Some implementations of a Fibonacci method in Swift

Fibonacci.swift

// The correct numbers

let expectedSequence = [1, 1, 2, 3, 5, 8, 13, 21, 34]

// Iterative

func digit(index: Int) -> Int {
    var digits = [1, 1]
    
    for i in (2...index) {
        let x = digits[i-1]
        let y = digits[i-2]
        digits.append(x + y)
    }
    
    return digits[index]
}

// Recursive

func digit(index: Int) -> Int {
    guard index > 1 else { return 1 }
    
    let x = digit(index: index - 1)
    let y = digit(index: index - 2)
    let result = x + y
    
    return result
}

// Recursive, with Caching

var cache = [Int:Int]()

func digit(index: Int) -> Int {
    guard index > 1 else { return 1 }
    
    if let cachedResult = cache[index] {
        return cachedResult
    }
    
    let x = digit(index: index - 1)
    let y = digit(index: index - 2)
    let result = x + y
    
    cache[index] = result
    
    return result
}

// Iterative, as an IteratorProtocol

struct Fibonacci: IteratorProtocol {
    let finalIndex: Int
    
    init(finalIndex: Int) {
        self.finalIndex = finalIndex
    }
    
    private var digits = [Int]()
    
    mutating func next() -> Int? {
        guard digits.count - 1 < finalIndex else {
            return nil
        }
        
        let result: Int
        
        if digits.count < 2 {
            result = 1
        } else {
            let x = digits[digits.count - 1]
            let y = digits[digits.count - 2]
            result = x + y
        }
        digits.append(result)
        
        return digits.last
    }
}

extension Fibonacci: Sequence { }

for i in Fibonacci(finalIndex: 10) {
    print(i)
}