Step 1: Divide the number into separate digits:
1641 => 1 6 4 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 6 => 5
6 to 4 => 2
4 to 1 => 3
1 to 1 => 0
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
5 => N
2 => N
3 => N
0 => E
Step 4: Combine the filters to a string:
N N N E = NNNE