Step 1: Divide the number into separate digits:
1655 => 1 6 5 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 6 => 5
6 to 5 => 1
5 to 5 => 0
5 to 1 => 4
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
5 => N
1 => C
0 => E
4 => N
Step 4: Combine the filters to a string:
N C E N = NCEN