Step 1: Divide the number into separate digits:
8999 => 8 9 9 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 9 = 1
9 to 9 = 0
9 to 9 = 0
9 to 8 = 1
Step 3: Combine the filters to a string:
1 0 0 1 = 1001