initial data and graph

server/scripts/modules/utils/polygon.mjs

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+// handle multi-polygon and holes
+const testPolygon = (point, _polygons) => {
+	// turn everything into a multi polygon for ease of processing
+	let polygons = [[..._polygons.coordinates]];
+	if (_polygons.type === 'MultiPolygon') polygons = [..._polygons.coordinates];
+
+	let inArea = false;
+
+	polygons.forEach((_polygon) => {
+		// copy the polygon
+		const polygon = [..._polygon];
+		// if a match has been found don't do anything more
+		if (inArea) return;
+
+		// polygons are defined as [[area], [optional hole 1], [optional hole 2], ...]
+		const area = polygon.shift();
+		// test if inside the initial area
+		inArea = pointInPolygon(point, area);
+
+		// if not in the area return false
+		if (!inArea) return;
+
+		// test the holes, if in any hole return false
+		polygon.forEach((hole) => {
+			if (pointInPolygon(point, hole)) {
+				inArea = false;
+			}
+		});
+	});
+	return inArea;
+};
+
+const pointInPolygon = (point, polygon) => {
+	// ray casting method from https://github.com/substack/point-in-polygon
+	const x = point[0];
+	const y = point[1];
+	let inside = false;
+	// eslint-disable-next-line no-plusplus
+	for (let i = 0, j = polygon.length - 1; i < polygon.length; j = i++) {
+		const xi = polygon[i][0];
+		const yi = polygon[i][1];
+		const xj = polygon[j][0];
+		const yj = polygon[j][1];
+		const intersect = ((yi > y) !== (yj > y))
+			&& (x < ((xj - xi) * (y - yi)) / (yj - yi) + xi);
+		if (intersect) inside = !inside;
+	}
+	return inside;
+};
+
+export default testPolygon;