swift---通过日期获取星座

/**
 通过日期获取星座
 
 - parameter date: 日期
 
 - returns: 星座名称
 */
class func calculateConstellationWithDate(date: NSDate) -> String {
    guard let calendar = NSCalendar(identifier: NSCalendarIdentifierGregorian) else {
        return ""
    }
    let components = calendar.components([.Month, .Day], fromDate: date)
    let month = components.month
    let day = components.day
    let astroString = "魔羯水瓶双鱼白羊金牛双子巨蟹狮子处女天秤天蝎射手魔羯"
    let astroFormat = "102123444543"
    
    guard (month < 1 || month > 12 || day < 1 || day > 31) == false else {
        return "错误日期格式"
    }
    guard (month == 2 && day > 29) == false else {
        return "错误日期格式"
    }
    guard ((month==4 || month==6 || month==9 || month==11) && day > 30) == false else {
        return "错误日期格式"
    }
    guard let astro: Int = Int(astroFormat[month - 1...month]) else {
        return "错误日期格式"
    }
    let isMore = day < (astro - (-19))
    
    let start = isMore ? (month * 2 - 1 * 2) : (month * 2 - 0 * 2)
    let result = astroString[start...(start + 1)] + "座"
    return result
}

这里需要用到一个String的extension

/**
 截取字符串
 
 - parameter r: 范围
 
 - returns: 返回结果
 */
subscript (r: Range<Int>) -> String {
    get {
        let startIndex = self.startIndex.advancedBy(r.startIndex)
        let endIndex = self.startIndex.advancedBy(r.endIndex)
        return self[Range(startIndex..<endIndex)]
    }
}