Step 1: Divide the number into separate digits:
8120 => 8 1 2 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 1 = 7
1 to 2 = 1
2 to 0 = 2
0 to 8 = 8
Step 3: Combine the filters to a string:
7 1 2 8 = 7128