Step 1: Divide the number into separate digits:
8165 => 8 1 6 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 1 => 7
1 to 6 => 5
6 to 5 => 1
5 to 8 => 3
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
7 => N
5 => N
1 => C
3 => N
Step 4: Combine the filters to a string:
N N C N = NNCN