Step 1: Divide the number into separate digits:
6999 => 6 9 9 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 9 = 3
9 to 9 = 0
9 to 9 = 0
9 to 6 = 3
Step 3: Combine the filters to a string:
3 0 0 3 = 3003