Step 1: Divide the number into separate digits:
1754 => 1 7 5 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 7 => 6
7 to 5 => 2
5 to 4 => 1
4 to 1 => 3
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
6 => N
2 => N
1 => C
3 => N
Step 4: Combine the filters to a string:
N N C N = NNCN