Parse user friendly time duration string in Swift

gistfile1.swift

import UIKit

extension NSDateComponents {
    subscript(unit: String) -> Int {
        get {
            switch unit{
            case "month":
                return self.month
            case "day":
                return self.day
            case "hour":
                return self.hour
            case "minute":
                return self.minute
            case "second":
                return self.second
            default:
                return 0
            }
        }
        set(newValue) {
            switch unit{
            case "month":
                self.month = newValue
            case "day":
                self.day = newValue
            case "hour":
                self.hour = newValue
            case "minute":
                self.minute = newValue
            case "second":
                self.second = newValue
            default:
                break
            }
        }
    }
}


func dateComponentFromNaturalLanguageString(var input: String) -> NSDateComponents {
    // Define the time units we can understand
    var timeUnits = [
        // Unit kind => [Unit aliases...]
        "month":    ["months", "month", "mon"],
        "week":     ["weeks", "week", "w"],
        "day":      ["days", "day", "d"],
        "hour":     ["hours", "hour", "hrs", "hr", "h"],
        "second":   ["seconds", "seconds", "secs", "sec", "s"]
    ]
    
    // Construct a reverse lookup table to go from unit alias => unit kind
    var unitKindTable = Dictionary<String, String>()
    for (kind, aliasArray) in timeUnits {
        for alias in aliasArray {
            unitKindTable[alias] = kind
        }
    }
   
    // Break input into individual tokens
    // This works well for English and other western languages only
    var tokens = input.componentsSeparatedByString(" ");
   
    var duration = NSDateComponents()
   
    // Process each of the tokens found
    for (index, element) in enumerate(tokens) {
        
        // Try to find a unit alias in the list of tokens
        var unitKind = unitKindTable[element]
        if(unitKind == nil || index == 0){
            continue
        }
       
        // Try to parse the token before the unit as a number
        var value = tokens[index-1].toInt()
        if(value == nil) {
            continue
        }
        
        // At this point, we've determine the unit type and the quantity
        
        // NSDateComponent doesn't deal with weeks the way we want.
        // Remap a week as 7 days
        if(unitKind == "week") {
            unitKind = "day"
            value! *= 7
        }
       
        // Set the component value through the extension
        duration[unitKind!] = value!
    }
    
    return duration
}

var duration = dateComponentFromNaturalLanguageString("1 week 1 hours")
var targetDate = NSCalendar.currentCalendar().dateByAddingComponents(duration, toDate: NSDate(), options: NSCalendarOptions(0))

println(targetDate)