>>1
If you know in advance the set of all strings that will be hashed, then you can experiment with different hashing functions until every string is assigned a unique int32 or int 64 value. Because of the birthday paradox, it'll get pretty difficult as number of strings gets larger.
http://en.wikipedia.org/wiki/Birthday_paradox
It is impossible if the number of strings is greater than the number of int32 or int64 values, but 2^32 and 2^64 are pretty large numbers.