require 'rgeo'
require 'geo_graf/polygon'

module GeoGraf
  class GeosNotInstalledError < StandardError; end

  class IntersectionCalculator
    def initialize(input_geodata)
      @geodata ||= input_geodata
        .map { |geodatum| {id: geodatum[:id], polygon: polygon_from_coords(geodatum[:polygon_coords])} }
        .sort_by { |geodatum| geodatum[:polygon].area } # always have smaller first
    end

    def intersections
      intersections = []

      geodata.combination(2) do |smaller, bigger|
        intersection_area = smaller[:polygon].intersection_area(bigger[:polygon])

        unless intersection_area.zero?
          intersections << {
            id: smaller[:id],
            contained_area_percentage: (intersection_area.to_f / smaller[:polygon].area * 100).round,
            container_id: bigger[:id]
          }
        end
      end

      intersections
    end

    private

    attr_reader :geodata

    def polygon_from_coords(coords)
      Polygon.new(coords, rgeo_factory)
    end

    def rgeo_factory
      raise(GeosNotInstalledError, 'The Geos library needs to be installed (see README.md).') unless RGeo::Geos.supported?
      @factory ||= RGeo::Geographic.simple_mercator_factory
    end
  end
end