Generating the next available unique name in C#

Question

If you were to have a naming system in your app where the app contains say 100 actions, which creates new objects, like:

Blur
Sharpen
Contrast
Darken
Matte
...

and each time you use one of these, a new instance is created with a unique editable name, like Blur01, Blur02, Blur03, Sharpen01, Matte01, etc. How would you generate the next available unique name, so that it's an O(1) operation or near constant time. Bear in mind that the user can also change the name to custom names, like RemoveFaceDetails, etc.

It's acceptable to have some constraints, like restricting the number of characters to 100, using letters, numbers, underscores, etc...

EDIT: You can also suggest solutions without "filling the gaps" that is without reusing the already used, but deleted names, except the custom ones of course.

Answer 1

You can easily do it in O(m) where m is the number of existing instances of the name (and not dependent on n, the number of items in the list.

1. Look up the string S in question. If S isn't in the list, you're done.
2. S exists, so construct S+"01" and check for that. Continue incrementing (e.g. next try S+"02" until it doesn't exist.

This gives you unique names but they're still "pretty" and human-readable.

Unless you expect a large number of duplicates, this should be "near-constant" time because m will be so small.

Caveat: What if the string naturally ends with e.g. "01"? In your case this sounds unlikely so perhaps you don't care. If you do care, consider adding more of a suffix, e.g. "_01" instead of just "01" so it's easier to tell them apart.

Answer 2

I would create a static integer in action class that gets incremented and assigned as part of each new instance of the class. For instance:

class Blur
{
    private static int count = 0;

    private string _name;
    public string Name
    {
        get { return _name; }
        set { _name = value; }
    }

    public Blur()
    {
        _name = "Blur" + count++.ToString();
    }
}

Since count is static, each time you create a new class, it will be incremented and appended to the default name. O(1) time.

EDIT

If you need to fill in the holes when you delete, I would suggest the following. It would automatically queue up numbers when items are renamed, but it would be more costly overall:

class Blur
    {
        private static int count = 0;
        private static Queue<int> deletions = new Queue<int>();

        private string _name;
        public string Name
        {
            get { return _name; }
            set
            {
                _name = value;
                Delete();
            }
        }

        private int assigned;

        public Blur()
        {
            if (deletions.Count > 0)
            {
                assigned = deletions.Dequeue();
            }
            else
            {
                assigned = count++;
            }
            _name = "Blur" + assigned.ToString();
        }

        public void Delete()
        {
            if (assigned >= 0)
            {
                deletions.Enqueue(assigned);
                assigned = -1;
            }
        }
    }

Also, when you delete an object, you'll need to call .Delete() on the object.

CounterClass Dictionary version

class CounterClass
{
   private int count;
   private Queue<int> deletions;

   public CounterClass()
   {
      count = 0;
      deletions = new Queue<int>();
   }

   public string GetNumber()
   {
      if (deletions.Count > 0)
      {
          return deletions.Dequeue().ToString();
      }
      return count++.ToString();
   }

   public void Delete(int num)
   {
      deletions.Enqueue(num);
   }
}

you can create a Dictionary to look up counters for each string. Just make sure you parse out the index and call .Delete(int) whenever you rename or delete a value.

Answer 3

If you want O(1) time then just track how many instances of each you have. Keep a hashtable with all of the possible objects, when you create an object, increment the value for that object and use the result in the name.

Answer 4

You're definitely not going to want to expose a GUID to the user interface.

Are you proposing an initial name like "Blur04", letting the user rename it, and then raising an error message if the user's custom name conflicts? Or silently renaming it to "CustomName01" or whatever?

You can use a Dictionary to check for duplicates in O(1) time. You can have incrementing counters for each effect type in the class that creates your new effect instances. Like Kevin mentioned, it gets more complex if you have to fill in gaps in the numbering when an effect is deleted.

Answer 5

I refer you to Michael A. Jackson's Two Rules of Program Optimization:

1. Don't do it.
2. For experts only: Don't do it yet.

Simple, maintainable code is far more important than optimizing for a speed problem that you think you might have later.

I would start simple: build a candidate name (e.g. "Sharpen01"), then loop through the existing filters to see if that name exists. If it does, increment and try again. This is O(N²), but until you get thousands of filters, that will be good enough.

If, sometime later, the O(N²) does become a problem, then I'd start by building a HashSet of existing names. Then you can check each candidate name against the HashSet, rather than iterating. Rebuild the HashSet each time you need a unique name, then throw it away; you don't need the complexity of maintaining it in the face of changes. This would leave your code easy to maintain, while only being O(N).

O(N) will be good enough. You do not need O(1). The user is not going to click "Sharpen" enough times for there to be any difference.

Answer 6

You could do something like this:

    private Dictionary<string, int> instanceCounts = new Dictionary<string, int>();

    private string GetNextName(string baseName)
    {
        int count = 1;

        if (instanceCounts.TryGetValue(baseName, out count))
        {
            // the thing already exists, so add one to it
            count++;
        }

        // update the dictionary with the new value
        instanceCounts[baseName] = count;

        // format the number as desired
        return baseName + count.ToString("00");
    }

You would then just use it by calling GetNextName(...) with the base name you wanted, such as string myNextName = GetNextName("Blur");

Using this, you wouldn't have to pre-init the dictionary. It would fill in as you used the various base words. Also, this is O(1).

Answer 7

I would create a dictionary with a string key and a integer value, storing the next number to use for a given action. This will be almost O(1) in practice.

private IDictionary<String, Int32> NextFreeActionNumbers = null;       

private void InitializeNextFreeActionNumbers()
{
   this.NextFreeActionNumbers = new Dictionary<String, Int32>();

   this.NextFreeActionNumbers.Add("Blur", 1);
   this.NextFreeActionNumbers.Add("Sharpen", 1);
   this.NextFreeActionNumbers.Add("Contrast", 1);
   // ... and so on ...
}

private String GetNextActionName(String action)
{
   Int32 number = this.NextFreeActionNumbers[action];

   this.NextFreeActionNumbers[action] = number + 1;

   return String.Format("{0} {1}", action, number);
}

And you will have to check against collisions with user edited values. Again a dictionary might be a smart choice. There is no way around that. What ever way you generate your names, the user can always change a existing name to the next one you generate unless you include all existing names into the generation schema. (Or use a special character that is not allowed in user edited names, but that would be not that nice.)

Because of the comments on reusing the holes I want to add it here, too. Don't resuse the holes generated be renaming or deletion. This will confuse the user because names he deleted or modified will suddenly reappear.

Answer 8

I would look for ways to simplify the problem.

Are there any constraints that can be applied? As an example, would it be good enough if each user can only have one (active) type of action? Then, the actions could be distinguished using the name (or ID) of the user.

• Blur (Ben F)
• Blur (Adrian H)
• Focus (Ben F)

Perhaps this is not an option in this case, but maybe something else would be possible. I would go to great lengths in order to avoid the complexity in some of the proposed solutions!