Step 1: Divide the number into separate digits:
43999 => 4 3 9 9 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 3 = 1
3 to 9 = 6
9 to 9 = 0
9 to 9 = 0
9 to 4 = 5
Step 3: Combine the filters to a string:
1 6 0 0 5 = 16005