Step 1: Divide the number into separate digits:
66901 => 6 6 9 0 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 6 = 0
6 to 9 = 3
9 to 0 = 9
0 to 1 = 1
1 to 6 = 5
Step 3: Combine the filters to a string:
0 3 9 1 5 = 03915