Step 1: Divide the number into separate digits:
3475 => 3 4 7 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 4 => 1
4 to 7 => 3
7 to 5 => 2
5 to 3 => 2
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
3 => N
2 => N
2 => N
Step 4: Combine the filters to a string:
C N N N = CNNN