/**
* @param vertices [[x1, y1], [x2, y2], …]
* @returns triangles [[[x1, y1], [x2, y2], [x3, y3]], …]
 */
d3.geom.delaunay = function(vertices) {
  var edges = vertices.map(function() { return []; }),
      triangles = [];

  // Use the Voronoi tessellation to determine Delaunay edges.
  d3_voronoi_tessellate(vertices, function(e) {
    edges[e.region.l.index].push(vertices[e.region.r.index]);
  });

  // Reconnect the edges into counterclockwise triangles.
  edges.forEach(function(edge, i) {
    var v = vertices[i],
        cx = v[0],
        cy = v[1];
    edge.forEach(function(v) {
      v.angle = Math.atan2(v[0] - cx, v[1] - cy);
    });
    edge.sort(function(a, b) {
      return a.angle - b.angle;
    });
    for (var j = 0, m = edge.length - 1; j < m; j++) {
      triangles.push([v, edge[j], edge[j + 1]]);
    }
  });

  return triangles;
};