Step 1: Divide the number into separate digits:
67391 => 6 7 3 9 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 7 => 1
7 to 3 => 4
3 to 9 => 6
9 to 1 => 8
1 to 6 => 5
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
4 => N
6 => N
8 => N
5 => N
Step 4: Combine the filters to a string:
C N N N N = CNNNN