How to convert MAC string to colon-separated string

Question

I have csv file with 200k rows and 3 types of MAC address defined as:

• ECE1A9312000
• E8:6D:52:75:2D:16
• 24-C9-A1-15-89-B0

My goal to stay only with colon-separated form.

So to convert - to : is not big deal:

mac = mac.replace("-", ":");

But how to convert ECE1A9312000 to EC:E1:A9:31:20:00.

I thought to use regex but its too expensive to use groups for so many data (~80k).

Do I need to run over each char and append :

like:

for(int i=0; i<mac.length(); i++){
    ch = mac.charAt(i);

    if(i % 2 == 0 && i != 0){
        tmp += ':';
    }
    tmp += ch;
}

or there is more efficient way?

Thank you,

Answer 1

I threw together a totally unoptimized program based on your discarded regex approach and timed it. It completed in 650 ms (250 ms with warmup). The slowest part doesn't involve the regex, but String.format. If we replace it with a straight StringBuilder approach, the time drops to 40 ms.

public class Test {
  static Pattern regex = Pattern.compile("(..)(..)(..)(..)(..)(..)");
  public static void main(String[] args) {
    final List<String> inMacs = new ArrayList<>(), outMacs = new ArrayList<>();
    for (int i = 0; i < 80_000; i++) inMacs.add(mac());
    final long start = System.nanoTime();
    for (String mac : inMacs) {
      final Matcher m = regex.matcher(mac);
      m.matches();
      outMacs.add(String.format("%s:%s:%s:%s:%s:%s",
          m.group(1), m.group(2), m.group(3), m.group(4), m.group(5), m.group(6)));
    }
    System.out.println("Took " + (System.nanoTime() - start)/1_000_000 + " milliseconds");
    final Iterator<String> it = outMacs.iterator();
    for (int i = 0; i < 100; i++) System.out.println(it.next());
  }

  static Random rnd = new Random();
  static String mac() {
    final long mac = (long) (rnd.nextDouble()*(1L<<48));
    return String.format("%012x", mac).toUpperCase();
  }
}

If you are really looking for a fast solution, then avoid the regex and use a simple test to detect your MAC format:

  static List<String> fixMacs(List<String> inMacs) {
    final List<String> outMacs = new ArrayList<>(inMacs.size());
    for (String mac : inMacs) outMacs.add(
          mac.charAt(2) == '-'? mac.replace("-", ":")
        : mac.charAt(2) != ':'? fixMac(mac)
        : mac);
    return outMacs;
  }

  static String fixMac(String inMac) {
    final StringBuilder b = new StringBuilder(18);
    for (int i = 0; i < inMac.length(); i++) {
      b.append(inMac.charAt(i));
      if (i%2 == 1 && i != inMac.length()-1) b.append(':');
    }
    return b.toString();
  }

With this approach I measured just 8 ms for your 80,000 MACs.