Fibonacci Sequence

Fibonacci.swift

import UIKit
import XCPlayground
import PlaygroundSupport

struct FibonacciIterator: IteratorProtocol {
    var (first, second) = (0,1)

    mutating func next() -> Int? {
        (first, second) = (second, first + second)
        return first
    }
}

struct FibonacciSequence: Sequence {
    func makeIterator() -> FibonacciIterator {
        return FibonacciIterator()
    }

    subscript(index: Int) -> Int {
        return Array(prefix(index + 1))[index]
    }
}

struct Fibonacci {
    let sequence = FibonacciSequence()

    func prefix(_ max: Int) -> [Int] {
        return Array(sequence.prefix(max))
    }

    func number(at index: Int) -> Int {
        return sequence[index - 1]
    }
}

let fibonacci = Fibonacci()
print(fibonacci.prefix(10)) // [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
print(fibonacci.number(at: 6)) // 8