Step 1: Divide the number into separate digits:
0321 => 0 3 2 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 3 = 3
3 to 2 = 1
2 to 1 = 1
1 to 0 = 1
Step 3: Combine the filters to a string:
3 1 1 1 = 3111