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Copy pathBridgeTraffic.java
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122 lines (108 loc) · 2.83 KB
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/*
* There is a bridge as shown in the picture,
* and the direction of traffic flow is shown by the arrow.
* Suppose two cars are not allowed to intersect on the bridge,
* but multiple cars are allowed to travel in the same direction.
* Traffic management on the bridge.
*/
public class BridgeTraffic {
public static void main(String[] args) {
Semaphores semaphores = new Semaphores();
Thread n2s = new Thread(new N2S(semaphores));
Thread s2n = new Thread(new S2N(semaphores));
n2s.start();
s2n.start();
}
}
class Semaphores {
public int countSN = 0;
public int countNS = 0;
public boolean mutexSN = true;
public boolean mutexNS = true;
public boolean bridge = true;
public synchronized boolean PSN() {
if (mutexSN) {
mutexSN = false;
return true;
} else {
return false;
}
}
public synchronized boolean PNS() {
if (mutexNS) {
mutexNS = false;
return true;
} else {
return false;
}
}
public synchronized boolean Pb() {
if (bridge) {
bridge = false;
return true;
} else {
return false;
}
}
public synchronized void VSN() {
mutexSN = true;
}
public synchronized void VNS() {
mutexNS = true;
}
public synchronized void Vb() {
bridge = true;
}
}
class S2N implements Runnable {
public Semaphores semaphores;
public S2N(Semaphores semaphores) {
this.semaphores = semaphores;
}
public void run() {
while (true) {
while (!semaphores.PSN()) {
}
if (semaphores.countSN == 0) {
while (!semaphores.Pb()) {
}
}
semaphores.countSN++;
semaphores.VSN();
System.out.println("From S to N.");
while (!semaphores.PSN()) {
}
semaphores.countSN--;
if (semaphores.countSN == 0) {
semaphores.Vb();
}
semaphores.VSN();
}
}
}
class N2S implements Runnable {
public Semaphores semaphores;
public N2S(Semaphores semaphores) {
this.semaphores = semaphores;
}
public void run() {
while (true) {
while (!semaphores.PNS()) {
}
if (semaphores.countNS == 0) {
while (!semaphores.Pb()) {
}
}
semaphores.countNS++;
semaphores.VNS();
System.out.println("From N to S.");
while (!semaphores.PNS()) {
}
semaphores.countNS--;
if (semaphores.countNS == 0) {
semaphores.Vb();
}
semaphores.VNS();
}
}
}