Step 1: Divide the number into separate digits:
7559 => 7 5 5 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 5 = 2
5 to 5 = 0
5 to 9 = 4
9 to 7 = 2
Step 3: Combine the filters to a string:
2 0 4 2 = 2042