neighbors.js

import bisect from "./bisect.js";

export default function(objects) {
  var indexesByArc = {}, // arc index -> array of object indexes
      neighbors = objects.map(function() { return []; });

  function line(arcs, i) {
    arcs.forEach(function(a) {
      if (a < 0) a = ~a;
      var o = indexesByArc[a];
      if (o) o.push(i);
      else indexesByArc[a] = [i];
    });
  }

  function polygon(arcs, i) {
    arcs.forEach(function(arc) { line(arc, i); });
  }

  function geometry(o, i) {
    if (o.type === "GeometryCollection") o.geometries.forEach(function(o) { geometry(o, i); });
    else if (o.type in geometryType) geometryType[o.type](o.arcs, i);
  }

  var geometryType = {
    LineString: line,
    MultiLineString: polygon,
    Polygon: polygon,
    MultiPolygon: function(arcs, i) { arcs.forEach(function(arc) { polygon(arc, i); }); }
  };

  objects.forEach(geometry);

  for (var i in indexesByArc) {
    for (var indexes = indexesByArc[i], m = indexes.length, j = 0; j < m; ++j) {
      for (var k = j + 1; k < m; ++k) {
        var ij = indexes[j], ik = indexes[k], n;
        if ((n = neighbors[ij])[i = bisect(n, ik)] !== ik) n.splice(i, 0, ik);
        if ((n = neighbors[ik])[i = bisect(n, ij)] !== ij) n.splice(i, 0, ij);
      }
    }
  }

  return neighbors;
}