Step 1: Divide the number into separate digits:
8210 => 8 2 1 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 2 = 6
2 to 1 = 1
1 to 0 = 1
0 to 8 = 8
Step 3: Combine the filters to a string:
6 1 1 8 = 6118