multithreading - Make Monte Carlo eval multithreaded for Hexgame C++ -

i trying make monte carlo eval function multithreaded computer player in hexgame faster. normal code (without multithreading) is:

using namespace std;  int getmontecarloeval(const state &board, player player, int trials) {     vector<move> moves = board.getmoves();     int wins = 0;      (int = 0; < trials; ++i)     {         state mcboard = board;          random_shuffle(moves.begin(), moves.end());         (const move &m : moves)             mcboard.domove(m);         if (mcboard.getwinner() == player)             wins++;     }      return wins - trials / 2; }  int getrandomeval(const state &board, player) {     return rand() % 101 - 50; } 

with multithreading have this:

using namespace std; vector<move> moves; atomic<int> wins = 0;  int getmontecarloeval(const state &board, player player, int trials) {     vector<move> moves = board.getmoves();     state mcboard = board;     size_t nr_threads = 8;     vector<thread> workers;      int delta = trials / nr_threads;     int remainder = trials % nr_threads;     int l = 0; int r = 0;      (int = 0; < nr_threads; i++) {         r = l + delta;         if (i == nr_threads - 1)             r += remainder;         workers.push_back(thread(mc, mcboard, player, l, r));         l = r;     }     (auto &thread : workers) {         thread.join();     }      return wins - trials / 2; }  int getrandomeval(const state &board, player) {     return rand() % 101 - 50; }  int mc(const state &board, player player, int l, int r) {     (int = l; < r; i++)     {         state mcboard = board;         random_shuffle(moves.begin(), moves.end());         (move m : moves)             mcboard.domove(m);         if (mcboard.getwinner() == player)             wins++;     } } 

the problem here is, computer player freaking stupid. without multithreading computer player plays slow. use alpha berta pruning searching best next step computer.

question: know doing wrong multithreading?