Your representation is base26, so the answer that you are going to get from a three-character value is not going to be 12 bits: it's going to be in the range 0..17575, inclusive, which requires 15 bits.
Recall that shifting left by k
bits is the same as multiplying by 2^k
. Hence, your x << 4
operations are equivalent to multiplying by 16. Also recall that when you convert a base-X number, you need to multiply its digits by a power of X, so your code should be multiplying by 26, rather than shifting the number left, like this:
int n = (chars.IndexOf(s[0]) * 26*26) +
(chars.IndexOf(s[1]) * 26) +
(chars.IndexOf(s[2]));