Step 1: Divide the number into separate digits:
6598 => 6 5 9 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 5 = 1
5 to 9 = 4
9 to 8 = 1
8 to 6 = 2
Step 3: Combine the filters to a string:
1 4 1 2 = 1412