Parse user friendly time duration string in Swift

TimeIntervalParsing.swift

extension DateComponents {
    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:
                fatalError("unknown unit \(unit)")
            }
        }
        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:
                fatalError("unknown unit \(unit)")
            }
        }
    }
}

func timeInterval(parsing input: String) -> TimeInterval {
    let components = dateComponentFromNaturalLanguageString(input)
    let reference = Date(timeIntervalSinceReferenceDate: 0)
    let date = Calendar(identifier: .gregorian).date(byAdding: components, to: reference)
    return date!.timeIntervalSinceReferenceDate
}

func dateComponentFromNaturalLanguageString(_ input: String) -> DateComponents {
    // Define the time units we can understand
    let timeUnits = [
        // Unit kind => [Unit aliases...]
        "month": ["months", "month", "mon"],
        "week": ["weeks", "week", "w"],
        "day": ["days", "day", "d"],
        "hour": ["hours", "hour", "hrs", "hr", "h"],
        "minute": ["minute", "minutes", "min", "m"],
        "second": ["seconds", "seconds", "secs", "sec", "s"]
    ]

    // Construct a reverse lookup table to go from unit alias => unit kind
    var unitKindTable = [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
    let tokens = input.components(separatedBy: " ")

    var duration = DateComponents()

    // Process each of the tokens found
    for (index, element) in tokens.enumerated() {
        // 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 = Int(tokens[index - 1])
        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
}

print(timeInterval(parsing: "1 h"))
print(timeInterval(parsing: "1 min"))
print(timeInterval(parsing: "1 day"))