Step 1: Divide the number into separate digits:
8019 => 8 0 1 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 0 = 8
0 to 1 = 1
1 to 9 = 8
9 to 8 = 1
Step 3: Combine the filters to a string:
8 1 8 1 = 8181