Ring Extraction

Step 1 of 4 of topology construction. From How To Infer Topology.

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

.mesh {
  fill: none;
  stroke: #333;
  stroke-width: .5px;
  stroke-linejoin: round;
}

.play circle {
  fill: white;
  stroke: black;
  stroke-width: 3px;
}

.play:hover path {
  fill: red;
}

.play.mousedown circle {
  fill: red;
}

.play.mousedown path {
  fill: white;
}

.play rect {
  fill: none;
  pointer-events: all;
  cursor: pointer;
}

.ring {
  fill: none;
  stroke: red;
  stroke-width: 3px;
  stroke-linejoin: round;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>

var width = 960,
    height = 500;

var path = d3.geo.path()
    .projection(null);

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

d3.json("us.json", function(error, us) {
  if (error) throw error;

svg.append("path")
      .datum(topojson.mesh(us))
      .attr("class", "mesh")
      .attr("d", path);

var play = svg.append("g")
      .attr("class", "play");

play.append("circle")
      .attr("r", 45)
      .attr("transform", "translate(" + width / 2 + "," + height / 2 + ")");

play.append("path")
      .attr("d", "M-22,-30l60,30l-60,30z")
      .attr("transform", "translate(" + width / 2 + "," + height / 2 + ")scale(.7)");

play.append("rect")
      .attr("width", width)
      .attr("height", height)
      .on("mousedown", function() {
        play.classed("mousedown", true);
        d3.select(window).on("mouseup", function() { play.classed("mousedown", false); });
      })
      .on("click", function() {
        resetAll();
        animation();
      });

function resetAll() {
    play.style("display", null);
  }

function animation() {
    play.style("display", "none");

var rings = d3.merge(topojson.feature(us, us.objects.states).features.map(function(d) {
      return d.geometry.type === "Polygon"
          ? d.geometry.coordinates
          : d3.merge(d.geometry.coordinates);
    }));

var interval = setInterval(function() {
      var ring = rings.pop();
      if (!ring) return clearInterval(interval), void setTimeout(resetAll, 750);
      svg.append("path")
          .datum({type: "Polygon", coordinates: [ring]})
          .attr("class", "ring")
          .attr("d", path)
        .transition()
          .duration(500)
          .style("stroke-opacity", 0)
          .remove();
    }, 100);
  }
});

</script>

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

https://d3js.org/topojson.v1.min.js