Haversine formula to calculate the distance between two points.

geo-distance--haversine.rb

module Geo
  class Point
    attr_accessor :latitude, :longitude
    def initialize(latitude:, longitude:)
      self.latitude = latitude
      self.longitude = longitude
    end
  end

  def self.radians(degrees)
    degrees * Math::PI / 180
  end

  class Distance
    # https://en.wikipedia.org/wiki/Geographical_distance
    EARTH_RADIUS_KM = 6371

    def initialize(p1:, p2:)
      self.p1 = p1
      self.p2 = p2
    end

    def value
      rad_lat1 = ::Geo.radians(p1.latitude)
      rad_lat2 = ::Geo.radians(p2.latitude)
      lat_half = (rad_lat2 - rad_lat1) / 2
      lon_half = Math::PI * (p2.longitude - p1.longitude) / 360

      a = Math.sin(lat_half)
      a *= a

      b = Math.sin(lat_half)
      b *= b * Math.cos(rad_lat1) * Math.cos(rad_lat2)

      central_angle = 2 * Math.atan2(Math.sqrt(a + b), Math.sqrt(1 - a - b))
      EARTH_RADIUS_KM * central_angle
    end

    private
    attr_accessor :p1, :p2
  end

end


p1 = Geo::Point.new(latitude: 42.8256, longitude: 13.7152)
p2 = Geo::Point.new(latitude: 42.8245, longitude: 13.7165)
distance = Geo::Distance.new(p1: p1, p2: p2)
distance.value # => 0.1516858383219569