Step 1: Divide the number into separate digits:
374 => 3 7 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 7 => 4
7 to 4 => 3
4 to 3 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
4 => N
3 => N
1 => C
Step 4: Combine the filters to a string:
N N C = NNC