Step 1: Divide the number into separate digits:
91738 => 9 1 7 3 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 1 => 8
1 to 7 => 6
7 to 3 => 4
3 to 8 => 5
8 to 9 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
8 => N
6 => N
4 => N
5 => N
1 => C
Step 4: Combine the filters to a string:
N N N N C = NNNNC