combine function

FeatureCollection<GeometryObject> combine(FeatureCollection<GeometryObject> collection)

Combines a FeatureCollection of Point, LineString, or Polygon features (and their Multi* counterparts) into MultiPoint, MultiLineString, or MultiPolygon features.

Features may be mixed: each non-empty geometry kind becomes one output feature. Other geometry types and features with null geometry are skipped.

Returns a FeatureCollection with up to three features, in Turf JS order: MultiLineString, then MultiPoint, then MultiPolygon (only groups that have coordinates are included).

Each output feature has properties.collectedProperties, a list of the source features' properties values (one entry per contributing feature; for a Multi* input, one entry applies to all coordinates merged from that feature).

See: https://turfjs.org/docs/#combine

Implementation

FeatureCollection<GeometryObject> combine(FeatureCollection collection) {
  // Buckets mirror @turf/combine: coordinates + parallel property list per group.
  final multiPointCoords = <Position>[];
  final multiPointProps = <Map<String, dynamic>?>[];

  final multiLineCoords = <List<Position>>[];
  final multiLineProps = <Map<String, dynamic>?>[];

  final multiPolyCoords = <List<List<Position>>>[];
  final multiPolyProps = <Map<String, dynamic>?>[];

  // Walk input; flatten Multi* into the same bucket as their single counterparts.
  featureEach(collection, (currentFeature, _) {
    final geometry = currentFeature.geometry;
    if (geometry == null) {
      return;
    }
    final Map<String, dynamic>? props = currentFeature.properties;

    if (geometry is Point) {
      multiPointCoords.add(geometry.coordinates);
      multiPointProps.add(props);
    } else if (geometry is MultiPoint) {
      multiPointCoords.addAll(geometry.coordinates);
      multiPointProps.add(props);
    } else if (geometry is LineString) {
      multiLineCoords.add(geometry.coordinates);
      multiLineProps.add(props);
    } else if (geometry is MultiLineString) {
      multiLineCoords.addAll(geometry.coordinates);
      multiLineProps.add(props);
    } else if (geometry is Polygon) {
      multiPolyCoords.add(geometry.coordinates);
      multiPolyProps.add(props);
    } else if (geometry is MultiPolygon) {
      multiPolyCoords.addAll(geometry.coordinates);
      multiPolyProps.add(props);
    }
  });

  // Emit one Feature per non-empty group (same key order as Object.keys(...).sort() in Turf).
  final features = <Feature<GeometryObject>>[];

  if (multiLineCoords.isNotEmpty) {
    features.add(
      Feature<GeometryObject>(
        geometry: MultiLineString(coordinates: multiLineCoords),
        properties: <String, dynamic>{'collectedProperties': multiLineProps},
      ),
    );
  }
  if (multiPointCoords.isNotEmpty) {
    features.add(
      Feature<GeometryObject>(
        geometry: MultiPoint(coordinates: multiPointCoords),
        properties: <String, dynamic>{'collectedProperties': multiPointProps},
      ),
    );
  }
  if (multiPolyCoords.isNotEmpty) {
    features.add(
      Feature<GeometryObject>(
        geometry: MultiPolygon(coordinates: multiPolyCoords),
        properties: <String, dynamic>{'collectedProperties': multiPolyProps},
      ),
    );
  }

  return FeatureCollection<GeometryObject>(features: features);
}