/*! Synchronization example with monitor on "old bridge" problem */

#include <pthread.h>
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>

#define THREADS  20

/* mutex and conditional variables, statically initialized */
static pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t s_n = PTHREAD_COND_INITIALIZER;
static pthread_cond_t n_s = PTHREAD_COND_INITIALIZER;
static pthread_cond_t *cq[2] = {&s_n, &n_s};

struct CarInfo {
	int id;
	int dir;
};
static char *sdir[] = {"south", "north"};

static int cars_on_bridge = 0;
static int dir_on_bridge = -1; /* 0: S->N; 1:N->S; -1:bridge empty */

void *car_thread(void *p)
{
	struct CarInfo *car = p;
	struct timespec t = {5, 0};

	pthread_mutex_lock(&m);

	while (cars_on_bridge > 2 ||
           (dir_on_bridge != -1 && dir_on_bridge != car->dir))
		pthread_cond_wait(cq[car->dir], &m);

	cars_on_bridge++;
	dir_on_bridge = car->dir;

	printf("Car %2d  on bridge, going %s, on bridge %d car(s) (to %s)\n",
		car->id, sdir[car->dir], cars_on_bridge, sdir[dir_on_bridge]);

	pthread_mutex_unlock(&m);

	nanosleep(&t, NULL);

	pthread_mutex_lock(&m);

	cars_on_bridge--;

	printf("Car %2d off bridge, going %s, on bridge %d car(s) (to %s)\n",
		car->id, sdir[car->dir], cars_on_bridge, sdir[dir_on_bridge]);

	if (cars_on_bridge > 0) {
		pthread_cond_signal(cq[car->dir]);
	}
	else {
		dir_on_bridge = -1;
		pthread_cond_broadcast(cq[1 - car->dir]);
	}

	pthread_mutex_unlock(&m);

	free(car);

	return NULL;
}

int main()
{
	pthread_t thr_id;
	pthread_attr_t attr;
	int i;
	struct CarInfo *car;
	struct timespec t = {2, 0};

	pthread_attr_init(&attr);
	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);

	for (i = 0; i < THREADS; i++) {
		car = malloc(sizeof(struct CarInfo));
		car->id = i+1;
		car->dir = rand() & 1;

		printf( "New car %d arrived from %s\n", i+1, sdir[1 - car->dir]);

		pthread_create(&thr_id, &attr, car_thread, (void *) car);

		nanosleep(&t, NULL);
	}

	return 0;
}

/* Example run:
$ gcc pthread_mutex.c -pthread -Wall
$ ./a.out
New car 1 arrived from south
Car  1  on bridge, going north, on bridge 1 car(s) (to north)
New car 2 arrived from north
New car 3 arrived from south
Car  3  on bridge, going north, on bridge 2 car(s) (to north)
Car  1 off bridge, going north, on bridge 1 car(s) (to north)
New car 4 arrived from south
Car  4  on bridge, going north, on bridge 2 car(s) (to north)
New car 5 arrived from south
Car  5  on bridge, going north, on bridge 3 car(s) (to north)
Car  3 off bridge, going north, on bridge 2 car(s) (to north)
New car 6 arrived from south
Car  6  on bridge, going north, on bridge 3 car(s) (to north)
Car  4 off bridge, going north, on bridge 2 car(s) (to north)
New car 7 arrived from north
Car  5 off bridge, going north, on bridge 1 car(s) (to north)
New car 8 arrived from north
Car  6 off bridge, going north, on bridge 0 car(s) (to north)
Car  2  on bridge, going south, on bridge 1 car(s) (to south)
...
*/