Concatenation of different datatypes and hashing it

Question

How can I concat two different datatypes and then hash it? for example: sha3(int8 + address)

Or it is also good for me hash them separately, and then hash together. like this: sha3(sha3(int8) + address)

How to do that most efficient way?

Answer 1

To concatenate the values, cast them into bytes, create an output bytes variable and then assign each byte

int8 _num;
address _addr;
bytes b0 = bytes(_num);
bytes b1 = bytes(_addr);
string s0 = new string(b0.length + b1.length);
bytes b2 = bytes(s0);
uint k = 0;
for (uint i = 0; i < b0.length(); i++) { b2[k++] = b0[i] }
for (i = 0; i < b1.length(); i++) {b2[k++] = b1[i] }

Then you can just hash the concatenated byte array. If you hashed these separately and then concatenated the two hashes it would probably be much less efficient (computationally) since you'd still have to write some code for concatenating them and they are much longer than the two variables.