Step 1: Divide the number into separate digits:
72138 => 7 2 1 3 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 2 = 5
2 to 1 = 1
1 to 3 = 2
3 to 8 = 5
8 to 7 = 1
Step 3: Combine the filters to a string:
5 1 2 5 1 = 51251