Step 1: Divide the number into separate digits:
1997 => 1 9 9 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 9 => 8
9 to 9 => 0
9 to 7 => 2
7 to 1 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
8 => N
0 => E
2 => N
6 => N
Step 4: Combine the filters to a string:
N E N N = NENN