Step 1: Divide the number into separate digits:
21151 => 2 1 1 5 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 = 1
1 to 1 = 0
1 to 5 = 4
5 to 1 = 4
1 to 2 = 1
Step 3: Combine the filters to a string:
1 0 4 4 1 = 10441