#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>

typedef struct {
    sem_t hSemaphore;
    sem_t oSemaphore;
    pthread_barrier_t barrier;
} H2O;

void initH2O(H2O* h2o) {
    sem_init(&h2o->hSemaphore, 0, 2);
    sem_init(&h2o->oSemaphore, 0, 1);
    pthread_barrier_init(&h2o->barrier, NULL, 3);
}

void hydrogen(H2O* h2o, void (*releaseHydrogen)()) {
    sem_wait(&h2o->hSemaphore);
    
    pthread_barrier_wait(&h2o->barrier);
    releaseHydrogen();
    
    sem_post(&h2o->hSemaphore);
}

void oxygen(H2O* h2o, void (*releaseOxygen)()) {
    sem_wait(&h2o->oSemaphore);
    
    pthread_barrier_wait(&h2o->barrier);
    releaseOxygen();
    
    sem_post(&h2o->oSemaphore);
}

void releaseH() { printf("H"); }
void releaseO() { printf("O"); }

void* hThread(void* arg) {
    H2O* h2o = (H2O*)arg;
    hydrogen(h2o, releaseH);
    return NULL;
}

void* oThread(void* arg) {
    H2O* h2o = (H2O*)arg;
    oxygen(h2o, releaseO);
    return NULL;
}

int main() {
    H2O h2o;
    initH2O(&h2o);
    
    pthread_t threads[6];
    char sequence[] = "HHOOHH";
    
    for (int i = 0; i < 6; i++) {
        if (sequence[i] == 'H') {
            pthread_create(&threads[i], NULL, hThread, &h2o);
        } else {
            pthread_create(&threads[i], NULL, oThread, &h2o);
        }
    }
    
    for (int i = 0; i < 6; i++) {
        pthread_join(threads[i], NULL);
    }
    
    return 0;
}