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Panda Nets is a client side library of many basic Internet protocols.
There is a newer version: 1.8.0
package panda.net.nntp;

/**
 * This is an implementation of a message threading algorithm, as originally devised by Zamie Zawinski.
 * See http://www.jwz.org/doc/threading.html for details.
 * For his Java implementation, see
 * 
 * http://lxr.mozilla.org/mozilla/source/grendel/sources/grendel/view/Threader.java
 */

import java.util.Arrays;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;

public class Threader {

	/**
	 * The client passes in a list of Threadable objects, and the Threader constructs a connected
	 * 'graph' of messages
	 * 
	 * @param messages list of messages to thread, must not be empty
	 * @return null if messages == null or root.child == null or messages list is empty
	 */
	public Threadable thread(List messages) {
		return thread((Iterable)messages);
	}

	/**
	 * The client passes in a list of Iterable objects, and the Threader constructs a connected
	 * 'graph' of messages
	 * 
	 * @param messages iterable of messages to thread, must not be empty
	 * @return null if messages == null or root.child == null or messages list is empty
	 */
	public Threadable thread(Iterable messages) {
		if (messages == null) {
			return null;
		}

		HashMap idTable = new HashMap();

		// walk through each Threadable element
		for (Threadable t : messages) {
			if (!t.isDummy()) {
				buildContainer(t, idTable);
			}
		}

		if (idTable.isEmpty()) {
			return null;
		}

		ThreadContainer root = findRootSet(idTable);
		idTable.clear();
		idTable = null;

		pruneEmptyContainers(root);

		root.reverseChildren();
		gatherSubjects(root);

		if (root.next != null) {
			throw new RuntimeException("root node has a next:" + root);
		}

		for (ThreadContainer r = root.child; r != null; r = r.next) {
			if (r.threadable == null) {
				r.threadable = r.child.threadable.makeDummy();
			}
		}

		Threadable result = (root.child == null ? null : root.child.threadable);
		root.flush();

		return result;
	}

	/**
	 * @param threadable
	 * @param idTable
	 */
	private void buildContainer(Threadable threadable, HashMap idTable) {
		String id = threadable.messageThreadId();
		ThreadContainer container = idTable.get(id);
		int bogusIdCount = 0;

		// A ThreadContainer exists for this id already. This should be a forward reference, but may
		// be a duplicate id, in which case we will need to generate a bogus placeholder id
		if (container != null) {
			if (container.threadable != null) { // oops! duplicate ids...
				id = "";
				container = null;
			}
			else {
				// The container just contained a forward reference to this message, so let's
				// fill in the threadable field of the container with this message
				container.threadable = threadable;
			}
		}

		// No container exists for that message Id. Create one and insert it into the hash table.
		if (container == null) {
			container = new ThreadContainer();
			container.threadable = threadable;
			idTable.put(id, container);
		}

		// Iterate through all of the references and create ThreadContainers for any references that
		// don't have them.
		ThreadContainer parentRef = null;
		{
			String[] references = threadable.messageThreadReferences();
			for (String refString : references) {
				ThreadContainer ref = idTable.get(refString);

				// if this id doesnt have a container, create one
				if (ref == null) {
					ref = new ThreadContainer();
					idTable.put(refString, ref);
				}

				// Link references together in the order they appear in the References: header,
				// IF they dont have a have a parent already &&
				// IF it will not cause a circular reference
				if ((parentRef != null) && (ref.parent == null) && (parentRef != ref) && !(ref.findChild(parentRef))) {
					// Link ref into the parent's child list
					ref.parent = parentRef;
					ref.next = parentRef.child;
					parentRef.child = ref;
				}
				parentRef = ref;
			}
		}

		// parentRef is now set to the container of the last element in the references field. make
		// that
		// be the parent of this container, unless doing so causes a circular reference
		if (parentRef != null && (parentRef == container || container.findChild(parentRef))) {
			parentRef = null;
		}

		// if it has a parent already, its because we saw this message in a References: field, and
		// presumed
		// a parent based on the other entries in that field. Now that we have the actual message,
		// we can
		// throw away the old parent and use this new one
		if (container.parent != null) {
			ThreadContainer rest, prev;

			for (prev = null, rest = container.parent.child; rest != null; prev = rest, rest = rest.next) {
				if (rest == container) {
					break;
				}
			}

			if (rest == null) {
				throw new RuntimeException("Didnt find " + container + " in parent" + container.parent);
			}

			// Unlink this container from the parent's child list
			if (prev == null) {
				container.parent.child = container.next;
			}
			else {
				prev.next = container.next;
			}

			container.next = null;
			container.parent = null;
		}

		// If we have a parent, link container into the parents child list
		if (parentRef != null) {
			container.parent = parentRef;
			container.next = parentRef.child;
			parentRef.child = container;
		}
	}

	/**
	 * Find the root set of all existing ThreadContainers
	 * 
	 * @param idTable
	 * @return root the ThreadContainer representing the root node
	 */
	private ThreadContainer findRootSet(HashMap idTable) {
		ThreadContainer root = new ThreadContainer();
		Iterator iter = idTable.keySet().iterator();

		while (iter.hasNext()) {
			Object key = iter.next();
			ThreadContainer c = idTable.get(key);
			if (c.parent == null) {
				if (c.next != null) {
					throw new RuntimeException("c.next is " + c.next.toString());
				}
				c.next = root.child;
				root.child = c;
			}
		}
		return root;
	}

	/**
	 * Delete any empty or dummy ThreadContainers
	 * 
	 * @param parent
	 */
	private void pruneEmptyContainers(ThreadContainer parent) {
		ThreadContainer container, prev, next;
		for (prev = null, container = parent.child, next = container.next; container != null; prev = container, container = next, next = (container == null ? null : container.next)) {

			// Is it empty and without any children? If so,delete it
			if (container.threadable == null && container.child == null) {
				if (prev == null) {
					parent.child = container.next;
				}
				else {
					prev.next = container.next;
				}

				// Set container to prev so that prev keeps its same value the next time through the
				// loop
				container = prev;
			}

			// Else if empty, with kids, and (not at root or only one kid)
			else if (container.threadable == null && container.child != null
					&& (container.parent != null || container.child.next == null)) {
				// We have an invalid/expired message with kids. Promote the kids to this level.
				ThreadContainer tail;
				ThreadContainer kids = container.child;

				// Remove this container and replace with 'kids'.
				if (prev == null) {
					parent.child = kids;
				}
				else {
					prev.next = kids;
				}

				// Make each child's parent be this level's parent -> i.e. promote the children.
				// Make the last child's next point to this container's next
				// i.e. splice kids into the list in place of container
				for (tail = kids; tail.next != null; tail = tail.next) {
					tail.parent = container.parent;
				}

				tail.parent = container.parent;
				tail.next = container.next;

				// next currently points to the item after the inserted items in the chain - reset
				// that so we process the newly
				// promoted items next time round
				next = kids;

				// Set container to prev so that prev keeps its same value the next time through the
				// loop
				container = prev;
			}
			else if (container.child != null) {
				// A real message , with kids
				// Iterate over the children
				pruneEmptyContainers(container);
			}
		}
	}

	/**
	 * If any two members of the root set have the same subject, merge them. This is to attempt to
	 * accomodate messages without References: headers.
	 * 
	 * @param root
	 */
	private void gatherSubjects(ThreadContainer root) {

		int count = 0;

		for (ThreadContainer c = root.child; c != null; c = c.next) {
			count++;
		}

		// TODO verify this will avoid rehashing
		HashMap subjectTable = new HashMap((int)(count * 1.2),
			(float)0.9);
		count = 0;

		for (ThreadContainer c = root.child; c != null; c = c.next) {
			Threadable threadable = c.threadable;

			// No threadable? If so, it is a dummy node in the root set.
			// Only root set members may be dummies, and they alway have at least 2 kids
			// Take the first kid as representative of the subject
			if (threadable == null) {
				threadable = c.child.threadable;
			}

			String subj = threadable.simplifiedSubject();

			if (subj == null || subj.length() == 0) {
				continue;
			}

			ThreadContainer old = subjectTable.get(subj);

			// Add this container to the table iff:
			// - There exists no container with this subject
			// - or this is a dummy container and the old one is not - the dummy one is
			// more interesting as a root, so put it in the table instead
			// - The container in the table has a "Re:" version of this subject, and
			// this container has a non-"Re:" version of this subject. The non-"Re:" version
			// is the more interesting of the two.
			if (old == null
					|| (c.threadable == null && old.threadable != null)
					|| (old.threadable != null && old.threadable.subjectIsReply() && c.threadable != null && !c.threadable
						.subjectIsReply())) {
				subjectTable.put(subj, c);
				count++;
			}
		}

		// If the table is empty, we're done
		if (count == 0) {
			return;
		}

		// subjectTable is now populated with one entry for each subject which occurs in the
		// root set. Iterate over the root set, and gather together the difference.
		ThreadContainer prev, c, rest;
		for (prev = null, c = root.child, rest = c.next; c != null; prev = c, c = rest, rest = (rest == null ? null : rest.next)) {
			Threadable threadable = c.threadable;

			// is it a dummy node?
			if (threadable == null) {
				threadable = c.child.threadable;
			}

			String subj = threadable.simplifiedSubject();

			// Dont thread together all subjectless messages
			if (subj == null || subj.length() == 0) {
				continue;
			}

			ThreadContainer old = subjectTable.get(subj);

			if (old == c) { // That's us
				continue;
			}

			// We have now found another container in the root set with the same subject
			// Remove the "second" message from the root set
			if (prev == null) {
				root.child = c.next;
			}
			else {
				prev.next = c.next;
			}
			c.next = null;

			if (old.threadable == null && c.threadable == null) {
				// both dummies - merge them
				ThreadContainer tail;
				for (tail = old.child; tail != null && tail.next != null; tail = tail.next) {
					// do nothing
				}

				if (tail != null) { // protect against possible NPE
					tail.next = c.child;
				}

				for (tail = c.child; tail != null; tail = tail.next) {
					tail.parent = old;
				}

				c.child = null;
			}
			else if (old.threadable == null
					|| (c.threadable != null && c.threadable.subjectIsReply() && !old.threadable.subjectIsReply())) {
				// Else if old is empty, or c has "Re:" and old does not ==> make this message a
				// child of old
				c.parent = old;
				c.next = old.child;
				old.child = c;
			}
			else {
				// else make the old and new messages be children of a new dummy container.
				// We create a new container object for old.msg and empty the old container
				ThreadContainer newc = new ThreadContainer();
				newc.threadable = old.threadable;
				newc.child = old.child;

				for (ThreadContainer tail = newc.child; tail != null; tail = tail.next) {
					tail.parent = newc;
				}

				old.threadable = null;
				old.child = null;

				c.parent = old;
				newc.parent = old;

				// Old is now a dummy- give it 2 kids , c and newc
				old.child = c;
				c.next = newc;
			}
			// We've done a merge, so keep the same prev
			c = prev;
		}

		subjectTable.clear();
		subjectTable = null;

	}

	// DEPRECATED METHODS - for API compatibility only - DO NOT USE

	/**
	 * The client passes in an array of Threadable objects, and the Threader constructs a connected
	 * 'graph' of messages
	 * 
	 * @param messages array of messages to thread, must not be empty
	 * @return null if messages == null or root.child == null or messages array is empty
	 * @deprecated (2.2) prefer {@link #thread(List)}
	 */
	@Deprecated
	public Threadable thread(Threadable[] messages) {
		if (messages == null) {
			return null;
		}
		return thread(Arrays.asList(messages));
	}

}