Fibonacci.swift

Fibonacci.swift

public struct Fibonacci<Number: FixedWidthInteger>: Sequence {
    public let startsAtZero: Bool

    public init(startAtZero: Bool = true) {
        startsAtZero = startAtZero
    }

    public func makeIterator() -> Iterator {
        return .first(startsAtZero: startsAtZero)
    }
}

extension Fibonacci {
    public enum Iterator: IteratorProtocol {
        public typealias Element = Number

        case first(startsAtZero: Bool)
        case second(last: Element)
        case any(secondLast: Element, last: Element)

        public mutating func next() -> Element? {
            let next: Element
            switch self {
            case .first(let startsAtZero):
                next = startsAtZero ? 0 : 1
                self = .second(last: next)
            case .second(let last):
                next = 1
                self = .any(secondLast: last, last: next)
            case .any(let secondLast, let last):
                // Prevent overflow
                guard case let (nextValue, overflow) = secondLast.addingReportingOverflow(last),
                    !overflow else { return nil }
                next = nextValue
                self = .any(secondLast: last, last: next)
            }
            return next
        }
    }
}

let fibonacci = Fibonacci<Int>()
for num in fibonacci where num < 100 { 
  print(num) 
}