Step 1: Divide the number into separate digits:
21431 => 2 1 4 3 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 = 1
1 to 4 = 3
4 to 3 = 1
3 to 1 = 2
1 to 2 = 1
Step 3: Combine the filters to a string:
1 3 1 2 1 = 13121