Step 1: Divide the number into separate digits:
2181 => 2 1 8 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 => 1
1 to 8 => 7
8 to 1 => 7
1 to 2 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
7 => N
7 => N
1 => C
Step 4: Combine the filters to a string:
C N N C = CNNC