Step 1: Divide the number into separate digits:
9931 => 9 9 3 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 9 => 0
9 to 3 => 6
3 to 1 => 2
1 to 9 => 8
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
0 => E
6 => N
2 => N
8 => N
Step 4: Combine the filters to a string:
E N N N = ENNN