#include <string>
#include <algorithm>
#if (__cplusplus != 201103)
#define nullptr NULL
#endif

#ifndef BOOST_RANDOM_HPP
#define RANDOM rand
#else
//or any other engine you'd like, I suppose
::boost::random::mt19937 __engine_a;
#define RANDOM __engine_a
#endif

using namespace std;
bool check_key(string &key)
{
    //simply runs through and checks to see if any characters 'mirror' each other
    for(size_t x = NULL, y = (key.size() - 1) ; x < key.size(); x++, y--)
    {
        if(key[x] == key[y])
            {return false;}
        else continue;
    }
    return true;
}
string* keygen(size_t size, string charset)
{
    if(size & 1) //can only do even number of elements
        {return nullptr;}
    else
    {
        //allocate the new string to return
        string* tmp = new string;
        tmp->resize(size, '\0');
        //initialize the string with random characters
        for(size_t x = NULL; x < size; x++)
            {(*tmp)[x] = charset[RANDOM() % charset.size()];}
        //goes through and on any matches replaces a character
        while(!check_key(*tmp))
        {
            for(size_t x = NULL, y = (size - 1); x < size; x++, y--)
            {
                if((*tmp)[x] == (*tmp)[y])
                    {(*tmp)[x] = charset[RANDOM() % charset.size()];}
                else continue;
            }
        }
        return tmp;
    }
    return nullptr; //in here because of compiler warnings about control structures reaching the end of function... blah.
}