cb.py

import threading
import random
import time


num_of_head_north = 0
num_of_head_south = 0
counter_lock = threading.Lock()
bridge_cond = threading.Condition()
bridge_sem = threading.BoundedSemaphore(3)

class car(threading.Thread):
    def __init__(self, car_id, direction):
        threading.Thread.__init__(self)
        self.direction = direction
        self.car_id = car_id

    def run(self):
        if self.direction == 1: # 1 表示方向向南
            self.cross_bridge_to_south()

        elif self.direction == 0: # 0 表示方向向北
            self.cross_bridge_to_north()

    def cross_bridge_to_north(self):
        global num_of_head_north
        global num_of_head_south
        counter_lock.acquire()
        if num_of_head_south != 0: # 如果有向南走的车, 就等待
            counter_lock.release()
            bridge_cond.acquire()
            bridge_cond.wait()
            bridge_cond.release()
            while True:
                counter_lock.acquire()
                if num_of_head_south == 0:
                    num_of_head_north += 1
                    counter_lock.release()
                    break
                else:
                    counter_lock.release()
                    bridge_cond.acquire()
                    bridge_cond.wait()
                    bridge_cond.wait()



        else:
            num_of_head_north += 1 # 桥上的车数加一
            counter_lock.release()

        bridge_sem.acquire() # 获取上桥资格
        print(">>> on the bridge ", str(self.car_id))
        counter_lock.acquire()
        num_of_head_north -= 1 # 桥上的车数减一
        print(">>>>> leave the bridge", str(self.car_id))
        bridge_sem.release() # 释放上桥资源
        if num_of_head_north == 0: # 如果桥上向北走的车数为零， 唤醒所有向南走的车。
            bridge_cond.acquire()
            bridge_cond.notifyAll()
            bridge_cond.release()
            counter_lock.release()
        else:
            counter_lock.release()


    def cross_bridge_to_south(self):
        global num_of_head_north
        global num_of_head_south
        counter_lock.acquire()
        if num_of_head_north != 0: # 如果有向北走的车, 等待
            counter_lock.release()
            bridge_cond.acquire()
            bridge_cond.wait()
            bridge_cond.release()
            while True:
                counter_lock.acquire()
                if num_of_head_north == 0:
                    num_of_head_south += 1
                    counter_lock.release()
                    break
                else:
                    counter_lock.release()
                    bridge_cond.acquire()
                    bridge_cond.wait()
                    bridge_cond.release()


        else:
            num_of_head_south += 1
            counter_lock.release()

        bridge_sem.acquire() # 获得上桥的资源
        print("<-<-<- on the bridge ", str(self.car_id))

        counter_lock.acquire()
        num_of_head_south -= 1
        print("<-<-<-<-<- leave the bridge ", str(self.car_id))
        bridge_sem.release() # 释放上桥的资源
        if num_of_head_south == 0: # 如果桥上向南走的车数为零， 唤醒等待向北走的。
            bridge_cond.acquire()
            bridge_cond.notifyAll()
            bridge_cond.release()
            counter_lock.release()
        else:
            counter_lock.release()


def main():
    cars = []
    random.seed()
    for x in range(20):
        cars.append(car(x, random.randint(0,1)))
    
    for x in cars:
        x.start()
    
    for x in cars:
        x.join()
    
    print("all cars has gone")

main()