Step 1: Divide the number into separate digits:
9909 => 9 9 0 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 9 => 0
9 to 0 => 9
0 to 9 => 9
9 to 9 => 0
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
0 => E
9 => C
9 => C
0 => E
Step 4: Combine the filters to a string:
E C C E = ECCE