Step 1: Divide the number into separate digits:
21666 => 2 1 6 6 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 => 1
1 to 6 => 5
6 to 6 => 0
6 to 6 => 0
6 to 2 => 4
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
5 => N
0 => E
0 => E
4 => N
Step 4: Combine the filters to a string:
C N E E N = CNEEN