index.mjs

import * as martinez from 'martinez-polygon-clipping';
import area from '@turf/area';
import { feature, multiPolygon, polygon } from '@turf/helpers';
import { getGeom } from '@turf/invariant';
import { flattenEach } from '@turf/meta';

/**
 * Finds the difference between two {@link Polygon|polygons} by clipping the second polygon from the first.
 *
 * @name difference
 * @param {Feature<Polygon|MultiPolygon>} polygon1 input Polygon feature
 * @param {Feature<Polygon|MultiPolygon>} polygon2 Polygon feature to difference from polygon1
 * @returns {Feature<Polygon|MultiPolygon>|null} a Polygon or MultiPolygon feature showing the area of `polygon1` excluding the area of `polygon2` (if empty returns `null`)
 * @example
 * var polygon1 = turf.polygon([[
 *   [128, -26],
 *   [141, -26],
 *   [141, -21],
 *   [128, -21],
 *   [128, -26]
 * ]], {
 *   "fill": "#F00",
 *   "fill-opacity": 0.1
 * });
 * var polygon2 = turf.polygon([[
 *   [126, -28],
 *   [140, -28],
 *   [140, -20],
 *   [126, -20],
 *   [126, -28]
 * ]], {
 *   "fill": "#00F",
 *   "fill-opacity": 0.1
 * });
 *
 * var difference = turf.difference(polygon1, polygon2);
 *
 * //addToMap
 * var addToMap = [polygon1, polygon2, difference];
 */
function difference(polygon1, polygon2) {
    var geom1 = getGeom(polygon1);
    var geom2 = getGeom(polygon2);
    var properties = polygon1.properties || {};

    // Issue #721 - JSTS/Martinez can't handle empty polygons
    geom1 = removeEmptyPolygon(geom1);
    geom2 = removeEmptyPolygon(geom2);
    if (!geom1) return null;
    if (!geom2) return feature(geom1, properties);

    var differenced = martinez.diff(geom1.coordinates, geom2.coordinates);
    if (differenced.length === 0) return null;
    if (differenced.length === 1) return polygon(differenced[0], properties);
    return multiPolygon(differenced, properties);
}

/**
 * Detect Empty Polygon
 *
 * @private
 * @param {Geometry<Polygon|MultiPolygon>} geom Geometry Object
 * @returns {Geometry<Polygon|MultiPolygon>|null} removed any polygons with no areas
 */
function removeEmptyPolygon(geom) {
    switch (geom.type) {
    case 'Polygon':
        if (area(geom) > 1) return geom;
        return null;
    case 'MultiPolygon':
        var coordinates = [];
        flattenEach(geom, function (feature) {
            if (area(feature) > 1) coordinates.push(feature.geometry.coordinates);
        });
        if (coordinates.length) return {type: 'MultiPolygon', coordinates: coordinates};
    }
}

export default difference;