ConcurrentSet.java

/******************************************************************************************
MIT License

Copyright (c) 2012 Benjamin Diedrichsen

Permission is hereby granted, free of charge, to any person obtaining a copy of this
software and associated documentation files (the "Software"), to deal in the Software
without restriction, including without limitation the rights to use, copy, modify, merge,
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The above copyright notice and this permission notice shall be included in all copies or
substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
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********************************************************************************************/
package net.engio.mbassy.common;


import java.lang.ref.WeakReference;
import java.util.Iterator;
import java.util.WeakHashMap;

/**
 * This data structure is optimized for non-blocking reads even when write operations occur.
 * Running read iterators will not be affected by add operations since writes always insert at the head of the
 * structure. Remove operations can affect any running iterator such that a removed element that has not yet
 * been reached by the iterator will not appear in that iterator anymore.
 * <p/>
 * The structure uses weak references to the elements. Iterators automatically perform cleanups of
 * garbage collected objects during iteration -> no dedicated maintenance operations need to be called or run in background.
 * <p/>
 * <p/>
 * <p/>
 *
 * @author bennidi
 *         Date: 2/12/12
 */
public class ConcurrentSet<T> implements Iterable<T>{

    private WeakHashMap<T, Entry<T>> entries = new WeakHashMap<T, Entry<T>>(); // maintain a map of entries for O(log n) lookup

    private Entry<T> head; // reference to the first element

    public ConcurrentSet<T> add(T element) {
        if (element == null || entries.containsKey(element)) return this;
        synchronized (this) {
            insert(element);
        }
        return this;
    }

    public boolean contains(T element){
        Entry<T> entry = entries.get(element);
        return entry != null && entry.getValue() != null;
    }

    private void insert(T element) {
        if (entries.containsKey(element)) return;
        if (head == null) {
            head = new Entry<T>(element);
        } else {
            head = new Entry<T>(element, head);
        }
        entries.put(element, head);
    }

    public int size(){
        return entries.size();
    }

    public ConcurrentSet<T> addAll(Iterable<T> elements) {
        synchronized (this) {
            for (T element : elements) {
                if (element == null || entries.containsKey(element)) return this;

                insert(element);
            }
        }
        return this;
    }

    public boolean remove(T element) {
        if (!entries.containsKey(element)) return false;
        synchronized (this) {
            Entry<T> listelement = entries.get(element);
            if(listelement == null)return false; //removed by other thread
            if (listelement != head) {
                listelement.remove();
            } else {
                Entry<T> oldHead = head;
                head = head.next();
                oldHead.next = null; // optimize for GC
            }
            entries.remove(element);
        }
        return true;
    }

    public Iterator<T> iterator() {
        return new Iterator<T>() {

            private Entry<T> current = head;

            public boolean hasNext() {
                if (current == null) return false;
                T value = current.getValue();
                if (value == null) {    // auto-removal of orphan references
                    remove();
                    return hasNext();
                } else {
                    return true;
                }
            }

            public T next() {
                if (current == null) return null;
                T value = current.getValue();
                if (value == null) {    // auto-removal of orphan references
                    remove();
                    return next();
                } else {
                    current = current.next();
                    return value;
                }
            }

            public void remove() {
                if (current == null) return;
                Entry<T> newCurrent = current.next();
                ConcurrentSet.this.remove(current.getValue());
                current = newCurrent;
            }
        };
    }


    public class Entry<T> {

        private WeakReference<T> value;

        private Entry<T> next;

        private Entry<T> predecessor;


        private Entry(T value) {
            this.value = new WeakReference<T>(value);
        }

        private Entry(T value, Entry<T> next) {
            this(value);
            this.next = next;
            next.predecessor = this;
        }

        public T getValue() {
            return value.get();
        }

        // not thread-safe! must be synchronized in enclosing context
        public void remove() {
            if (predecessor != null) {
                predecessor.next = next;
                if(next != null)next.predecessor = predecessor;
            } else if (next != null) {
                next.predecessor = null;
            }
            next = null;
            predecessor = null;
        }

        public Entry<T> next() {
            return next;
        }


    }
}