Precompute the Cartesian length of shared borders in a topojson-powered map

get_county_borders.js

var fs = require("fs");
var d3 = require("d3");
var topojson = require("topojson");
var point = require('point-at-length');
var ProgressBar = require('progress');

var us = require("./us-10m.v1.json");

var path = d3.geoPath().projection(null); // pre-projected

var counties = topojson.feature(us, us.objects.counties).features,
	neighbors = topojson.neighbors(us.objects.counties.geometries);

function border(id0, id1) {
	return function(a, b) {
		return a.id === id0 && b.id === id1 || a.id === id1 && b.id === id0;
	};
}

var start = new Date();

var bar = new ProgressBar(':bar (:current / :total counties)', { total: counties.length, width: 50 });

var distances = {};

for (var n = 0; n < counties.length; n += 1) {
	var county = counties[n];
	neighbors[n].forEach(function(neighbor_index) {
		var neighbor = counties[neighbor_index];
		// getting border-to-self for 31087
		if (county.id == neighbor.id) {
			return;
		}

		var id = county.id + "_" + neighbor.id;
		var b = topojson.mesh(us, us.objects.counties, border(county.id, neighbor.id));
		var p = path(b);
		var d = point(p).length();
		distances[id] = parseFloat(d.toFixed(2));
	});
	bar.tick();
}

var end = new Date();

var elapsed = Math.round((end.getTime() - start.getTime()) / 1000);

console.log("Completed in", elapsed, "seconds");

fs.writeFileSync("border_distances.json", JSON.stringify(distances));

package.json

{
  "name": "compute_borders",
  "version": "0.0.1",
  "description": "Precompute the Cartesian length of shared borders in a topojson-powered map",
  "main": "get_county_borders.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [
    "topojson",
    "borders"
  ],
  "author": "Chris Wilson",
  "license": "MIT",
  "dependencies": {
    "d3": "^4.5.0",
    "point-at-length": "^1.0.2",
    "progress": "^1.1.8",
    "topojson": "^2.2.0"
  }
}

us-10m.v1.json