Step 1: Divide the number into separate digits:
2588 => 2 5 8 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 5 = 3
5 to 8 = 3
8 to 8 = 0
8 to 2 = 6
Step 3: Combine the filters to a string:
3 3 0 6 = 3306