Step 1: Divide the number into separate digits:
25611 => 2 5 6 1 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 5 => 3
5 to 6 => 1
6 to 1 => 5
1 to 1 => 0
1 to 2 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
3 => N
1 => C
5 => N
0 => E
1 => C
Step 4: Combine the filters to a string:
N C N E C = NCNEC