Step 1: Divide the number into separate digits:
2128 => 2 1 2 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 = 1
1 to 2 = 1
2 to 8 = 6
8 to 2 = 6
Step 3: Combine the filters to a string:
1 1 6 6 = 1166