Step 1: Divide the number into separate digits:
0997 => 0 9 9 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 9 => 9
9 to 9 => 0
9 to 7 => 2
7 to 0 => 7
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
9 => C
0 => E
2 => N
7 => N
Step 4: Combine the filters to a string:
C E N N = CENN