Step 1: Divide the number into separate digits:
7544 => 7 5 4 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 5 => 2
5 to 4 => 1
4 to 4 => 0
4 to 7 => 3
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
1 => C
0 => E
3 => N
Step 4: Combine the filters to a string:
N C E N = NCEN