Step 1: Divide the number into separate digits:
6874 => 6 8 7 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 8 => 2
8 to 7 => 1
7 to 4 => 3
4 to 6 => 2
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
1 => C
3 => N
2 => N
Step 4: Combine the filters to a string:
N C N N = NCNN