Write a function that guarantees to never return the same value twice

Write a function that is guaranteed to never return the same value twice. Assume that this function will be accessed by multiple machines concurrently.

1. Just toss a simple (threadsafe) counter behind some communication endpoint:

   long x = long.MinValue;
   public long ID(){
      return Interlocked.Increment(ref x);
   }

2. Let interviewer be the one that follow up with those problems:
   • Does it need to survive reboots?
   • What about hard drive failure?
   • What about nuclear war?
   • Does it need to be random?
   • How random?
3. If they made it clear that it has to be unique across reboots and across different machines, I’d give them a function that calls into the standard mechanism for creating a new GUID, whatever that happens to be in the language being used. This is basically the problem that guids solve. Producing a duplicate Guid, no matter its format, is the most difficult lottery on the planet.