Step 1: Divide the number into separate digits:
1749 => 1 7 4 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 7 => 6
7 to 4 => 3
4 to 9 => 5
9 to 1 => 8
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
6 => N
3 => N
5 => N
8 => N
Step 4: Combine the filters to a string:
N N N N = NNNN