Step 1: Divide the number into separate digits:
5645 => 5 6 4 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 6 => 1
6 to 4 => 2
4 to 5 => 1
5 to 5 => 0
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
2 => N
1 => C
0 => E
Step 4: Combine the filters to a string:
C N C E = CNCE