Step 1: Divide the number into separate digits:
87521 => 8 7 5 2 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 7 = 1
7 to 5 = 2
5 to 2 = 3
2 to 1 = 1
1 to 8 = 7
Step 3: Combine the filters to a string:
1 2 3 1 7 = 12317