");
  var g_circles = svg.append("g")
    .attr("class","circles");
  var g_midpoints = svg.append("g")
    .attr("class","midpoints");

  //SHADOW DEFINITION
  createDefs(svg.append('svg:defs'));

  $.each(circles, function(i, d) {
    g_circles.append("circle")
      .attr('filter', 'url(#dropShadow)')
      .attr("class","circle")
      .attr("id", "circle" + i)
      .attr("r", radius)
      .attr("cx", d[0])
      .attr("cy", d[1])
      .call(drag);
  });

  g_lines.selectAll(".link_line").data(links).enter().append("path")
    .attr("class", "link_line")
    .attr("fill", function(d) { return d3color(color_scale(d.strength)); })
    .attr("id", function(i, d) { return "link_line" + d; } )
    .attr("d", function(d){ return drawCurve(d); });

  function dragmove(dragged) {
    var x = d3.select(dragged).attr("cx");
    var y = d3.select(dragged).attr("cy");
    d3.select(dragged)
      .attr("cx", Math.max(radius, Math.min(width - radius, +x + d3.event.dx)))
      .attr("cy", Math.max(radius, Math.min(height - radius, +y + d3.event.dy)));
    $.each(links, function(i, link){
      if (link.source == dragged.id.match(/\d+/)[0] || link.target == dragged.id.match(/\d+/)[0]) {
        d3.select('#link_line' + i).attr("d", function(d){ return drawCurve(d); });
      }
    });
  }

  function drawCurve(d) {
    var slope = Math.atan2((+d3.select('#circle' + d.target).attr("cy") - d3.select('#circle' + d.source).attr("cy")), (+d3.select('#circle' + d.target).attr("cx") - d3.select('#circle' + d.source).attr("cx")));
    var slopePlus90 = Math.atan2((+d3.select('#circle' + d.target).attr("cy") - d3.select('#circle' + d.source).attr("cy")), (+d3.select('#circle' + d.target).attr("cx") - d3.select('#circle' + d.source).attr("cx"))) + (Math.PI/2);

    var sourceX = +d3.select('#circle' + d.source).attr("cx");
    var sourceY = +d3.select('#circle' + d.source).attr("cy");
    var targetX = +d3.select('#circle' + d.target).attr("cx");
    var targetY = +d3.select('#circle' + d.target).attr("cy");

    var points = [];

    var bothDirections = -1;
    $.each(links, function(index, value) {
      if(value.source == d.target && value.target == d.source) {
        bothDirections = index;
        return false;
      }
    });

    if(bothDirections >= 0) {
      points.push([sourceX - 0.5*radius*Math.cos(slope), sourceY - 0.5*radius*Math.sin(slope)]);
      points.push([targetX + radius*Math.cos(slope) + (strength_scale(links[bothDirections].strength)) * Math.cos(slopePlus90), targetY + radius*Math.sin(slope) + (strength_scale(links[bothDirections].strength)) * Math.sin(slopePlus90)]);
      points.push([sourceX - 0.5*radius*Math.cos(slope) + 2 * strength_scale(links[bothDirections].strength) * Math.cos(slopePlus90), sourceY - 0.5*radius*Math.sin(slope) + 2 * strength_scale(links[bothDirections].strength) * Math.sin(slopePlus90)]);
    } else {
      points.push([sourceX - radius*Math.cos(slope) - strength_scale(d.strength) * Math.cos(slopePlus90), sourceY - radius*Math.sin(slope) - strength_scale(d.strength) * Math.sin(slopePlus90)]);
      points.push([targetX  + radius * Math.cos(slope), targetY + radius * Math.sin(slope)]);
      points.push([sourceX - radius*Math.cos(slope) + strength_scale(d.strength) * Math.cos(slopePlus90), sourceY - radius*Math.sin(slope) + strength_scale(d.strength) * Math.sin(slopePlus90)]);
    }


    return d3LineLinear(points) + "Z";
  }