Fast intersection of two sorted integer arrays

Question

I need to find the intersection of two sorted integer arrays and do it very fast.

Right now, I am using the following code:

int i = 0, j = 0;

while (i          


        

Answer 1

C# doesn't support SIMD. Additionally, and I haven't yet figured out why, DLL's that use SSE aren't any faster when called from C# than the non-SSE equivalent functions. Also, all SIMD extensions that I know of don't work with branching anyway, ie your "if" statements.

If you're using .net 4.0, you can use Parallel For to gain speed if you have multiple cores. Otherwise you can write a multithreaded version if you have .net 3.5 or less.

Here is a method similar to yours:

    IList intersect(int[] arr1, int[] arr2)
    {
        IList intersect = new List();
        int i = 0, j = 0;
        int iMax = arr1.Length - 1, jMax = arr2.Length - 1;
        while (i < iMax && j < jMax)
        {
            while (i < iMax && arr1[i] < arr2[j]) i++;
            if (arr1[i] == arr2[j]) intersect.Add(arr1[i]);
            while (i < iMax && arr1[i] == arr2[j]) i++; //prevent reduntant entries
            while (j < jMax && arr2[j] < arr1[i]) j++;
            if (arr1[i] == arr2[j]) intersect.Add(arr1[i]);
            while (j < jMax && arr2[j] == arr1[i]) j++; //prevent redundant entries
        }
        return intersect;
    }

This one also prevents any entry from appearing twice. With 2 sorted arrays both of size 10 million, it completed in about a second. The compiler is supposed to remove array bounds checks if you use array.Length in a For statement, I don't know if that works in a while statement though.