Step 1: Divide the number into separate digits:
13577 => 1 3 5 7 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 3 => 2
3 to 5 => 2
5 to 7 => 2
7 to 7 => 0
7 to 1 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
2 => N
2 => N
0 => E
6 => N
Step 4: Combine the filters to a string:
N N N E N = NNNEN