Step 1: Divide the number into separate digits:
42515 => 4 2 5 1 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 2 = 2
2 to 5 = 3
5 to 1 = 4
1 to 5 = 4
5 to 4 = 1
Step 3: Combine the filters to a string:
2 3 4 4 1 = 23441