LoopIndex fails when given an index that is less than -List.Count.

[pflannery]

You probably know this already but I just wanted to point out that the LoopIndex method fails when given an index that is less than -List.Count.

For example this fails:

// fails
int count = 360;
int index = -361;
int invalidIndex = (index + count) % count;
Console.WriteLine(invalidIndex); // writes -1

Here is a solution that preserves the original function output but also supports indexes less than -List.Count

// solution
int count = 360;
int index = -361;
int i = index % count;
if ((i < 0 && count > 0) || (i > 0 && count < 0)) i = i + count;
Console.WriteLine(i); // writes 359

The solution only performs i+count when the signs of i and count are different.

[j4ra]

Better solution would be:

//better solution
int count = 360;
int index = -721;
i = ((index % count) + count) % count;
Console.WriteLine(i); // writes 359

But I think that since the method is private and you are using it only in edge cases (eg. index = Count + 1 or index = -1) the original method is better because it would be faster.