String Calculator in Swift

StringCalculator.playground

import Foundation

class StringCalculator {
    func add(string:String) -> Int {
        let numbers = string.componentsSeparatedByString(",")
        var total = 0
        for number in numbers {
            total += (number as NSString).integerValue
        }
        return total
    }
}

var x = StringCalculator()

assert(x.add("1,2,3") == 6, "it handles lists of numbers.")
assert(x.add("1") == 1, "it handles single numbers.")

assert(x.add("") == 0, "it handles empty strings.")
assert(x.add("1,,2") == 3, "it handles empty strings between numbers.")
assert(x.add(",1,2,") == 3, "it handles empty strings on the ends.")

assert(x.add(" ") == 0, "it handles whitespace")
assert(x.add(" 1 ") == 1, "it handles whitespace around numbers")
assert(x.add(" 1, 2 ,3 ") == 6, "it handles whitespace around commas")