Step 1: Divide the number into separate digits:
1531 => 1 5 3 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 5 => 4
5 to 3 => 2
3 to 1 => 2
1 to 1 => 0
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
4 => N
2 => N
2 => N
0 => E
Step 4: Combine the filters to a string:
N N N E = NNNE