module PathCompare
    require 'json'
    # p1 and p2 given as geojson strings
    def self.compare(p1, p2)
        p1 = JSON.parse(p1)['coordinates'].first
        p2 = JSON.parse(p2)['coordinates'].first
        p1 = even_split(p1, 10)
        p2 = even_split(p2, 10)

        point_distances = []
        p1.each_with_index do |coord, index|
            unless p2.size < index + 1  #TODO: Handle different length paths
                point_distances << Math.sqrt((coord[0] - p2[index][0]) ** 2) + Math.sqrt((coord[1] - p2[index][1]) ** 2)
            end
        end
        return point_distances.sum / point_distances.size
    end

    # Splits a linestring in to points split_size apart. (Not quite but it should be close)
    def self.even_split(coords, split_size)
        new_coords = []
        current_distance = 0
        target_distance = split_size
        last_coord = coords.first
        coords.each do |coord|
            if current_distance < target_distance
                current_distance += distance(coord[0], coord[1], last_coord[0], last_coord[1])
            else
                new_coords << last_coord #TODO: Add a new point at exactly the right spot instead of using last point
                target_distance += split_size
            end
            last_coord = coord
        end
        new_coords
    end

    RAD_PER_DEG = Math::PI / 180
    EARTH_DIAMETER_KM = 6371
    def self.distance(lat1, lon1, lat2, lon2)
        dlon = lon2 - lon1
        dlat = lat2 - lat1

        a = calc(dlat, lat1, lat2, dlon)

        2 * Math.atan2( Math.sqrt(a), Math.sqrt(1-a)) * EARTH_DIAMETER_KM * 1000
    end
    def self.deg_to_rad(num)
        num * RAD_PER_DEG
    end
    def self.calc(dlat, lat1, lat2, dlon)
        (Math.sin(deg_to_rad(dlat)/2))**2 + Math.cos(deg_to_rad(lat1)) * Math.cos((deg_to_rad(lat2))) * (Math.sin(deg_to_rad(dlon)/2))**2
    end
end