OpenClosePrinciple.swift

protocol Shipping {
    func getCost(forOrder order: Order) -> Double
}

class GroundShipping: Shipping {
    
    func getCost(forOrder order: Order) -> Double {
        if order.getTotal() > 100 {
            return 0
        }
        // $1.5 per kilogram, but $10 minimum
        return max(10, order.getTotalWeight() * 1.5)
    }
}

class AirShipping: Shipping {
    
    func getCost(forOrder order: Order) -> Double {
        return max(20, getTotalWeight() * 3)
    }
}

struct Order {
    
    let lineItems: [String]
    let shipping: Shipping
    
    func getShippingCost() -> Double {
        return shipping.getCost(forOrder: self)
    }
    
    func getTotal() -> Double {
        // calculate the total
        return 0.0
    }
    
    func getTotalWeight() -> Double {
        // calculate the total weight
        return 0.0
    }
}