N-Body Problem

A recreation of an earlier Protovis example.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

body {
  background: #000;
}

</style>
<canvas width="960" height="500"></canvas>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var canvas = document.querySelector("canvas"),
    context = canvas.getContext("2d"),
    width = canvas.width,
    height = canvas.height;

var centerXScale = d3.scale.linear().range([-width/3, width/3])
var centerYScale = d3.scale.linear().range([-height/3, height/3])

var n = 200,
    tau = 2 * Math.PI,
    nodes = d3.range(n).map(function() { return {radius: Math.random() * 8 + 2}; })
              .concat(d3.range(30).map(function () { 
                var x = centerXScale(Math.random())
                var y = centerYScale(Math.random())
                var angle = (Math.atan2(y, x) - 0.01 + tau) % tau
                
                return {
                  radius: Math.random() * 8 + 2,
                  x, 
                  y,
                  px: x * Math.cos(angle),
                  py: y * Math.sin(angle)
                }
                
              }));

var force = d3.layout.force()
    .charge(0.5)
    .gravity(0.1)
    .friction(1)
    .nodes(nodes)
    .size([width, height])
    .start();

var stroke = d3.scale.linear()
    .domain([0, 1])
    .range(["steelblue", "brown"]);

force.on("tick", function(e) {
  var q = d3.geom.quadtree(nodes), node, i, vx, vy;
  context.clearRect(0, 0, width, height);
  context.lineWidth = 1.5;

for (i = 0; i < n; ++i) {
    q.visit(collide(nodes[i]));
  }

for (i = 0; i < n; ++i) {
    node = nodes[i];
    context.beginPath();
    context.arc(node.x, node.y, node.radius - 0.75, 0, tau);
    context.strokeStyle = stroke((vx = node.x - node.px) * vx + (vy = node.y - node.py) * vy);
    context.stroke();
  }

context.beginPath();
  for (i = 0; i < n; ++i) {
    node = nodes[i];
    context.moveTo(node.x, node.y);
    context.lineTo(node.x + (node.x - node.px) * 3, node.y + (node.y - node.py) * 3);
  }
  context.strokeStyle = "#fff";
  context.stroke();

force.resume();
});

</script>

https://d3js.org/d3.v3.min.js