Step 1: Divide the number into separate digits:
23708 => 2 3 7 0 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 3 => 1
3 to 7 => 4
7 to 0 => 7
0 to 8 => 8
8 to 2 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
4 => N
7 => N
8 => N
6 => N
Step 4: Combine the filters to a string:
C N N N N = CNNNN