Step 1: Divide the number into separate digits:
1291 => 1 2 9 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 2 => 1
2 to 9 => 7
9 to 1 => 8
1 to 1 => 0
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
7 => N
8 => N
0 => E
Step 4: Combine the filters to a string:
C N N E = CNNE