lineSliceAlongRaw function

LineString lineSliceAlongRaw(LineString line, double startDist, double stopDist, [ Unit unit = Unit.kilometers, ])

Takes a line, at a start distance startDist and a stop distance stopDist and returns a subsection of the line in-between those distances.

If startDist and stopDist 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 distances on the route.

Implementation

LineString lineSliceAlongRaw(
  LineString line,
  double startDist,
  double stopDist, [
  Unit unit = Unit.kilometers,
]) {
  List<Position> coords = line.coordinates;
  var slice = <Position>[];

  var origCoordsLength = coords.length;
  double travelled = 0;
  double? overshot;
  double direction = 0;
  Position? interpolated;
  for (var i = 0; i < coords.length; i++) {
    if (startDist >= travelled && i == coords.length - 1) {
      break;
    } else if (travelled > startDist && slice.isEmpty) {
      overshot = startDist - travelled;
      if (overshot == 0) {
        slice.add(coords[i]);
        return LineString(coordinates: slice);
      }
      direction = bearingRaw(coords[i], coords[i - 1]) - 180;
      interpolated = destinationRaw(coords[i], overshot, direction, unit);
      slice.add(interpolated);
    }

    if (travelled >= stopDist) {
      overshot = stopDist - travelled;
      if (overshot == 0) {
        slice.add(coords[i]);
        return LineString(coordinates: slice);
      }
      direction = bearingRaw(coords[i], coords[i - 1]) - 180;
      interpolated = destinationRaw(coords[i], overshot, direction, unit);
      slice.add(interpolated);
      return LineString(coordinates: slice);
    }

    if (travelled >= startDist) {
      slice.add(coords[i]);
    }

    if (i == coords.length - 1) {
      return LineString(coordinates: slice);
    }

    travelled += distanceRaw(coords[i], coords[i + 1], unit);
  }

  if (travelled < startDist && coords.length == origCoordsLength) {
    throw Exception("Start position is beyond line");
  }

  final last = coords[coords.length - 1];
  return LineString(coordinates: [last, last]);
}