Extension methods that perform circular-addressing math on 0-index based arrays

CircleOffset.cs

namespace Unosquare.Utilities
{
    public static class Extensions
    {
        static public int CircleOffset(this int currentIndex, int offset, int lastIndex)
        {
            var newIndex = currentIndex + offset;
            return newIndex.CircleIndex(lastIndex);
        }

        static public int CircleIndex(this int currentIndex, int lastIndex)
        {
            var newIndex = currentIndex;
            if (currentIndex > lastIndex)
            {
                newIndex = currentIndex % (lastIndex + 1);
            }
            else if (currentIndex < 0)
            {
                newIndex = lastIndex + currentIndex + 1;
                newIndex = newIndex.CircleIndex(lastIndex);
            }

            return newIndex;
        }
    }
}