Sunday, August 14, 2011

%%Lockfree Unbounded Stack(generic) using AtomicReference

import static java.lang.System.*;
import java.util.concurrent.atomic.AtomicReference;

public class LockfreeUnboundedStack<V> {
                final private AtomicReference<Node<V>> arTop = new AtomicReference<Node<V>>();
                @SuppressWarnings("unchecked")
                public LockfreeUnboundedStack() {
                                this.arTop.set((Node<V>) Node.theTerminator);
                }

                public static void main(String[] oipwuir) {
                                LockfreeUnboundedStack<String> stack = new LockfreeUnboundedStack<String>();
                                Node<String> a = stack.pop();
                                a = stack.pop();
                                stack.push("1");
                                a = stack.pop();
                                a = stack.pop();
                                stack.push("2");
                                a = stack.pop();
                                a = stack.pop();
                }
                /**
                * @return a terminator node if empty stack
                */
                public Node<V> pop() {
                                boolean done;
                                Node<V> oldTopNode;
                                do {
                                                oldTopNode = this.arTop.get();
                                                if (oldTopNode.isTerminator()) {
                                                                out.println("returning terminator");
                                                                return oldTopNode;
                                                }
                                                Node<V> newTopNode = oldTopNode.previous;
                                                done = this.arTop.compareAndSet(oldTopNode, newTopNode);
                                } while (!done);
                                out.println("returning " + oldTopNode);
                                return oldTopNode;
                }
                public void push(V item) {
                                assert item != null;
                                Node<V> newNode = new Node<V>(item);
                                boolean done;
                                do {
                                                Node<V> replacedTopNode = this.arTop.get();
                                                newNode.previous = replacedTopNode;
                                                done = arTop.compareAndSet(replacedTopNode, newNode);
                                } while (!done);
                }
}

class Node<V> {
                final V payload;
                Node<V> previous = null;
                public Node(V pay) {
                                assert pay != null;
                                this.payload = pay;
                }

                /**
                * callable only by the anonymous nested class
                */
                private Node() {
                                this.payload = null;
                }

                public boolean isTerminator() {
                                return false;
                }
                public String toString() {
                                String p = "an invalid payload";
                                if (this.payload != null) p = this.payload.toString();
                                return "Node holding " + this.payload.toString();
                }

                static final Node<?> theTerminator = new Node<Object>() {
                                @Override
                                final public boolean isTerminator() {
                                                return true;
                                }
                };
}

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