Step 1: Divide the number into separate digits:
9976 => 9 9 7 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 9 = 0
9 to 7 = 2
7 to 6 = 1
6 to 9 = 3
Step 3: Combine the filters to a string:
0 2 1 3 = 0213