A FibonacciSequence type

FibonacciSequence.playground

import Foundation

struct FibsIterator: IteratorProtocol {
    private var state:(UInt, UInt) = (0, 1)
    private var upTo: Int
    private var counter = 0
    
    init(upTo: Int) {
        self.upTo = upTo
    }
    
    mutating func next() -> UInt? {
        guard upTo > counter else {return nil}
        guard upTo > 0 else {return nil}
        
        let upcomingNumber = state.0
        state = (state.1, state.0 + state.1)
        counter += 1
        
        return upcomingNumber
    }
}

struct FibsSequence: Sequence {
    private var upTo: Int
    
    init(upTo: Int) {
        self.upTo = upTo
    }
    
    func makeIterator() -> FibsIterator {
        return FibsIterator(upTo: upTo)
    }
}

for (index, fib) in FibsSequence(upTo: 60).enumerated() {
    print("fib: \(fib), index: \(index + 1)")
}