/* A bunch of cars are racing on a track shaped like the number 8. RoadTile number 0 is the intersection of the 8 shaped race track. Semaphores ensure that there are no accidents. Not more than 2 cars can drive through same RoadTile at the same time. 2 cars limit enables drivers to pass in front of each other but 3 would be too dangerous as the road is too narrow. */ #include #include #include #include #include #include #include #include using namespace std; ///////////////////////////////////////// class Semaphore { public: Semaphore(int max=1):count(max){} void release(); void acquire(); bool try_acquire(); private: mutex mtx; condition_variable cv; int count; }; void Semaphore::release(){ lock_guardlock(mtx); ++count; cv.notify_one(); } void Semaphore::acquire(){ unique_locklock(mtx); while(!count) cv.wait(lock); --count; } bool Semaphore::try_acquire(){ lock_guardlock(mtx); return count?count--:false; } mutex message_mtx; void message(string m){ lock_guard lock(message_mtx); cout< next=nullptr; shared_ptr data; Node(shared_ptr d):data(d){} }; class DeadlyEight{ public: DeadlyEight(unsigned n); const shared_ptr & start(){ return head; } ~DeadlyEight(){head=head->next=nullptr;} private: void addNodeToBack(shared_ptr n); void intersectAt(unsigned index); shared_ptr head; }; DeadlyEight::DeadlyEight(unsigned n){ for(unsigned i=0;i(make_shared(i))); intersectAt(n/2); } void DeadlyEight::intersectAt(unsigned index){ shared_ptr crossing=make_shared(head->data); shared_ptr cursor = head; while(index--) cursor = cursor->next; shared_ptr next2 = cursor->next; cursor->next = crossing; crossing->next = next2; cursor = next2; while(cursor->next!=nullptr) cursor=cursor->next; cursor -> next = head; } void DeadlyEight::addNodeToBack(shared_ptr n){ if(head==nullptr && (head=n)) return; shared_ptr cursor = head; while(cursor->next!=nullptr) cursor=cursor->next; cursor->next = n; } ///////////////////////////////////////// chrono::system_clock::time_point start_time; void car(string name,const shared_ptr & start){ shared_ptrcurrent=start; const int sleep=name=="Gilles Villeneuve"?1:rand()%5+2; while(true){ current->data->acquire(); message(name+" drives through tile "+to_string(current->data->id)); this_thread::sleep_for(chrono::milliseconds(sleep)); if(current->next->data->id==0&¤t->data->id==20){ auto now=chrono::system_clock::now(); message(name+" finished a lap 🏁 after "+to_string(chrono::duration_cast(now-start_time).count())+" ms"); } current->data->release(); current=current->next; } } ///////////////////////////////////////// int main() { DeadlyEight track(21); start_time = chrono::system_clock::now(); thread car1(car,"Gilles Villeneuve",track.start()); thread car2(car,"Michael Schumaker",track.start()); thread car3(car,"Mika Hakinen",track.start()); thread car4(car,"Jacques Villeneuve",track.start()); thread car5(car,"David Coulthard",track.start()); thread car6(car,"Alain Prost",track.start()); car1.join(); car2.join(); car3.join(); car4.join(); car5.join(); car6.join(); return 0; }

Deadly 8 - Semaphores - Threads