Step 1: Divide the number into separate digits:
661 => 6 6 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 6 => 0
6 to 1 => 5
1 to 6 => 5
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
0 => E
5 => N
5 => N
Step 4: Combine the filters to a string:
E N N = ENN