filter.js

import prune from "./prune.js";

export default function(topology, filter) {
  var oldObjects = topology.objects,
      newObjects = {},
      key;

  if (filter == null) filter = filterTrue;

  function filterGeometry(input) {
    var output, arcs;
    switch (input.type) {
      case "Polygon": {
        arcs = filterRings(input.arcs);
        output = arcs ? {type: "Polygon", arcs: arcs} : {type: null};
        break;
      }
      case "MultiPolygon": {
        arcs = input.arcs.map(filterRings).filter(filterIdentity);
        output = arcs.length ? {type: "MultiPolygon", arcs: arcs} : {type: null};
        break;
      }
      case "GeometryCollection": {
        arcs = input.geometries.map(filterGeometry).filter(filterNotNull);
        output = arcs.length ? {type: "GeometryCollection", geometries: arcs} : {type: null};
        break;
      }
      default: return input;
    }
    if (input.id != null) output.id = input.id;
    if (input.bbox != null) output.bbox = input.bbox;
    if (input.properties != null) output.properties = input.properties;
    return output;
  }

  function filterRings(arcs) {
    return arcs.length && filterExteriorRing(arcs[0]) // if the exterior is small, ignore any holes
        ? [arcs[0]].concat(arcs.slice(1).filter(filterInteriorRing))
        : null;
  }

  function filterExteriorRing(ring) {
    return filter(ring, false);
  }

  function filterInteriorRing(ring) {
    return filter(ring, true);
  }

  for (key in oldObjects) {
    newObjects[key] = filterGeometry(oldObjects[key]);
  }

  return prune({
    type: "Topology",
    bbox: topology.bbox,
    transform: topology.transform,
    objects: newObjects,
    arcs: topology.arcs
  });
}

function filterTrue() {
  return true;
}

function filterIdentity(x) {
  return x;
}

function filterNotNull(geometry) {
  return geometry.type != null;
}