lineSlice function

Feature<LineString> lineSlice(Feature<Point> startPt, Feature<Point> stopPt, Feature<LineString> line)

Takes a line, at a start point startPt, and a stop point stopPt and returns a subsection of the line in-between those points. The start & stop points don't need to fall exactly on the line.

If startPt and stopPt resolve to the same point on line, null is returned as the resolved line would only contain one point which isn't supported by LineString.

This can be useful for extracting only the part of a route between waypoints.

Implementation

Feature<LineString> lineSlice(
    Feature<Point> startPt, Feature<Point> stopPt, Feature<LineString> line) {
  final coords = line.geometry;
  final startPtGeometry = startPt.geometry;
  final stopPtGeometry = stopPt.geometry;
  if (coords == null) {
    throw Exception('line has no geometry');
  }
  if (startPtGeometry == null) {
    throw Exception('startPt has no geometry');
  }
  if (stopPtGeometry == null) {
    throw Exception('stopPt has no geometry');
  }

  final startVertex = nearestPointOnLine(coords, startPtGeometry);
  final stopVertex = nearestPointOnLine(coords, stopPtGeometry);
  late final List<Feature<Point>> ends;
  if (startVertex.properties!['index'] <= stopVertex.properties!['index']) {
    ends = [startVertex, stopVertex];
  } else {
    ends = [stopVertex, startVertex];
  }
  final List<Position> clipCoords = [getCoord(ends[0])];
  for (var i = ends[0].properties!['index'] + 1;
      i < ends[1].properties!['index'];
      i++) {
    clipCoords.add(coords.coordinates[i]);
  }
  clipCoords.add(getCoord(ends[1]));
  return Feature<LineString>(
    geometry: LineString(coordinates: clipCoords),
    properties: line.properties,
  );
}