C# Flags - Getting all possible combinations where a given flag can be present

Question

Consider the following Enum:

Enum AnimalType
{
  Dog = 1,
  Cat = 2,
  Bird = 4,
  Wolf = 8
}

Now suppose we want to find all possible flag combinations where Dog is active, for example.

I divised the following method to do this:

public static int[] testss(int value)
    {
        var animalTypes = (AnimalType)value;
        List<int> possibleValues = new List<int>();
        possibleValues.Add(value);

        int totalEnumValues = Enum.GetNames(typeof(AnimalType)).Length;
        List<int> helper = new List<int>();
        int cnt = 0;

        for (int i = 1; i < totalEnumValues; i++)
        {
            foreach (Enum val in Enum.GetValues(animalTypes.GetType()))
            {
                if (cnt >= i)
                    break;

                if (i == 1)
                {
                    if ((AnimalType)val != (AnimalType)value)
                    {
                        possibleValues.Add((int)(AnimalType)val + value);
                    }
                }
                else
                {                        
                    if ((AnimalType)val != (AnimalType)value && (cnt < i))
                    {
                        helper.Add((int)(AnimalType)val);
                        cnt += 1;
                    }                        
                }
            }

            if (cnt > 0)
            {
                possibleValues.Add(helper.Sum() + value);

                helper = new List<int>();
                cnt = 0;
            }
        }         

        return possibleValues.ToArray();
    }

This method will build an array with all the possible numeric representations containing a given flag as input.

It works only partially, if you test it for Dog (1) for example, you'll see that 2 values are missing from the possibleValues array.

Can you help me realizing where I went wrong?

@Baldrick, To search and process entries given their active flags. They are stored in the database with their numeric representation (for instance, 15) and I need to know, for a given flag, all possible numeric representations to fetch from the DB and process. — Hallaghan, Jun 22 '15 at 09:45
Take the binary representation and use logical AND with the binary representation of the given flag. If it's not zero, you are looking for it. — TGlatzer, Jun 22 '15 at 09:48
Do you have control over the DB? If so, might be easier to add a stored proc that will return what you want. Otherwise, you're doing dozens of DB queries when the work would be better done DB-side. — Baldrick, Jun 22 '15 at 09:49
@DGibbs, code edited (leftovers when masking my original variables) — Hallaghan, Jun 22 '15 at 09:51
@Baldrick, I have no control over the DB, I have to do it this way. — Hallaghan, Jun 22 '15 at 09:52
This seems to be a problem of calculating all the possible combinations and then filtering from them... http://stackoverflow.com/q/7802822/613130 Clearly it could be optimized to pre-skip the combinations not containing the chosen flag, but post-filtering is easier. — xanatos, Jun 22 '15 at 09:55
Why you can't use bitwise operators in your query to database? — Oleg, Jun 22 '15 at 10:05

score 2 · Accepted Answer · answered Jun 22 '15 at 10:08

You could use the [Flags] enum attribute, then an extension method. Something like this:

[Flags]
enum AnimalType
{
  Dog = 1,
  Cat = 2,
  Bird = 4,
  Wolf = 8
} 

static class Program
{
    static void Main(string[] args)
    {
        var test = AnimalType.Dog.AllContaining();
    }

    public static int[] AllContaining(this AnimalType thisAnimal)
    {
        List<int> retVal = new List<int>();
        var possibleEnums = Enum.GetValues(typeof(AnimalType)).Length;
        var maxValue = (int)Math.Pow(2, possibleEnums);

        for (int i = 0; i < maxValue; i++)
        {
            if (((AnimalType)i).HasFlag(thisAnimal))
            {
                retVal.Add(i);
            }
        }
        return retVal.ToArray();
    }
}

It spins through all possible integer values of the enum, and sees if the 'flag' for the supplied animal is present. If it is, it adds to the return array.

in the end the solution was less complicated than I had imagined it. Thanks for showing me the way! — Hallaghan, Jun 22 '15 at 11:11
You're welcome. You can probably improve on the `Math.Pow` bit by looking at the other answer. The Max() * 2 approach is cleaner, i.m.o. — Baldrick, Jun 22 '15 at 11:27

score 1 · Answer 2 · answered Jun 22 '15 at 10:08

Assuming there's not a lot of enum values, you can try:

        private static void Main(string[] args)
        {
            var type = AnimalType.Wolf;

            foreach (var x in GetAllPossibleCombinationWith(type))
            {
                Console.WriteLine(x);
            }


            Console.ReadLine();
        }


        public static IEnumerable<AnimalType> GetAllPossibleCombinationWith(AnimalType type) // Bird
        {
            var maxValue = Enum.GetValues(typeof(AnimalType)).Cast<int>().Max();
            var combinationValue =2* maxValue - 1;
            for (int i = 0; i < combinationValue; i++)
            {
                var val = (AnimalType) i;
                if ((val & type) == type) yield return val;
            }
        }

        [Flags]
        public enum AnimalType
        {
            Dog = 1,
            Cat = 2,
            Bird = 4,
            Wolf = 8,
            Fish = 16
        }

It assumes there's no "hole" in the flags values.

C# Flags - Getting all possible combinations where a given flag can be present

2 Answers2