Step 1: Divide the number into separate digits:
6686 => 6 6 8 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 6 => 0
6 to 8 => 2
8 to 6 => 2
6 to 6 => 0
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
0 => E
2 => N
2 => N
0 => E
Step 4: Combine the filters to a string:
E N N E = ENNE