Step 1: Divide the number into separate digits:
7998 => 7 9 9 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 9 => 2
9 to 9 => 0
9 to 8 => 1
8 to 7 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
0 => E
1 => C
1 => C
Step 4: Combine the filters to a string:
N E C C = NECC