Step 1: Divide the number into separate digits:
31665 => 3 1 6 6 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 1 => 2
1 to 6 => 5
6 to 6 => 0
6 to 5 => 1
5 to 3 => 2
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
5 => N
0 => E
1 => C
2 => N
Step 4: Combine the filters to a string:
N N E C N = NNECN