Step 1: Divide the number into separate digits:
6801 => 6 8 0 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 8 => 2
8 to 0 => 8
0 to 1 => 1
1 to 6 => 5
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
8 => N
1 => C
5 => N
Step 4: Combine the filters to a string:
N N C N = NNCN