Step 1: Divide the number into separate digits:
9158 => 9 1 5 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 1 = 8
1 to 5 = 4
5 to 8 = 3
8 to 9 = 1
Step 3: Combine the filters to a string:
8 4 3 1 = 8431