Step 1: Divide the number into separate digits:
9102 => 9 1 0 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 1 = 8
1 to 0 = 1
0 to 2 = 2
2 to 9 = 7
Step 3: Combine the filters to a string:
8 1 2 7 = 8127