// http://bl.ocks.org/mbostock/3916621
function path_tween(d1, precision) {
  return function() {
    var path0 = this,
      path1 = path0.cloneNode(),
      n0 = path0.getTotalLength() || 0,
      n1 = (path1.setAttribute("d", d1), path1).getTotalLength() || 0;

    // Uniform sampling of distance based on specified precision.
    var distances = [0],
      i = 0,
      dt = precision / Math.max(n0, n1);
    while ((i += dt) < 1) distances.push(i);
    distances.push(1);

    // Compute point-interpolators at each distance.
    var points = distances.map(function(t) {
      var p0 = path0.getPointAtLength(t * n0),
        p1 = path1.getPointAtLength(t * n1);
      return d3.interpolate([p0.x, p0.y], [p1.x, p1.y]);
    });

    return function(t) {
      return t < 1 ? "M" + points.map(function(p) {
        return p(t);
      }).join("L") : d1;
    };
  };
}

MG.path_tween = path_tween;