[Swift] Abbreviate number class, such as number of 1500 convert to 1.5k

AbbreviateNumber.swift

    func abbreviateNumber(num: NSNumber) -> String {
        // less than 1000, no abbreviation
        if num < 1000 {
            return "\(num)"
        }
        
        // less than 1 million, abbreviate to thousands
        if num < 1000000 {
            var n = Double(num);
            n = Double( floor(n/100)/10 )
            return "\(n.description)k"
        }
        
        // more than 1 million, abbreviate to millions
        var n = Double(num)
        n = Double( floor(n/100000)/10 )
        return "\(n.description)m"
    }