We Keep Coding

Two counters on different threads counting until one is done

Make 2 thread counters - counting from start to end value. The value of the counter should be displayed for each count. One thread is set to count to a smaller value than the other. When one of the threads counts to the setpoint, the other stops and completes its execution.
This is my task so I made a solution with a AtomicBoolean run = new AtomicBoolean(true);
/**
* Implements {#link Runnable} and his method {#link Runnable#run} to run until stop is set to true
* and count is less than maxCount.
*/
public class Counter implements Runnable {
private static final Logger logger = LoggerFactory.getLogger(Counter.class);
private static final String MAX_COUNT_EXCEPTION_MESSAGE = "maxCount should be bigger than zero!";
private static final int NUMBER_TO_CHECK_MAX_COUNT_VALIDATION = 1;
private AtomicBoolean run;
private int count;
private int maxCount;
/**
* Constructs a counter with zero count and run equal to true with a specified maxCount.
*
* #param maxCount of the counter
* #throws IllegalArgumentException if maxCount is smaller than 1
*/
Counter(int maxCount, AtomicBoolean run) {
if (maxCount < NUMBER_TO_CHECK_MAX_COUNT_VALIDATION) {
throw new IllegalArgumentException(MAX_COUNT_EXCEPTION_MESSAGE);
}
this.maxCount = maxCount;
this.run = run;
}
/**
* Entry point.
* <p>
* Runs until run is set to false and count is less than maxCount.
* <p>
* On every loop {#link Counter} increment count with 1 and print the count.
*/
#Override
public void run() {
while (count < maxCount && run.get()) {
incrementCount();
logger.info(this + " : " + count);
}
run.set(false);
}
/**
* Used for obtaining current value for the count.
*
* #return count.
*/
int getCount() {
return count;
}
/**
* Increment the count.
*/
private void incrementCount() {
count++;
}
}
/**
* Runs two counters and print the counting for both of them until one reach his maxCount then both are stopped.
*/
public class RunnerTwoCounters {
private static final Logger logger = LoggerFactory.getLogger(RunnerTwoCounters.class);
public static void main(String[] args) throws InterruptedException {
AtomicBoolean run = new AtomicBoolean(true);
Counter counter1 = new Counter(2, run);
Thread thread1 = new Thread(counter1);
Counter counter2 = new Counter(10, run);
Thread thread2 = new Thread(counter2);
thread1.start();
thread2.start();
thread1.join();
thread2.join();
logger.info(counter1 + " " + counter1.getCount());
logger.info(counter2 + " " + counter2.getCount());
}
}
How can I make the same thing to work but without Atomicboolean just to work with Object lock and some synchronization on it?

This does what your recent comment says. Both threads start counting. When one reaches the setPoint, they both stop.
public class ThreadCounting {
int value1;
int value2;
volatile boolean flag = true;
public static void main(String[] args) {
new ThreadCounting().start();
}
public void start() {
int setPoint = 100;
Thread begin = new Thread(() -> {
while(flag && ((value1 = counter1()) <= setPoint)) {
System.out.println(Thread.currentThread().getName() + " counter1 = " + value1);
}
flag = false;
});
Thread finish = new Thread(() -> {
while(flag && ((value2 = counter2()) <= setPoint)) {
System.out.println(Thread.currentThread().getName() +" counter2 = " + value2);
}
flag = false;
});
begin.start();
finish.start();
}
int val1 = 1;
public int counter1() {
return val1++;
}
int val2 = 1;
public int counter2() {
return val2++;
}
}

Error in my Queue simulator

The program simulates a customer service operation in places, e.g., call center, bank, store, airport, with customers being served by tellers. The customers arrive at random time and wait in a line until a teller is available to serve them. The waiting line is implemented with queue data structure. However im getting two minor errors 1.) my enqueue method is not applicable for the argument and 2.)cannot cast from int to customers. Here is the code. The error is in the bolded lines towards the end
import java.util.Random;
class MyQueue<E> {
private int maxSize;
private int[] queArray;
private int front;
private int rear;
public MyQueue(int s) // constructor
{
maxSize = s+1; // array is 1 cell larger
queArray = new int[maxSize]; // than requested
front = 0;
rear = -1;
}
public void enqueue(int j) // put item at rear of queue
{
if(rear == maxSize-1)
rear = -1;
queArray[++rear] = j;
}
public int dequeue() // take item from front of queue
{
int temp = queArray[front++];
if(front == maxSize)
front = 0;
return temp;
}
public int peek() // peek at front of queue
{
return queArray[front];
}
public boolean isEmpty() // true if queue is empty
{
return ( rear+1==front || (front+maxSize-1==rear) );
}
public boolean isFull() // true if queue is full
{
return ( rear+2==front || (front+maxSize-2==rear) );
}
public int size() // (assumes queue not empty)
{
if(rear >= front) // contiguous sequence
return rear-front+1;
else // broken sequence
return (maxSize-front) + (rear+1);
}
}
class Customer { int arrive; // Time point that the customer arrived. int processTime; // Time duration that the customer will need to be served.
/**
* Default constructor
*/
public Customer() {
arrive = 0;
processTime = 0;
}
/**
* Set the arrival time point of the customer.
*
* #param Time
* point
*/
public Customer(int arrivalTime) {
arrive = arrivalTime;
// We set the processing time as a random integer between 1 and 3.
processTime = (int) (Math.random() * 3 + 1);
}
/**
* #return the arrival time point of the customer.
*/
public int getArrivalTime() {
return arrive;
}
/**
* #return the processing time of the customer.
*/
public int getProcTime() {
return processTime;
}
}
public class Simulator { /** * The main method of the class. * * #param args * Command line arguments. */ public static void main(String[] args) {
if (args.length != 3) {
System.out.println("usage: " + Simulator.class.getSimpleName()
+ " qCapacity simHours customPerHour");
System.out.println("Example: " + Simulator.class.getSimpleName()
+ " 10 1 30");
System.exit(-1);
}
// maximum size of queue
int qCapacity = Integer.parseInt(args[0]);
// number of simulation hours
int simHours = Integer.parseInt(args[1]);
// average number of customers per hour
int custPerHour = Integer.parseInt(args[2]);
// Run simulation
simulation(qCapacity, simHours, custPerHour);
}
private static void simulation(int qCapacity, int simHours, int custPerHour) {
// Constant
final int MIN_PER_HR = 60;
// A queue that will hold and manage objects of type Customer.
MyQueue<Customer> line = new MyQueue<Customer>(qCapacity);
// For how many cycles should the simulation run. We assume that each
// cycle takes one minute.
int cycleLimit = MIN_PER_HR * simHours;
// The average number of customers can arrive per minute
float custPerMin = ((float) custPerHour) / MIN_PER_HR;
// The number of customers that were turned away because the line
// (queue)
// was full at the time they arrived.
int turnAways = 0;
// Number of customers that arrived.
int customers = 0;
// Number of customers that were served.
int served = 0;
// Total number of customers that entered the line (queue).
int sumLine = 0;
// Waiting time until the next customer is served.
int waitTime = 0;
// Total time that all the customers waited in the line.
int lineWait = 0;
// Simulation
for (int cycle = 0; cycle < cycleLimit; cycle++) {
float j = custPerMin;
while (j > 0) {
if (newCustomer(j)) {
if (line.isFull()) {
turnAways++;
} else {
customers++;
Customer customer = new Customer(cycle);
**line.enqueue(customer);**
}
}
j = j - 1;
}
if (waitTime <= 0 && !line.isEmpty())
{
**Customer customer = (Customer) line.dequeue();**
waitTime = customer.getProcTime();
lineWait += cycle - customer.getArrivalTime();
served++;
}
if (waitTime > 0) {
waitTime--;
}
sumLine += line.size();
}
// Print the simulation results.
if (customers > 0) {
System.out.println("\nCustomers accepted: " + customers);
System.out.println(" Customers served: " + served);
System.out.println(" Customers waiting: " + line.size());
System.out.println(" Turnaways: " + turnAways);
System.out.println("Average queue size: " + (float) sumLine
/ cycleLimit);
System.out.println(" Average wait time: " + (float) lineWait
/ served + " minutes");
} else {
System.out.println("No customers!");
}
}
private static boolean newCustomer(float j) {
if(j > 1)
return true;
else
return (j > Math.random() );
}

It looks like your problem is with these two methods:
public void enqueue(int j) // put item at rear of queue
{
if(rear == maxSize-1)
rear = -1;
queArray[++rear] = j;
}
public int dequeue() // take item from front of queue
{
int temp = queArray[front++];
if(front == maxSize)
front = 0;
return temp;
}
If you had intended on the Queue to hold anything but integers, then you'll need to change the argument type / return type to reflect that.

**line.enqueue(customer);**
// 1.) my enqueue method is not applicable for the argument
Your enqueue method takes an int argmuent, yet you're trying to pass a Customer type to it. Maybe you want something like this: line.enqueue(customer.getSomething());. I can't really tell from your code.
**Customer customer = (Customer) line.dequeue();**
// 2.)cannot cast from int to customers
(Customer) line.dequeue();. Here you're casting Customer to int-line.dequeue()
Your dequque method return am int so basically you're saying that a Customer equals and int, which is impossible unless Customer isint, which it isn't
You want this:
Customer customer = new Customer(line.dequeue)
// Customer constructor takes an int value

Thread-safe circular buffer in Java

Consider a few web server instances running in parallel. Each server holds a reference to a single shared "Status keeper", whose role is keeping the last N requests from all servers.
For example (N=3):
Server a: "Request id = ABCD" Status keeper=["ABCD"]
Server b: "Request id = XYZZ" Status keeper=["ABCD", "XYZZ"]
Server c: "Request id = 1234" Status keeper=["ABCD", "XYZZ", "1234"]
Server b: "Request id = FOO" Status keeper=["XYZZ", "1234", "FOO"]
Server a: "Request id = BAR" Status keeper=["1234", "FOO", "BAR"]
At any point in time, the "Status keeper" might be called from a monitoring application that reads these last N requests for an SLA report.
What's the best way to implement this producer-consumer scenario in Java, giving the web servers higher priority than the SLA report?
CircularFifoBuffer seems to be the appropriate data structure to hold the requests, but I'm not sure what's the optimal way to implement efficient concurrency.

Buffer fifo = BufferUtils.synchronizedBuffer(new CircularFifoBuffer());

Here's a lock-free ring buffer implementation. It implements a fixed-size buffer - there is no FIFO functionality. I would suggest you store a Collection of requests for each server instead. That way your report can do the filtering rather than getting your data structure to filter.
/**
* Container
* ---------
*
* A lock-free container that offers a close-to O(1) add/remove performance.
*
*/
public class Container<T> implements Iterable<T> {
// The capacity of the container.
final int capacity;
// The list.
AtomicReference<Node<T>> head = new AtomicReference<Node<T>>();
// TESTING {
AtomicLong totalAdded = new AtomicLong(0);
AtomicLong totalFreed = new AtomicLong(0);
AtomicLong totalSkipped = new AtomicLong(0);
private void resetStats() {
totalAdded.set(0);
totalFreed.set(0);
totalSkipped.set(0);
}
// TESTING }
// Constructor
public Container(int capacity) {
this.capacity = capacity;
// Construct the list.
Node<T> h = new Node<T>();
Node<T> it = h;
// One created, now add (capacity - 1) more
for (int i = 0; i < capacity - 1; i++) {
// Add it.
it.next = new Node<T>();
// Step on to it.
it = it.next;
}
// Make it a ring.
it.next = h;
// Install it.
head.set(h);
}
// Empty ... NOT thread safe.
public void clear() {
Node<T> it = head.get();
for (int i = 0; i < capacity; i++) {
// Trash the element
it.element = null;
// Mark it free.
it.free.set(true);
it = it.next;
}
// Clear stats.
resetStats();
}
// Add a new one.
public Node<T> add(T element) {
// Get a free node and attach the element.
totalAdded.incrementAndGet();
return getFree().attach(element);
}
// Find the next free element and mark it not free.
private Node<T> getFree() {
Node<T> freeNode = head.get();
int skipped = 0;
// Stop when we hit the end of the list
// ... or we successfully transit a node from free to not-free.
while (skipped < capacity && !freeNode.free.compareAndSet(true, false)) {
skipped += 1;
freeNode = freeNode.next;
}
// Keep count of skipped.
totalSkipped.addAndGet(skipped);
if (skipped < capacity) {
// Put the head as next.
// Doesn't matter if it fails. That would just mean someone else was doing the same.
head.set(freeNode.next);
} else {
// We hit the end! No more free nodes.
throw new IllegalStateException("Capacity exhausted.");
}
return freeNode;
}
// Mark it free.
public void remove(Node<T> it, T element) {
totalFreed.incrementAndGet();
// Remove the element first.
it.detach(element);
// Mark it as free.
if (!it.free.compareAndSet(false, true)) {
throw new IllegalStateException("Freeing a freed node.");
}
}
// The Node class. It is static so needs the <T> repeated.
public static class Node<T> {
// The element in the node.
private T element;
// Are we free?
private AtomicBoolean free = new AtomicBoolean(true);
// The next reference in whatever list I am in.
private Node<T> next;
// Construct a node of the list
private Node() {
// Start empty.
element = null;
}
// Attach the element.
public Node<T> attach(T element) {
// Sanity check.
if (this.element == null) {
this.element = element;
} else {
throw new IllegalArgumentException("There is already an element attached.");
}
// Useful for chaining.
return this;
}
// Detach the element.
public Node<T> detach(T element) {
// Sanity check.
if (this.element == element) {
this.element = null;
} else {
throw new IllegalArgumentException("Removal of wrong element.");
}
// Useful for chaining.
return this;
}
public T get () {
return element;
}
#Override
public String toString() {
return element != null ? element.toString() : "null";
}
}
// Provides an iterator across all items in the container.
public Iterator<T> iterator() {
return new UsedNodesIterator<T>(this);
}
// Iterates across used nodes.
private static class UsedNodesIterator<T> implements Iterator<T> {
// Where next to look for the next used node.
Node<T> it;
int limit = 0;
T next = null;
public UsedNodesIterator(Container<T> c) {
// Snapshot the head node at this time.
it = c.head.get();
limit = c.capacity;
}
public boolean hasNext() {
// Made into a `while` loop to fix issue reported by #Nim in code review
while (next == null && limit > 0) {
// Scan to the next non-free node.
while (limit > 0 && it.free.get() == true) {
it = it.next;
// Step down 1.
limit -= 1;
}
if (limit != 0) {
next = it.element;
}
}
return next != null;
}
public T next() {
T n = null;
if ( hasNext () ) {
// Give it to them.
n = next;
next = null;
// Step forward.
it = it.next;
limit -= 1;
} else {
// Not there!!
throw new NoSuchElementException ();
}
return n;
}
public void remove() {
throw new UnsupportedOperationException("Not supported.");
}
}
#Override
public String toString() {
StringBuilder s = new StringBuilder();
Separator comma = new Separator(",");
// Keep counts too.
int usedCount = 0;
int freeCount = 0;
// I will iterate the list myself as I want to count free nodes too.
Node<T> it = head.get();
int count = 0;
s.append("[");
// Scan to the end.
while (count < capacity) {
// Is it in-use?
if (it.free.get() == false) {
// Grab its element.
T e = it.element;
// Is it null?
if (e != null) {
// Good element.
s.append(comma.sep()).append(e.toString());
// Count them.
usedCount += 1;
} else {
// Probably became free while I was traversing.
// Because the element is detached before the entry is marked free.
freeCount += 1;
}
} else {
// Free one.
freeCount += 1;
}
// Next
it = it.next;
count += 1;
}
// Decorate with counts "]used+free".
s.append("]").append(usedCount).append("+").append(freeCount);
if (usedCount + freeCount != capacity) {
// Perhaps something was added/freed while we were iterating.
s.append("?");
}
return s.toString();
}
}
Note that this is close to O1 put and get. A Separator just emits "" first time around and then its parameter from then on.
Edit: Added test methods.
// ***** Following only needed for testing. *****
private static boolean Debug = false;
private final static String logName = "Container.log";
private final static NamedFileOutput log = new NamedFileOutput("C:\\Junk\\");
private static synchronized void log(boolean toStdoutToo, String s) {
if (Debug) {
if (toStdoutToo) {
System.out.println(s);
}
log(s);
}
}
private static synchronized void log(String s) {
if (Debug) {
try {
log.writeLn(logName, s);
} catch (IOException ex) {
ex.printStackTrace();
}
}
}
static volatile boolean testing = true;
// Tester object to exercise the container.
static class Tester<T> implements Runnable {
// My name.
T me;
// The container I am testing.
Container<T> c;
public Tester(Container<T> container, T name) {
c = container;
me = name;
}
private void pause() {
try {
Thread.sleep(0);
} catch (InterruptedException ex) {
testing = false;
}
}
public void run() {
// Spin on add/remove until stopped.
while (testing) {
// Add it.
Node<T> n = c.add(me);
log("Added " + me + ": " + c.toString());
pause();
// Remove it.
c.remove(n, me);
log("Removed " + me + ": " + c.toString());
pause();
}
}
}
static final String[] strings = {
"One", "Two", "Three", "Four", "Five",
"Six", "Seven", "Eight", "Nine", "Ten"
};
static final int TEST_THREADS = Math.min(10, strings.length);
public static void main(String[] args) throws InterruptedException {
Debug = true;
log.delete(logName);
Container<String> c = new Container<String>(10);
// Simple add/remove
log(true, "Simple test");
Node<String> it = c.add(strings[0]);
log("Added " + c.toString());
c.remove(it, strings[0]);
log("Removed " + c.toString());
// Capacity test.
log(true, "Capacity test");
ArrayList<Node<String>> nodes = new ArrayList<Node<String>>(strings.length);
// Fill it.
for (int i = 0; i < strings.length; i++) {
nodes.add(i, c.add(strings[i]));
log("Added " + strings[i] + " " + c.toString());
}
// Add one more.
try {
c.add("Wafer thin mint!");
} catch (IllegalStateException ise) {
log("Full!");
}
c.clear();
log("Empty: " + c.toString());
// Iterate test.
log(true, "Iterator test");
for (int i = 0; i < strings.length; i++) {
nodes.add(i, c.add(strings[i]));
}
StringBuilder all = new StringBuilder ();
Separator sep = new Separator(",");
for (String s : c) {
all.append(sep.sep()).append(s);
}
log("All: "+all);
for (int i = 0; i < strings.length; i++) {
c.remove(nodes.get(i), strings[i]);
}
sep.reset();
all.setLength(0);
for (String s : c) {
all.append(sep.sep()).append(s);
}
log("None: " + all.toString());
// Multiple add/remove
log(true, "Multi test");
for (int i = 0; i < strings.length; i++) {
nodes.add(i, c.add(strings[i]));
log("Added " + strings[i] + " " + c.toString());
}
log("Filled " + c.toString());
for (int i = 0; i < strings.length - 1; i++) {
c.remove(nodes.get(i), strings[i]);
log("Removed " + strings[i] + " " + c.toString());
}
c.remove(nodes.get(strings.length - 1), strings[strings.length - 1]);
log("Empty " + c.toString());
// Multi-threaded add/remove
log(true, "Threads test");
c.clear();
for (int i = 0; i < TEST_THREADS; i++) {
Thread t = new Thread(new Tester<String>(c, strings[i]));
t.setName("Tester " + strings[i]);
log("Starting " + t.getName());
t.start();
}
// Wait for 10 seconds.
long stop = System.currentTimeMillis() + 10 * 1000;
while (System.currentTimeMillis() < stop) {
Thread.sleep(100);
}
// Stop the testers.
testing = false;
// Wait some more.
Thread.sleep(1 * 100);
// Get stats.
double added = c.totalAdded.doubleValue();
double skipped = c.totalSkipped.doubleValue();
//double freed = c.freed.doubleValue();
log(true, "Stats: added=" + c.totalAdded + ",freed=" + c.totalFreed + ",skipped=" + c.totalSkipped + ",O(" + ((added + skipped) / added) + ")");
}

Maybe you want to look at Disruptor - Concurrent Programming Framework.
Find a paper describing the alternatives, design and also a performance comparement to java.util.concurrent.ArrayBlockingQueue here: pdf
Consider to read the first three articles from BlogsAndArticles
If the library is too much, stick to java.util.concurrent.ArrayBlockingQueue

I would have a look at ArrayDeque, or for a more concurrent implementation have a look at the Disruptor library which is one of the most sophisticated/complex ring buffer in Java.
An alternative is to use an unbounded queue which is more concurrent as the producer never needs to wait for the consumer. Java Chronicle
Unless your needs justify the complexity, an ArrayDeque may be all you need.

Also have a look at java.util.concurrent.
Blocking queues will block until there is something to consume or (optionally) space to produce:
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/BlockingQueue.html
Concurrent linked queue is non-blocking and uses a slick algorithm that allows a producer and consumer to be active concurrently:
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ConcurrentLinkedQueue.html

Hazelcast's Queue offers almost everything you ask for, but doesn't support circularity. But from your description I am not sure if you actually need it.

If it were me, I would use the CircularFIFOBuffer as you indicated, and synchronize around the buffer when writing (add). When the monitoring application wants to read the buffer, synchronize on the buffer, and then copy or clone it to use for reporting.
This suggestion is predicated on the assumption that latency is minimal to copy/clone the buffer to a new object. If there are large number of elements, and copying time is slow, then this is not a good idea.
Pseudo-Code example:
public void writeRequest(String requestID) {
synchronized(buffer) {
buffer.add(requestID);
}
}
public Collection<String> getRequests() {
synchronized(buffer) {
return buffer.clone();
}
}

Since you specifically ask to give writers (that is web servers) higher priority than the reader (that is monitoring), I would suggest the following design.
Web servers add request information to a concurrent queue which is read by a dedicated thread, which adds requests to a thread-local (therefore non-synchronized) queue that overwrites the oldest element, like EvictingQueue or CircularFifoQueue.
This same thread checks a flag which indicates if a report has been requested after every request processed, and if positive, produces a report from all elements present in the thread-local queue.

how to implement multithreaded Breadth-first search in java?

I've done the breadth-first search in a normal way.
now I'm trying to do it in a multithreaded way.
i have one queue which is shared between the threads.
i use synchronize(LockObject) when i remove a node from the queue ( FIFI queue )
so what I'm trying to do is that.
when i thread finds a solution all the other threads will stop immediately.

i assume you are traversing a tree for your BFS.
create a thread pool.
for each unexplored children in the node, retrieve a thread from the thread pool (perhaps using a Semaphore). mark the child node as 'explored' and explore the node's children in a BFS manner. when you have found a solution or done exploring all the nodes, release the semaphore.
^ i've never done this before so i might have missed out something.

Assuming you want to do this iteratively (see note at the bottom why there may be better closed solutions), this is not a great problem for exercising multi threading. The problem is that multithreading is great if you don't depend on previous results, but here you want the minimum amount of coins.
As you point out, a breadth first solution guarantees that once you reach the desired amount, you won't have any further solutions with less coins in a single threaded environment. However, in a multithreaded environment, once you start calculating a solution, you cannot guarantee that it will finish before some other solution. Let's imagine for the value 21: it can be a 20c coin and a 1c or four 5c coins and a 1c; if both are calculating simultaneously, you cannot guarantee that the first (and correct) solution will finish first. In practice, it is unlikely the situation will happen, but when you work with multithreading you want the solution to work in theory, because multithreads always fail in the demonstration, no matter if they should not have failed until the death heat of the universe.
Now you have 2 possible solutions: one is to introduce choke points at the beginning of each level; you don't start that level until the previous level is finished. The other is once you reach a solution continue doing all the calculations with a lower level than the current result (which means you cannot purge the others). Probably with all the synchronization needed you get better performance by going single threaded, but let's go on.
For the first solution, the natural form is to iterate increasing the level. You can use the solution provided by happymeal, with a Semaphore. An alternative is to use the new classes provided by java.
CoinSet getCoinSet(int desiredAmount) throws InterruptedException {
// Use whatever number of threads you prefer or another member of Executors.
final ExecutorService executor = Executors.newFixedThreadPool(10);
ResultContainer container = new ResultContainer();
container.getNext().add(new Producer(desiredAmount, new CoinSet(), container));
while (container.getResult() == null) {
executor.invokeAll(container.setNext(new Vector<Producer>()));
}
return container.getResult();
}
public class Producer implements Callable<CoinSet> {
private final int desiredAmount;
private final CoinSet data;
private final ResultContainer container;
public Producer(int desiredAmount, CoinSet data, ResultContainer container) {
this.desiredAmount = desiredAmount;
this.data = data;
this.container = container;
}
public CoinSet call() {
if (data.getSum() == desiredAmount) {
container.setResult(data);
return data;
} else {
Collection<CoinSet> nextSets = data.addCoins();
for (CoinSet nextSet : nextSets) {
container.getNext().add(new Producer(desiredAmount, nextSet, container));
}
return null;
}
}
}
// Probably it is better to split this class, but you then need to pass too many parameters
// The only really needed part is to create a wrapper around getNext, since invokeAll is
// undefined if you modify the list of tasks.
public class ResultContainer {
// I use Vector because it is synchronized.
private Vector<Producer> next = new Vector<Producer>();
private CoinSet result = null;
// Note I return the existing value.
public Vector<Producer> setNext(Vector<Producer> newValue) {
Vector<Producer> current = next;
next = newValue;
return current;
}
public Vector<Producer> getNext() {
return next;
}
public synchronized void setResult(CoinSet newValue) {
result = newValue;
}
public synchronized CoinSet getResult() {
return result;
}
}
This still has the problem that existing tasks are executed; however, it is simple to fix that; pass the thread executor into each Producer (or the container). Then, when you find a result, call executor.shutdownNow. The threads that are executing won't be interrupted, but the operation in each thread is trivial so it will finish fast; the runnables that have not started won't start.
The second option means you have to let all the current tasks finish, unless you keep track of how many tasks you have run at each level. You no longer need to keep track of the levels, though, and you don't need the while cycle. Instead, you just call
executor.submit(new Producer(new CoinSet(), desiredAmount, container)).get();
And then, the call method is pretty similar (assume you have executor in the Producer):
public CoinSet call() {
if (container.getResult() != null && data.getCount() < container.getResult().getCount()) {
if (data.getSum() == desiredAmount)) {
container.setResult(data);
return data;
} else {
Collection<CoinSet> nextSets = data.addCoins();
for (CoinSet nextSet : nextSets) {
executor.submit(new Producer(desiredAmount, nextSet, container));
}
return null;
}
}
}
and you also have to modify container.setResult, since you cannot depend that between the if and setting the value it has not been set by some other threads (threads are really annoying, aren't they?)
public synchronized void setResult(CoinSet newValue) {
if (newValue.getCount() < result.getCount()) {
result = newValue;
}
}
In all previous answers, CoinSet.getSum() returns the sum of the coins in the set, CoinSet.getCount() returns the number of coins, and CoinSet.addCoins() returns a Collection of CoinSet in which each element is the current CoinSet plus one coin of each possible different value
Note: For the problem of the coins with the values 1, 5, 10 and 20, the simplest solution is take the amount and divide it by the largest coin. Then take the modulus of that and use the next largest value and so on. That is the minimum amount of coins you are going to need. This rule applies (AFAICT) when the following property if true: if for all consecutive pairs of coin values (i.e. in this case, 1-5, 5-10, 10-20) you can reach any int multiple of the lower element in the pair with with a smaller number of coins using the larger element and whatever coins are necessary. You only need to prove it to the min common multiple of both elements in the pair (after that it repeats itself)

I gather from your comment on happymeal's anwer that you are trying to find how to reach a specific amount of money by adding coins of 1c, 5c, 10c and 20c.
Since each coin denomination divides the denomination of the next bigger coin, this can be solved in constant time as follows:
int[] coinCount(int amount) {
int[] coinValue = {20, 10, 5, 1};
int[] coinCount = new int[coinValue.length];
for (int i = 0; i < coinValue.length; i++) {
coinCount[i] = amount / coinValue[i];
amount -= coinCount[i] * coinValue[i];
}
return coinCount;
}
Take home message: Try to optimize your algorithm before resorting to multithreading, because algorithmic improvements can yield much greater improvements.

I've successfully implemented it.
what i did is that i took all the nodes in the first level, let's say 4 nodes.
then i had 2 threads. each one takes 2 nodes and generate their children. whenever a node finds a solution it has to report the level that it found the solution in and limit the searching level so other threads don't exceed the level.
only the reporting method should be synchronized.
i did the code for the coins change problem. this is my code for others to use
Main Class (CoinsProblemBFS.java)
package coinsproblembfs;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
import java.util.Scanner;
/**
*
* #author Kassem M. Bagher
*/
public class CoinsProblemBFS
{
private static List<Item> MoneyList = new ArrayList<Item>();
private static Queue<Item> q = new LinkedList<Item>();
private static LinkedList<Item> tmpQ;
public static Object lockLevelLimitation = new Object();
public static int searchLevelLimit = 1000;
public static Item lastFoundNode = null;
private static int numberOfThreads = 2;
private static void InitializeQueu(Item Root)
{
for (int x = 0; x < MoneyList.size(); x++)
{
Item t = new Item();
t.value = MoneyList.get(x).value;
t.Totalvalue = MoneyList.get(x).Totalvalue;
t.Title = MoneyList.get(x).Title;
t.parent = Root;
t.level = 1;
q.add(t);
}
}
private static int[] calculateQueueLimit(int numberOfItems, int numberOfThreads)
{
int total = 0;
int[] queueLimit = new int[numberOfThreads];
for (int x = 0; x < numberOfItems; x++)
{
if (total < numberOfItems)
{
queueLimit[x % numberOfThreads] += 1;
total++;
}
else
{
break;
}
}
return queueLimit;
}
private static void initializeMoneyList(int numberOfItems, Item Root)
{
for (int x = 0; x < numberOfItems; x++)
{
Scanner input = new Scanner(System.in);
Item t = new Item();
System.out.print("Enter the Title and Value for item " + (x + 1) + ": ");
String tmp = input.nextLine();
t.Title = tmp.split(" ")[0];
t.value = Double.parseDouble(tmp.split(" ")[1]);
t.Totalvalue = t.value;
t.parent = Root;
MoneyList.add(t);
}
}
private static void printPath(Item item)
{
System.out.println("\nSolution Found in Thread:" + item.winnerThreadName + "\nExecution Time: " + item.searchTime + " ms, " + (item.searchTime / 1000) + " s");
while (item != null)
{
for (Item listItem : MoneyList)
{
if (listItem.Title.equals(item.Title))
{
listItem.counter++;
}
}
item = item.parent;
}
for (Item listItem : MoneyList)
{
System.out.println(listItem.Title + " x " + listItem.counter);
}
}
public static void main(String[] args) throws InterruptedException
{
Item Root = new Item();
Root.Title = "Root Node";
Scanner input = new Scanner(System.in);
System.out.print("Number of Items: ");
int numberOfItems = input.nextInt();
input.nextLine();
initializeMoneyList(numberOfItems, Root);
System.out.print("Enter the Amount of Money: ");
double searchValue = input.nextDouble();
int searchLimit = (int) Math.ceil((searchValue / MoneyList.get(MoneyList.size() - 1).value));
System.out.print("Number of Threads (Muste be less than the number of items): ");
numberOfThreads = input.nextInt();
if (numberOfThreads > numberOfItems)
{
System.exit(1);
}
InitializeQueu(Root);
int[] queueLimit = calculateQueueLimit(numberOfItems, numberOfThreads);
List<Thread> threadList = new ArrayList<Thread>();
for (int x = 0; x < numberOfThreads; x++)
{
tmpQ = new LinkedList<Item>();
for (int y = 0; y < queueLimit[x]; y++)
{
tmpQ.add(q.remove());
}
BFS tmpThreadObject = new BFS(MoneyList, searchValue, tmpQ);
Thread t = new Thread(tmpThreadObject);
t.setName((x + 1) + "");
threadList.add(t);
}
for (Thread t : threadList)
{
t.start();
}
boolean finish = false;
while (!finish)
{
Thread.sleep(250);
for (Thread t : threadList)
{
if (t.isAlive())
{
finish = false;
break;
}
else
{
finish = true;
}
}
}
printPath(lastFoundNode);
}
}
Item Class (Item.java)
package coinsproblembfs;
/**
*
* #author Kassem
*/
public class Item
{
String Title = "";
double value = 0;
int level = 0;
double Totalvalue = 0;
int counter = 0;
Item parent = null;
long searchTime = 0;
String winnerThreadName="";
}
Threads Class (BFS.java)
package coinsproblembfs;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
/**
*
* #author Kassem M. Bagher
*/
public class BFS implements Runnable
{
private LinkedList<Item> q;
private List<Item> MoneyList;
private double searchValue = 0;
private long start = 0, end = 0;
public BFS(List<Item> monyList, double searchValue, LinkedList<Item> queue)
{
q = new LinkedList<Item>();
MoneyList = new ArrayList<Item>();
this.searchValue = searchValue;
for (int x = 0; x < queue.size(); x++)
{
q.addLast(queue.get(x));
}
for (int x = 0; x < monyList.size(); x++)
{
MoneyList.add(monyList.get(x));
}
}
private synchronized void printPath(Item item)
{
while (item != null)
{
for (Item listItem : MoneyList)
{
if (listItem.Title.equals(item.Title))
{
listItem.counter++;
}
}
item = item.parent;
}
for (Item listItem : MoneyList)
{
System.out.println(listItem.Title + " x " + listItem.counter);
}
}
private void addChildren(Item node, LinkedList<Item> q, boolean initialized)
{
for (int x = 0; x < MoneyList.size(); x++)
{
Item t = new Item();
t.value = MoneyList.get(x).value;
if (initialized)
{
t.Totalvalue = 0;
t.level = 0;
}
else
{
t.parent = node;
t.Totalvalue = MoneyList.get(x).Totalvalue;
if (t.parent == null)
{
t.level = 0;
}
else
{
t.level = t.parent.level + 1;
}
}
t.Title = MoneyList.get(x).Title;
q.addLast(t);
}
}
#Override
public void run()
{
start = System.currentTimeMillis();
try
{
while (!q.isEmpty())
{
Item node = null;
node = (Item) q.removeFirst();
node.Totalvalue = node.value + node.parent.Totalvalue;
if (node.level < CoinsProblemBFS.searchLevelLimit)
{
if (node.Totalvalue == searchValue)
{
synchronized (CoinsProblemBFS.lockLevelLimitation)
{
CoinsProblemBFS.searchLevelLimit = node.level;
CoinsProblemBFS.lastFoundNode = node;
end = System.currentTimeMillis();
CoinsProblemBFS.lastFoundNode.searchTime = (end - start);
CoinsProblemBFS.lastFoundNode.winnerThreadName=Thread.currentThread().getName();
}
}
else
{
if (node.level + 1 < CoinsProblemBFS.searchLevelLimit)
{
addChildren(node, q, false);
}
}
}
}
} catch (Exception e)
{
e.printStackTrace();
}
}
}
Sample Input:
Number of Items: 4
Enter the Title and Value for item 1: one 1
Enter the Title and Value for item 2: five 5
Enter the Title and Value for item 3: ten 10
Enter the Title and Value for item 4: twenty 20
Enter the Amount of Money: 150
Number of Threads (Muste be less than the number of items): 2

Circular ShortBuffer for Audio in Java

I'm implementing an audio track class and I'm in need of a good circular buffer implementation. I'm using shorts for my audio samples, so I would prefer to use a ShortBuffer class for the actual buffer. This track will need to be thread-safe but I can guarantee that only one thread will read and another will write on the track.
My current implementation looks like this (it doesn't handle wrapping).
public class Track {
//sample rate 44100, 2 channels with room for 4 seconds
private volatile ShortBuffer buffer = ShortBuffer.allocate((44100 * 2) * 4);
//keep count of the samples in the buffer
private AtomicInteger count = new AtomicInteger(0);
private ReentrantLock lock = new ReentrantLock(true);
private int readPosition = 0;
public int getSampleCount() {
int i = count.get();
return i > 0 ? i / 2 : 0;
}
public short[] getSamples(int sampleCount) {
short[] samples = new short[sampleCount];
try {
lock.tryLock(10, TimeUnit.MILLISECONDS);
int writePosition = buffer.position();
buffer.position(readPosition);
buffer.get(samples);
//set new read position
readPosition = buffer.position();
// set back to write position
buffer.position(writePosition);
count.addAndGet(-sampleCount);
} catch (InterruptedException e) {
System.err.println("Exception getting samples" + e);
} finally {
if (lock.isHeldByCurrentThread()) {
lock.unlock();
}
}
return samples;
}
public void pushSamples(short[] samples) {
try {
lock.tryLock(10, TimeUnit.MILLISECONDS);
buffer.put(samples);
count.addAndGet(samples.length);
} catch (InterruptedException e) {
System.err.println("Exception getting samples" + e);
} finally {
if (lock.isHeldByCurrentThread()) {
lock.unlock();
}
}
}
}

Here's the solution that I have come up with http://pastebin.com/2St01Wzf I decided it was easier to use a head and tail property with a short array, instead of just the read position with a ShortBuffer. I also took an idea from the Java collections classes to detect when the buffer is full. Here is the source, just in case the pastebin disappears:
public class Track {
private static Logger log = LoggerFactory.getLogger(Track.class);
private final long id = System.nanoTime();
// number of channels
private int channelCount;
// maximum seconds to buffer
private int bufferedSeconds = 5;
private AtomicInteger count = new AtomicInteger(0);
private ReentrantLock lock;
private volatile short[] buffer;
private int capacity = 0;
private int head = 0;
private int tail = 0;
public Track(int samplingRate, int channelCount) {
// set the number of channels
this.channelCount = channelCount;
// size the buffer
capacity = (samplingRate * channelCount) * bufferedSeconds;
buffer = new short[capacity];
// use a "fair" lock
lock = new ReentrantLock(true);
}
/**
* Returns the number of samples currently in the buffer.
*
* #return
*/
public int getSamplesCount() {
int i = count.get();
return i > 0 ? i / channelCount : 0;
}
/**
* Removes and returns the next sample in the buffer.
*
* #return single sample or null if a buffer underflow occurs
*/
public Short remove() {
Short sample = null;
if (count.get() > 0) {
// decrement sample counter
count.addAndGet(-1);
// reposition the head
head = (head + 1) % capacity;
// get the sample at the head
sample = buffer[head];
} else {
log.debug("Buffer underflow");
}
return sample;
}
/**
* Adds a sample to the buffer.
*
* #param sample
* #return true if added successfully and false otherwise
*/
public boolean add(short sample) {
boolean result = false;
if ((count.get() + 1) < capacity) {
// increment sample counter
count.addAndGet(1);
// reposition the tail
tail = (tail + 1) % capacity;
// add the sample to the tail
buffer[tail] = sample;
// added!
result = true;
} else {
log.debug("Buffer overflow");
}
return result;
}
/**
* Offers the samples for addition to the buffer, if there is enough capacity to
* contain them they will be added.
*
* #param samples
* #return true if the samples can be added and false otherwise
*/
public boolean offer(short[] samples) {
boolean result = false;
if ((count.get() + samples.length) <= capacity) {
pushSamples(samples);
result = true;
}
return result;
}
/**
* Adds an array of samples to the buffer.
*
* #param samples
*/
public void pushSamples(short[] samples) {
log.trace("[{}] pushSamples - count: {}", id, samples.length);
try {
lock.tryLock(10, TimeUnit.MILLISECONDS);
for (short sample : samples) {
log.trace("Position at write: {}", tail);
if (!add(sample)) {
log.warn("Sample could not be added");
break;
}
}
} catch (InterruptedException e) {
log.warn("Exception getting samples", e);
} finally {
if (lock.isHeldByCurrentThread()) {
lock.unlock();
}
}
}
/**
* Returns a single from the buffer.
*
* #return
*/
public Short popSample(int channel) {
log.trace("[{}] popSample - channel: {}", id, channel);
Short sample = null;
if (channel < channelCount) {
log.trace("Position at read: {}", head);
try {
lock.tryLock(10, TimeUnit.MILLISECONDS);
sample = remove();
} catch (InterruptedException e) {
log.warn("Exception getting sample", e);
} finally {
if (lock.isHeldByCurrentThread()) {
lock.unlock();
}
}
}
return sample;
}
}