Step 1: Divide the number into separate digits:
81099 => 8 1 0 9 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 1 = 7
1 to 0 = 1
0 to 9 = 9
9 to 9 = 0
9 to 8 = 1
Step 3: Combine the filters to a string:
7 1 9 0 1 = 71901