Step 1: Divide the number into separate digits:
7870 => 7 8 7 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 8 => 1
8 to 7 => 1
7 to 0 => 7
0 to 7 => 7
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
1 => C
7 => N
7 => N
Step 4: Combine the filters to a string:
C C N N = CCNN