get MD5 in Swift 3

gistfile1.txt

extension String {
    var md5: String? {
        guard let str = cString(using: .utf8) else { return nil }
        let strLen = CC_LONG(lengthOfBytes(using: .utf8))
        let digestLen = Int(CC_MD5_DIGEST_LENGTH)
        let result = UnsafeMutablePointer<CUnsignedChar>.allocate(capacity: digestLen)
        
        CC_MD5(str, strLen, result)
        let hash = (0 ..< digestLen).map({String(format:"%02x", result[$0])}).joined()
        
        result.deallocate(capacity: digestLen)
        return hash as String
    }
}