Question

I’m wondering if anyone knows a better (as in faster) algorithm/solution to solve my problem:

In my program I have an array of uints, from which I want to remove the entries contained in another uint array. However, I cannot use the union of the sets, because I need to keep duplicate values. Badly worded explaination, but the example should make it a bit clearer:

    uint[] array_1 = new uint[7] { 1, 1, 1, 2, 3, 4, 4};
    uint[] array_2 = new uint[4] { 1, 2, 3, 4 };

    uint[] result = array_1 .RemoveRange(array_2);
    // result should be: { 1, 1, 4 }

This is my current best idea; but it’s fairly slow:

    public static uint[] RemoveRange(this uint[] source_array, uint[] entries_to_remove)
    {
        int current_source_length = source_array.Length;
        for (int i = 0; i < entries_to_remove.Length; i++)
        {
            for (int j = 0; j < current_source_length; j++)
            {
                if (entries_to_remove[i] == source_array[j])
                {
                    // Shifts the entries in the source_array.
                    Buffer.BlockCopy(source_array, (j + 1)* 4 , source_array, j * 4, (current_source_length - j) * 4);
                    current_source_length--;
                    break;
                }
            }
        }
        uint[] new_array = new uint[current_source_length];
        Buffer.BlockCopy(source_array, 0, new_array, 0, current_source_length * 4);
        return new_array;
    }

So again, can someone come up with a more clever approach to achieve what I want?

Thanks!

Answer 1

This would do what you want as far as I understand it, they key is reference counting of the number of occurrences and then using the remaining reference count (if > 0) as the number of times a number has to be emitted:

public static uint[] RemoveRange(this uint[] source_array, uint[] entries_to_remove)
{
    var referenceCount = new Dictionary<uint, int>();
    foreach (uint n in source_array)
    {
        if (!referenceCount.ContainsKey(n))
            referenceCount[n] = 1;
        else
            referenceCount[n]++;
    }
    foreach (uint n in entries_to_remove)
    {
        if (referenceCount.ContainsKey(n))
            referenceCount[n]--;
    }
    return referenceCount.Where(x => x.Value > 0)
                         .Select(x => Enumerable.Repeat(x.Key, x.Value))
                         .SelectMany( x => x)
                         .ToArray();
}