Step 1: Divide the number into separate digits:
401 => 4 0 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 0 => 4
0 to 1 => 1
1 to 4 => 3
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
4 => N
1 => C
3 => N
Step 4: Combine the filters to a string:
N C N = NCN