Step 1: Divide the number into separate digits:
3120 => 3 1 2 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 1 = 2
1 to 2 = 1
2 to 0 = 2
0 to 3 = 3
Step 3: Combine the filters to a string:
2 1 2 3 = 2123