Step 1: Divide the number into separate digits:
6815 => 6 8 1 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 8 => 2
8 to 1 => 7
1 to 5 => 4
5 to 6 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
7 => N
4 => N
1 => C
Step 4: Combine the filters to a string:
N N N C = NNNC