Step 1: Divide the number into separate digits:
3976 => 3 9 7 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 = 6
9 to 7 = 2
7 to 6 = 1
6 to 3 = 3
Step 3: Combine the filters to a string:
6 2 1 3 = 6213