Step 1: Divide the number into separate digits:
17992 => 1 7 9 9 2
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 9 => 0
9 to 2 => 7
2 to 1 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
6 => N
2 => N
0 => E
7 => N
1 => C
Step 4: Combine the filters to a string:
N N E N C = NNENC