Step 1: Divide the number into separate digits:
39817 => 3 9 8 1 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 => 6
9 to 8 => 1
8 to 1 => 7
1 to 7 => 6
7 to 3 => 4
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
6 => N
1 => C
7 => N
6 => N
4 => N
Step 4: Combine the filters to a string:
N C N N N = NCNNN