Step 1: Divide the number into separate digits:
3941 => 3 9 4 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 = 6
9 to 4 = 5
4 to 1 = 3
1 to 3 = 2
Step 3: Combine the filters to a string:
6 5 3 2 = 6532