Step 1: Divide the number into separate digits:
17908 => 1 7 9 0 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 7 => 6
7 to 9 => 2
9 to 0 => 9
0 to 8 => 8
8 to 1 => 7
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
6 => N
2 => N
9 => C
8 => N
7 => N
Step 4: Combine the filters to a string:
N N C N N = NNCNN