infinite sequences via generators & take

take.swift

class EvenNaturalNumbers: SequenceType {
    typealias GeneratorType = EvenNaturalNumbersGenerator
    
    func generate() -> EvenNaturalNumbersGenerator {
        return EvenNaturalNumbersGenerator()
    }
}

class EvenNaturalNumbersGenerator : GeneratorType {
    var current = 2
    
    typealias Element = Int
    
    func next() -> Int? {
        let ret = current
        current += 2
        return ret
    }
}

class Fibonacci : SequenceType {
    typealias GeneratorType = FibonacciGenerator
    
    func generate() -> FibonacciGenerator {
        return FibonacciGenerator()
    }
}

class FibonacciGenerator : GeneratorType {
    var current = 0, nextValue = 1
    
    typealias Element = Int
    
    func next() -> Int? {
        let ret = current
        current = nextValue
        nextValue = nextValue + ret
        return ret
    }
}

func take<T, S : SequenceType where S.Generator.Element == T>(n: Int, sequence: S) -> [T] {
    var gen = sequence.generate()
    var values = [T]()
    for _ in (1...n) {
        if let value = gen.next() {
            values.append(value)
        }
    }
    return values
}

take(5, [1,2,5,12,31,4,2])
take(10, EvenNaturalNumbers())
take(10, Fibonacci())