module NSWTopo::Declination

def get_features
  @params["fill"] ||= @params["stroke"]
  declination = @angle || @map.declination
  col_spacing = @spacing
  row_spacing = @arrows * 0.5
  col_offset = @offset % @spacing
  radius = 0.5 * @map.neatline.bounds.transpose.then do |bl, tr|
    Vector[*tr] - Vector[*bl]
  end.norm
  j_max = (radius / col_spacing).ceil
  i_max = (radius / row_spacing).ceil
  (-j_max..j_max).each.with_object(GeoJSON::Collection.new(projection: @map.neatline.projection)) do |j, collection|
    x = j * col_spacing + col_offset
    coordinates = [radius, -radius].map do |y|
      Vector[x, y].rotate_by_degrees(declination - @map.rotation) + Vector[*@map.dimensions] / 2
    end
    collection.add_linestring coordinates
    (-i_max..i_max).reject(&j.even? ? :even? : :odd?).map do |i|
      Vector[x, i * row_spacing].rotate_by_degrees(declination - @map.rotation) + Vector[*@map.dimensions] / 2
    end.each do |coordinates|
      collection.add_point coordinates, "rotation" => declination
    end
  end
end