Simple question about a more efficient way to implement Expander Function in S-DES

Question

SO, I'm implementing (Simplified)-DES in C++, and I've got a working Expander function. The expander function is defined as, given a 6 entry input (say, {1,2,3,4,5,6}, we output a new, 8 entry output in the following form: {1,2,4,3,4,3,5,6}. I have this implemented as follows, but I'm sure there is a much better, and likely faster way to implement this. Any suggestions?

void expandchars(std::vector<int> &vec){
std::vector<int> temp=vec;
vec={temp[0],temp[1],temp[3],temp[2],temp[3],temp[2],temp[4],temp[5]};
}

Answer 1

A faster way would be to avoid allocating a temp vector. If vec.capacity() < 8 then your code is almost optimal. To make it optimal:

void realloc_expandchars(std::vector<int> &vec){
  // Optimal if vec.capacity() <8, since a reallocation is unavoidable
  std::vector<int> temp=std::move(vec);
  vec={temp[0],temp[1],temp[3],temp[2],temp[3],temp[2],temp[4],temp[5]};
}

If vec.capacity() >= 8 then you probably want to avoid creating a new vector. For that, you might want to do something like:

void inplace_expandchars(std::vector<int> &vec){
    vec.insert(vec.begin() + 4, 2 /*count*/, 0 /* value */);
    vec[4] = vec[3];
    vec[5] = vec[2];
    vec[2] = vec[4];
    vec[3] = vec[5];
}

Possibly slightly slower, but more readable would be to use std::array:

void inplace_expandchars(std::vector<int> &vec){
    std::array<int, 8> temp = {vec[0], vec[1], vec[3], vec[2], vec[3], vec[2], vec[4], vec[5]}; 
    vec.clear();
    vec.insert(vec.begin(), temp.begin(), temp.end());
}

And then combine it all:

   void expandchars(std::vector<int> &vec){
     if (vec.capacity() < 8)
         realloc_expandchars(vec);
     else
         inplace_expandchars(vec);
   }