Step 1: Divide the number into separate digits:
77421 => 7 7 4 2 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 7 = 0
7 to 4 = 3
4 to 2 = 2
2 to 1 = 1
1 to 7 = 6
Step 3: Combine the filters to a string:
0 3 2 1 6 = 03216