Step 1: Divide the number into separate digits:
680 => 6 8 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 8 => 2
8 to 0 => 8
0 to 6 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
8 => N
6 => N
Step 4: Combine the filters to a string:
N N N = NNN