Step 1: Divide the number into separate digits:
3923 => 3 9 2 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 => 6
9 to 2 => 7
2 to 3 => 1
3 to 3 => 0
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
6 => N
7 => N
1 => C
0 => E
Step 4: Combine the filters to a string:
N N C E = NNCE