TemplateMethod.swift

struct Donor {
    let title: String
    let firstName: String
    let familyName: String
    let lastDonation: Float
    
    init(_ title:String, _ first:String, _ family:String, _ last:Float) {
        self.title = title
        self.firstName = first
        self.familyName = family
        self.lastDonation = last
    }
}

class DonorDatabase {
    private var donors: [Donor]
    
    init() {
        donors = [
            Donor("Ms",   "Anne",  "Jones", 0),
            Donor("Mr",   "Bob",   "Smith", 100),
            Donor("Dr",   "Alice", "Doe",   200),
            Donor("Prof", "Joe",   "Davis", 320)
        ]
    }
    
    func generateGalaInvitations(maxNumber: Int) -> [String] {
        var targetDonors: [Donor] = donors.filter({ $0.lastDonation > 0 })
        targetDonors.sort({ $0.lastDonation > $1.lastDonation })
        if (targetDonors.count > maxNumber) {
            targetDonors = Array(targetDonors[0..<maxNumber])
        }
        return targetDonors.map({ donor in
            return "Dear \(donor.title). \(donor.familyName)"
            })
    }
}

let donorDb = DonorDatabase()
let galaInvitations = donorDb.generateGalaInvitations(2)
for invite in galaInvitations {
    print(invite)
}