greiner-hormann/src/clip.leaflet.js

Greiner-Hormann clipping algorithm

import Polygon from './polygon';

/**
 * Clip driver
 * @param  {L.Polygon} polygonA
 * @param  {L.Polygon} polygonB
 * @param  {Boolean} sourceForwards
 * @param  {Boolean} clipForwards
 * @return {Array.<L.LatLng>|null}
 */
export default function (polygonA, polygonB, sourceForwards, clipForwards) {
  let source = [], clip = [];

  let latlngs = polygonA['_latlngs'][0];
  for (let i = 0, len = latlngs.length; i < len; i++) {
      source.push([latlngs[i]['lng'], latlngs[i]['lat']]);
  }
  latlngs = polygonB['_latlngs'][0];
  for (let i = 0, len = latlngs.length; i < len; i++) {
      clip.push([latlngs[i]['lng'], latlngs[i]['lat']]);
  }

  source = new Polygon(source),
  clip = new Polygon(clip);

  const result = source.clip(clip, sourceForwards, clipForwards);
  if (result && result.length > 0) {
    for (let i = 0, len = result.length; i < len; i++) {
      result[i] = toLatLngs(result[i]);
    }

    if (result) {
      if (result.length === 1) return result[0];
      else                     return result;
    } else return null;
  } else return null;
}

function toLatLngs(poly) {
  let result = poly;

  if (result) {
    if (result[0][0] === result[result.length - 1][0] &&
        result[0][1] === result[result.length - 1][1]) {
      result = result.slice(0, result.length - 1);
    }

    for (let i = 0, len = result.length; i < len; i++) {
      result[i] = [result[i][1], result[i][0]];
    }
    return result;
  } else {
    return null;
  }
}