// This example is from the book _Java Threads_ by Scott Oaks and Henry Wong. 
// Written by Scott Oaks and Henry Wong.
// Copyright (c) 1997 O'Reilly & Associates.
// You may study, use, modify, and distribute this example for any purpose.
// This example is provided WITHOUT WARRANTY either expressed or implied.

// Sample CircularList -- Chapter 5, p. 90.

class CircularListNode {
    Object o;
    CircularListNode next;
    CircularListNode prev;
}

public class CircularList {
    private CircularListNode current;

    public synchronized void insert(Object o) {
        CircularListNode tn = new CircularListNode();
        tn.o = o;
        if (current == null) {
            tn.next = tn.prev = tn;
            current = tn;
        } else {                            // Add Before Current Node
            tn.next = current;
            tn.prev = current.prev;
            current.prev.next = tn;
            current.prev = tn;
        }
    }

    public synchronized void delete(Object o) {
        CircularListNode p = find(o);
        CircularListNode next = p.next;
        CircularListNode prev = p.prev;
        if (p == p.next) {       // Last Object on the list
            current = null;
            return;
        }
        prev.next = next;
        next.prev = prev;
        if (current == p) current = next;
    }

    private CircularListNode find(Object o) {
        CircularListNode p = current;
        if (p == null)
            throw new IllegalArgumentException();
        do {
            if (p.o == o) return p;
            p = p.next;
        } while (p != current);
        throw new IllegalArgumentException();
    }

    public synchronized Object locate(Object o) {
        CircularListNode p = current;
        do {
            if (p.o.equals(o)) return p.o;
            p = p.next;
        } while (p != current);
        throw new IllegalArgumentException();
    }

    public synchronized Object getNext() {
        if (current == null)
            return null;
        current = current.next;
        return current.o;
    }
}