Step 1: Divide the number into separate digits:
91059 => 9 1 0 5 9
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 5 = 5
5 to 9 = 4
9 to 9 = 0
Step 3: Combine the filters to a string:
8 1 5 4 0 = 81540