Step 1: Divide the number into separate digits:
99153 => 9 9 1 5 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 9 => 0
9 to 1 => 8
1 to 5 => 4
5 to 3 => 2
3 to 9 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
0 => E
8 => N
4 => N
2 => N
6 => N
Step 4: Combine the filters to a string:
E N N N N = ENNNN