Memoization in Swift

gistfile1.swift

class Memo<T> {
    private let closure: () -> T;
    private var result: T?

    var value: T {
        if let value = result {
            return value
        } else {
            let value = closure()
            println("computed \(value)")
            result = value
            return value
        }
    }

    init(_ closure: () -> T) {
        self.closure = closure
    }

    deinit {
        println("memo destroyed")
    }

    func invalidate() {
        result = nil
    }
}

private func factorial(n: Int) -> Int {
    if n <= 0 {
        return 1
    } else {
        return n * factorial(n - 1)
    }
}

class Sample {
    lazy var memo: Memo<Int> = Memo { [unowned self] in factorial(self.degree) }

    var degree: Int = 3 {
        didSet {
            if degree != oldValue {
                memo.invalidate()
            }
        }
    }

    init() {}

    deinit {
        println("memo owner destroyed")
    }
}

var sample: Sample! = Sample()
println(sample.memo.value)
println(sample.memo.value)
println(sample.memo.value)

sample.degree = 20
println(sample.memo.value)
println(sample.memo.value)
println(sample.memo.value)

sample.degree = 5
println(sample.memo.value)
println(sample.memo.value)
println(sample.memo.value)

sample = nil