Source: rewind2.js

  1. import { check } from "./helpers/check.js";
  3. /**
  4.  * @function rewind2
  5.  * @summary Rewind a geoJSON (Mapbox). The function allows to rewind the winding order of a GeoJSON object. The winding order of a polygon is the order in which the vertices are visited by the path that defines the polygon. The winding order of a polygon is significant because it determines the interior of the polygon. The winding order of a polygon is typically either clockwise or counterclockwise.
  6.  * @description  Adapted from MapBox geojson-rewind code (https://github.com/mapbox/grojson-rewind) under ISC license.
  7.  * @param {object|array} data - A GeoJSON FeatureCollection, an array of features, an array of geometries, a single feature or a single geometry.
  8.  * @property {boolean} [options.outer = false] - rewind Rings Outer
  9.  * @returns {object|array} - A GeoJSON FeatureCollection, an array of features, an array of geometries, a single feature or a single geometry (it depends on what you've set as `data`)
  10.  * @example
  11.  * geotoolbox.rewind2(*a geojson*)
  12.  */
  14. export function rewind2(data, { outer = false } = {}) {
  15.   const handle = check(data);
  16.   let geo = handle.import(data);
  18.   for (let i = 0; i < geo.features.length; i++) {
  19.     if (geo.features[i].geometry.type === "Polygon") {
  20.       rewindRings(geo.features[i].geometry.coordinates, outer);
  21.     } else if (geo.features[i].geometry.type === "MultiPolygon") {
  22.       for (let j = 0; j < geo.features[i].geometry.coordinates.length; j++) {
  23.         rewindRings(geo.features[i].geometry.coordinates[j], outer);
  24.       }
  25.     }
  26.   }
  27.   return handle.export(geo);
  28. }
  30. function rewindRings(rings, outer) {
  31.   if (rings.length === 0) return;
  32.   rewindRing(rings[0], outer);
  33.   for (let i = 1; i < rings.length; i++) {
  34.     rewindRing(rings[i], !outer);
  35.   }
  36. }
  38. function rewindRing(ring, dir) {
  39.   let tArea = 0;
  40.   let err = 0;
  41.   for (let i = 0, len = ring.length, j = len - 1; i < len; j = i++) {
  42.     const k = (ring[i][0] - ring[j][0]) * (ring[j][1] + ring[i][1]);
  43.     const m = tArea + k;
  44.     err += Math.abs(tArea) >= Math.abs(k) ? tArea - m + k : k - m + tArea;
  45.     tArea = m;
  46.   }
  47.   if (tArea + err >= 0 !== !!dir) ring.reverse();
  48. }