phase1.swift

import Foundation

"4 5 + 3 +"

func calculator(_ input: String) -> Double {
    return input |> split |> reversePolishNotation
}

func reversePolishNotation(_ input: [String]) -> Double {
    var stack: [Double] = []

    for s in input {
        if let double = Double(s) {
            stack = [double] + stack
        }
        else {
            stack = applyOperator(stack, s)
        }
    }

    return stack.first!
}

func applyOperator(_ input: [Double], _ op: String) -> [Double] {
    switch op {
    case "+":
        return [input[0] + input[1]] + input[1..<input.count]
    default:
        return input
    }
}

func fake<T>(_ input: T) -> Double { return 0 }

func split(_ input: String) -> [String] {
    return input.components(separatedBy: " ")
}



calculator("4 5 +")