Step 1: Divide the number into separate digits:
6671 => 6 6 7 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 6 = 0
6 to 7 = 1
7 to 1 = 6
1 to 6 = 5
Step 3: Combine the filters to a string:
0 1 6 5 = 0165