/************************************************************
  File:          bridge.cc
  Description:   A program that simulates a one-lane bridge
                 and some cars trying to get through it.

                 Implementation of the bridge monitor.
                                                         
  Author:        Dana Vrajitoru                          
  Organization:  IUSB                                    
  Date:          December 7, 2004
**************************************************************/

#include "bridge.h"

#include <iostream>
using namespace std;

// Simple init function setting all the values of the variables to 0.
void Bridge_monitor::init()
{
  crossing = 0;
  waiting_left = 0;
  waiting_right = 0;
  pthread_mutex_init(&mutex, NULL);
  pthread_cond_init(&cond_left, NULL);
  pthread_cond_init(&cond_right, NULL);
}

// A tread is calling these when it wants to cross the bridge to the
// left or right respectively. If the bridge is not free, the thread
// should be waiting on the appropriate conditional variable to be
// allowed to cross the bridge.
void Bridge_monitor::request_left()
{
  pthread_mutex_lock(&mutex);
  if (crossing || waiting_left || waiting_right) {
    waiting_left++;
    pthread_cond_wait(&cond_left, &mutex);
  }
  crossing++;
  pthread_mutex_unlock(&mutex);
}

void Bridge_monitor::request_right()
{
  // To be completed by the student.
}

// A thread is calling these after it has finished crossing the
// bridge. The last thread to cross should broadcast the conditional
// variable in the opposite direction if any thread is waiting on it,
// or the one in the same direction if not.
void Bridge_monitor::release_left()
{
  pthread_mutex_lock(&mutex);
  crossing--;
  if (crossing == 0)
    if (waiting_right) {
      pthread_cond_broadcast(&cond_right);
      waiting_right = 0;
    }
    else {
      pthread_cond_broadcast(&cond_left);
      waiting_left = 0;
    }
  pthread_mutex_unlock(&mutex);
}

void Bridge_monitor::release_right()
{
  // To be completed by the student.
}