module GeoGraf
  class Polygon
    def initialize(coordinates, factory)
      @rgeo_polygons = [create_polygon(coordinates, factory)]

      create_shadow_polygon_if_required(coordinates, factory)
    end

    def intersection_area(other)
      intersection = intersection(other)

      if intersection.respond_to?(:area)
        intersection.area
      elsif intersection.respond_to?(:map)
        sum_of_areas_in(intersection)
      else
        0.0
      end
    end

    def area
      rgeo_polygons.first.area
    end

    protected

    attr_reader :rgeo_polygons

    private

    def intersection(other)
      intersection = nil

      # If you wonder why for a seemingly simple intersection of two
      # polygons we are actually intersecting multiple rgeo_polygons
      # with other's rgeo_polygons, check out the comment above
      # #create_shadow_polygon_if_required. Peace.

      rgeo_polygons.each do |polygon|
        other.rgeo_polygons.each do |other_polygon|
          intersection = polygon.intersection(other_polygon)
          return intersection if intersection && !intersection.is_empty?
        end
      end 
      intersection
    end

    def create_polygon(coordinates, factory, longitude_offset: 0)
      points = coordinates.map { |c| factory.point(c[1] + longitude_offset, c[0]) }
      ring = factory.linear_ring(points)
      factory.polygon(ring)
    end

    def sum_of_areas_in(collection)
      collection
      .select { |i| i.respond_to?(:area) }
      .map(&:area)
      .inject(0, :+)
    end

    # Ah, the shadow polygon... We use it because RGeo is kind of retarded
    # and doesn't deal with situations in which shapes wrap around
    # the 180th meridian correctly. Therefore we create a fake shape with
    # an offset of either 360 or -360 degrees to ensure that we catch
    # the intersection if there is one. And, the name is awesome, right?

    def create_shadow_polygon_if_required(coordinates, factory)
      if line_180(factory).intersects?(rgeo_polygons.first)
        rgeo_polygons << create_polygon(coordinates, factory, longitude_offset: -360)
      elsif line_minus_180(factory).intersects?(rgeo_polygons.first)
        rgeo_polygons << create_polygon(coordinates, factory, longitude_offset: 360)
      end
    end

    def line_180(factory)
      factory.line(factory.point(180, 90), factory.point(180, -90))
    end

    def line_minus_180(factory)
      factory.line(factory.point(-180, 90), factory.point(-180, -90))
    end
  end
end