A Swift 2 function that calculates a fibonacci number, then cached it in a dictionary for later use. Max value of `Double` is 1.79769313486232e+308. Note: there is a faster way to calculate this if you need to jump later numbers. It's called "fast doubling", see https://vinayakgarg.wordpress.com/2012/11/07/fastest-way-to-compute-fibonacci-number/

CachedFibonacci.swift

    typealias NumType = Double
    
    var calculatedFibs = [NumType: NumType]()
    
    func fib(num: NumType) -> NumType {
        guard num > 1 else { // when num is 0, 1, just return 0, 1.
            return num
        }
        if let cached = calculatedFibs[num] { // read cache
            return cached
        }
        
        // calculate recursively
        let result = fib(num - 1) + fib(num - 2)
        
        // write cache
        calculatedFibs[num] = result
        
        return result
    }