/*
 * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package com.sun.tools.jdi;

import com.sun.jdi.*;
import com.sun.jdi.event.*;
import com.sun.jdi.request.*;

import java.util.*;
enum EventDestination {UNKNOWN_EVENT, INTERNAL_EVENT, CLIENT_EVENT};

/*
 * An EventSet is normally created by the transport reader thread when
 * it reads a JDWP Composite command.  The constructor doesn't unpack
 * the events contained in the Composite command and create EventImpls
 * for them because that process might involve calling back into the back-end
 * which should not be done by the transport reader thread.  Instead,
 * the raw bytes of the packet are read and stored in the EventSet.
 * The EventSet is then added to each EventQueue. When an EventSet is
 * removed from an EventQueue, the EventSetImpl.build() method is called.
 * This method reads the packet bytes and creates the actual EventImpl objects.
 * build() also filters out events for our internal handler and puts them in
 * their own EventSet.  This means that the EventImpls that are in the EventSet
 * that is on the queues are all for client requests.
 */
public class EventSetImpl extends ArrayList<Event> implements EventSet {
    private static final long serialVersionUID = -4857338819787924570L;
    private VirtualMachineImpl vm; // we implement Mirror
    private Packet pkt;
    private byte suspendPolicy;
    private EventSetImpl internalEventSet;

    public String toString() {
        String string = "event set, policy:" + suspendPolicy +
                        ", count:" + this.size() + " = {";
        boolean first = true;
        for (Event event : this) {
            if (!first) {
                string += ", ";
            }
            string += event.toString();
            first = false;
        }
        string += "}";
        return string;
    }

    abstract class EventImpl extends MirrorImpl implements Event {

        private final byte eventCmd;
        private final int requestID;
        // This is set only for client requests, not internal requests.
        private final EventRequest request;

        
Constructor for events.
/**
         * Constructor for events.
         */
        protected EventImpl(JDWP.Event.Composite.Events.EventsCommon evt,
                            int requestID) {
            super(EventSetImpl.this.vm);
            this.eventCmd = evt.eventKind();
            this.requestID = requestID;
            EventRequestManagerImpl ermi = EventSetImpl.this.
                vm.eventRequestManagerImpl();
            this.request =  ermi.request(eventCmd, requestID);
        }

        /*
         * Override superclass back to default equality
         */
        public boolean equals(Object obj) {
            return this == obj;
        }

        public int hashCode() {
            return System.identityHashCode(this);
        }

        
Constructor for VM disconnected events.
/**
         * Constructor for VM disconnected events.
         */
        protected EventImpl(byte eventCmd) {
            super(EventSetImpl.this.vm);
            this.eventCmd = eventCmd;
            this.requestID = 0;
            this.request = null;
        }

        public EventRequest request() {
            return request;
        }

        int requestID() {
            return requestID;
        }

        EventDestination destination() {
            /*
             * We need to decide if this event is for
             * 1. an internal request
             * 2. a client request that is no longer available, ie
             *    it has been deleted, or disabled and re-enabled
             *    which gives it a new ID.
             * 3. a current client request that is disabled
             * 4. a current enabled client request.
             *
             * We will filter this set into a set
             * that contains only 1s for our internal queue
             * and a set that contains only 4s for our client queue.
             * If we get an EventSet that contains only 2 and 3
             * then we have to resume it if it is not SUSPEND_NONE
             * because no one else will.
             */
            if (requestID == 0) {
                /* An unsolicited event.  These have traditionally
                 * been treated as client events.
                 */
                return EventDestination.CLIENT_EVENT;
            }

            // Is this an event for a current client request?
            if (request == null) {
                // Nope.  Is it an event for an internal request?
                EventRequestManagerImpl ermi = this.vm.getInternalEventRequestManager();
                if (ermi.request(eventCmd, requestID) != null) {
                    // Yep
                    return EventDestination.INTERNAL_EVENT;
                }
                return EventDestination.UNKNOWN_EVENT;
            }

            // We found a client request
            if (request.isEnabled()) {
                return EventDestination.CLIENT_EVENT;
            }
            return EventDestination.UNKNOWN_EVENT;
        }

        abstract String eventName();

        public String toString() {
            return eventName();
        }

    }

    abstract class ThreadedEventImpl extends EventImpl {
        private ThreadReference thread;

        ThreadedEventImpl(JDWP.Event.Composite.Events.EventsCommon evt,
                          int requestID, ThreadReference thread) {
            super(evt, requestID);
            this.thread = thread;
        }

        public ThreadReference thread() {
            return thread;
        }

        public String toString() {
            return eventName() + " in thread " + thread.name();
        }
    }

    abstract class LocatableEventImpl extends ThreadedEventImpl
                                            implements Locatable {
        private Location location;

        LocatableEventImpl(JDWP.Event.Composite.Events.EventsCommon evt,
                           int requestID,
                           ThreadReference thread, Location location) {
            super(evt, requestID, thread);
            this.location = location;
        }

        public Location location() {
            return location;
        }

        
For MethodEntry and MethodExit
/**
         * For MethodEntry and MethodExit
         */
        public Method method() {
            return location.method();
        }

        public String toString() {
            return eventName() + "@" +
                   ((location() == null) ? " null" : location().toString()) +
                   " in thread " + thread().name();
        }
    }

    class BreakpointEventImpl extends LocatableEventImpl
                            implements BreakpointEvent {
        BreakpointEventImpl(JDWP.Event.Composite.Events.Breakpoint evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
        }

        String eventName() {
            return "BreakpointEvent";
        }
    }

    class StepEventImpl extends LocatableEventImpl implements StepEvent {
        StepEventImpl(JDWP.Event.Composite.Events.SingleStep evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
        }

        String eventName() {
            return "StepEvent";
        }
    }

    class MethodEntryEventImpl extends LocatableEventImpl
                            implements MethodEntryEvent {
        MethodEntryEventImpl(JDWP.Event.Composite.Events.MethodEntry evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
        }

        String eventName() {
            return "MethodEntryEvent";
        }
    }

    class MethodExitEventImpl extends LocatableEventImpl
                            implements MethodExitEvent {
        private Value returnVal = null;

        MethodExitEventImpl(JDWP.Event.Composite.Events.MethodExit evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
        }

        MethodExitEventImpl(JDWP.Event.Composite.Events.MethodExitWithReturnValue evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            returnVal = evt.value;
        }

        String eventName() {
            return "MethodExitEvent";
        }

        public Value returnValue() {
            if (!this.vm.canGetMethodReturnValues()) {
                throw new UnsupportedOperationException(
                "target does not support return values in MethodExit events");
            }
            return returnVal;
        }

    }

    class MonitorContendedEnterEventImpl extends LocatableEventImpl
                            implements MonitorContendedEnterEvent {
        private ObjectReference monitor = null;

        MonitorContendedEnterEventImpl(JDWP.Event.Composite.Events.MonitorContendedEnter evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            this.monitor = evt.object;
        }

        String eventName() {
            return "MonitorContendedEnter";
        }

        public ObjectReference  monitor() {
            return monitor;
        };

    }

    class MonitorContendedEnteredEventImpl extends LocatableEventImpl
                            implements MonitorContendedEnteredEvent {
        private ObjectReference monitor = null;

        MonitorContendedEnteredEventImpl(JDWP.Event.Composite.Events.MonitorContendedEntered evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            this.monitor = evt.object;
        }

        String eventName() {
            return "MonitorContendedEntered";
        }

        public ObjectReference  monitor() {
            return monitor;
        };

    }

    class MonitorWaitEventImpl extends LocatableEventImpl
                            implements MonitorWaitEvent {
        private ObjectReference monitor = null;
        private long timeout;

        MonitorWaitEventImpl(JDWP.Event.Composite.Events.MonitorWait evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            this.monitor = evt.object;
            this.timeout = evt.timeout;
        }

        String eventName() {
            return "MonitorWait";
        }

        public ObjectReference  monitor() {
            return monitor;
        };

        public long timeout() {
            return timeout;
        }
    }

    class MonitorWaitedEventImpl extends LocatableEventImpl
                            implements MonitorWaitedEvent {
        private ObjectReference monitor = null;
        private boolean timed_out;

        MonitorWaitedEventImpl(JDWP.Event.Composite.Events.MonitorWaited evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            this.monitor = evt.object;
            this.timed_out = evt.timed_out;
        }

        String eventName() {
            return "MonitorWaited";
        }

        public ObjectReference  monitor() {
            return monitor;
        };

        public boolean timedout() {
            return timed_out;
        }
    }

    class ClassPrepareEventImpl extends ThreadedEventImpl
                            implements ClassPrepareEvent {
        private ReferenceType referenceType;

        ClassPrepareEventImpl(JDWP.Event.Composite.Events.ClassPrepare evt) {
            super(evt, evt.requestID, evt.thread);
            referenceType = this.vm.referenceType(evt.typeID, evt.refTypeTag,
                                                  evt.signature);
            ((ReferenceTypeImpl)referenceType).setStatus(evt.status);
        }

        public ReferenceType referenceType() {
            return referenceType;
        }

        String eventName() {
            return "ClassPrepareEvent";
        }
    }

    class ClassUnloadEventImpl extends EventImpl implements ClassUnloadEvent {
        private String classSignature;

        ClassUnloadEventImpl(JDWP.Event.Composite.Events.ClassUnload evt) {
            super(evt, evt.requestID);
            this.classSignature = evt.signature;
        }

        public String className() {
            return classSignature.substring(1, classSignature.length()-1)
                .replace('/', '.');
        }

        public String classSignature() {
            return classSignature;
        }

        String eventName() {
            return "ClassUnloadEvent";
        }
    }

    class ExceptionEventImpl extends LocatableEventImpl
                                             implements ExceptionEvent {
        private ObjectReference exception;
        private Location catchLocation;

        ExceptionEventImpl(JDWP.Event.Composite.Events.Exception evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            this.exception = evt.exception;
            this.catchLocation = evt.catchLocation;
        }

        public ObjectReference exception() {
            return exception;
        }

        public Location catchLocation() {
            return catchLocation;
        }

        String eventName() {
            return "ExceptionEvent";
        }
    }

    class ThreadDeathEventImpl extends ThreadedEventImpl
                                        implements ThreadDeathEvent {
        ThreadDeathEventImpl(JDWP.Event.Composite.Events.ThreadDeath evt) {
            super(evt, evt.requestID, evt.thread);
        }

        String eventName() {
            return "ThreadDeathEvent";
        }
    }

    class ThreadStartEventImpl extends ThreadedEventImpl
                                        implements ThreadStartEvent {
        ThreadStartEventImpl(JDWP.Event.Composite.Events.ThreadStart evt) {
            super(evt, evt.requestID, evt.thread);
        }

        String eventName() {
            return "ThreadStartEvent";
        }
    }

    class VMStartEventImpl extends ThreadedEventImpl
                                        implements VMStartEvent {
        VMStartEventImpl(JDWP.Event.Composite.Events.VMStart evt) {
            super(evt, evt.requestID, evt.thread);
        }

        String eventName() {
            return "VMStartEvent";
        }
    }

    class VMDeathEventImpl extends EventImpl implements VMDeathEvent {

        VMDeathEventImpl(JDWP.Event.Composite.Events.VMDeath evt) {
            super(evt, evt.requestID);
        }

        String eventName() {
            return "VMDeathEvent";
        }
    }

    class VMDisconnectEventImpl extends EventImpl
                                         implements VMDisconnectEvent {

        VMDisconnectEventImpl() {
            super((byte)JDWP.EventKind.VM_DISCONNECTED);
        }

        String eventName() {
            return "VMDisconnectEvent";
        }
    }

    abstract class WatchpointEventImpl extends LocatableEventImpl
                                            implements WatchpointEvent {
        private final ReferenceTypeImpl refType;
        private final long fieldID;
        private final ObjectReference object;
        private Field field = null;

        WatchpointEventImpl(JDWP.Event.Composite.Events.EventsCommon evt,
                            int requestID,
                            ThreadReference thread, Location location,
                            byte refTypeTag, long typeID, long fieldID,
                            ObjectReference object) {
            super(evt, requestID, thread, location);
            this.refType = this.vm.referenceType(typeID, refTypeTag);
            this.fieldID = fieldID;
            this.object = object;
        }

        public Field field() {
            if (field == null) {
                field = refType.getFieldMirror(fieldID);
            }
            return field;
        }

        public ObjectReference object() {
            return object;
        }

        public Value valueCurrent() {
            if (object == null) {
                return refType.getValue(field());
            } else {
                return object.getValue(field());
            }
        }
    }

    class AccessWatchpointEventImpl extends WatchpointEventImpl
                                            implements AccessWatchpointEvent {

        AccessWatchpointEventImpl(JDWP.Event.Composite.Events.FieldAccess evt) {
            super(evt, evt.requestID, evt.thread, evt.location,
                  evt.refTypeTag, evt.typeID, evt.fieldID, evt.object);
        }

        String eventName() {
            return "AccessWatchpoint";
        }
    }

    class ModificationWatchpointEventImpl extends WatchpointEventImpl
                           implements ModificationWatchpointEvent {
        Value newValue;

        ModificationWatchpointEventImpl(
                        JDWP.Event.Composite.Events.FieldModification evt) {
            super(evt, evt.requestID, evt.thread, evt.location,
                  evt.refTypeTag, evt.typeID, evt.fieldID, evt.object);
            this.newValue = evt.valueToBe;
        }

        public Value valueToBe() {
            return newValue;
        }

        String eventName() {
            return "ModificationWatchpoint";
        }
    }

    
Events are constructed on the thread which reads all data from the transport. This means that the packet cannot be converted to real JDI objects as that may involve further communications with the back end which would deadlock. Hence the build() method below called by EventQueue. /**
     * Events are constructed on the thread which reads all data from the
     * transport. This means that the packet cannot be converted to real
     * JDI objects as that may involve further communications with the
     * back end which would deadlock.
     *
     * Hence the {@link #build()} method below called by EventQueue.
     */
    EventSetImpl(VirtualMachine aVm, Packet pkt) {
        super();

        // From "MirrorImpl":
        // Yes, its a bit of a hack. But by doing it this
        // way, this is the only place we have to change
        // typing to substitute a new impl.
        vm = (VirtualMachineImpl)aVm;

        this.pkt = pkt;
    }

    
Constructor for special events like VM disconnected
/**
     * Constructor for special events like VM disconnected
     */
    EventSetImpl(VirtualMachine aVm, byte eventCmd) {
        this(aVm, null);
        suspendPolicy = JDWP.SuspendPolicy.NONE;
        switch (eventCmd) {
            case JDWP.EventKind.VM_DISCONNECTED:
                addEvent(new VMDisconnectEventImpl());
                break;

            default:
                throw new InternalException("Bad singleton event code");
        }
    }

    private void addEvent(EventImpl evt) {
        // Note that this class has a public add method that throws
        // an exception so that clients can't modify the EventSet
        super.add(evt);
    }

    /*
     * Complete the construction of an EventSet.  This is called from
     * an event handler thread.  It upacks the JDWP events inside
     * the packet and creates EventImpls for them.  The EventSet is already
     * on EventQueues when this is called, so it has to be synch.
     */
    synchronized void build() {
        if (pkt == null) {
            return;
        }
        PacketStream ps = new PacketStream(vm, pkt);
        JDWP.Event.Composite compEvt = new JDWP.Event.Composite(vm, ps);
        suspendPolicy = compEvt.suspendPolicy;
        if ((vm.traceFlags & VirtualMachine.TRACE_EVENTS) != 0) {
            switch(suspendPolicy) {
                case JDWP.SuspendPolicy.ALL:
                    vm.printTrace("EventSet: SUSPEND_ALL");
                    break;

                case JDWP.SuspendPolicy.EVENT_THREAD:
                    vm.printTrace("EventSet: SUSPEND_EVENT_THREAD");
                    break;

                case JDWP.SuspendPolicy.NONE:
                    vm.printTrace("EventSet: SUSPEND_NONE");
                    break;
            }
        }

        ThreadReference fix6485605 = null;
        for (int i = 0; i < compEvt.events.length; i++) {
            EventImpl evt = createEvent(compEvt.events[i]);
            if ((vm.traceFlags & VirtualMachine.TRACE_EVENTS) != 0) {
                try {
                    vm.printTrace("Event: " + evt);
                } catch (VMDisconnectedException ee) {
                    // ignore - see bug 6502716
                }
            }

            switch (evt.destination()) {
                case UNKNOWN_EVENT:
                    // Ignore disabled, deleted, unknown events, but
                    // save the thread if there is one since we might
                    // have to resume it.  Note that events for different
                    // threads can't be in the same event set.
                    if (evt instanceof ThreadedEventImpl &&
                        suspendPolicy == JDWP.SuspendPolicy.EVENT_THREAD) {
                        fix6485605 = ((ThreadedEventImpl)evt).thread();
                    }
                    continue;
                case CLIENT_EVENT:
                    addEvent(evt);
                    break;
                case INTERNAL_EVENT:
                    if (internalEventSet == null) {
                        internalEventSet = new EventSetImpl(this.vm, null);
                    }
                    internalEventSet.addEvent(evt);
                    break;
                default:
                    throw new InternalException("Invalid event destination");
            }
        }
        pkt = null; // No longer needed - free it up

        // Avoid hangs described in 6296125, 6293795
        if (super.size() == 0) {
            // This set has no client events.  If we don't do
            // needed resumes, no one else is going to.
            if (suspendPolicy == JDWP.SuspendPolicy.ALL) {
                vm.resume();
            } else if (suspendPolicy == JDWP.SuspendPolicy.EVENT_THREAD) {
                // See bug 6485605.
                if (fix6485605 != null) {
                    fix6485605.resume();
                } else {
                    // apparently, there is nothing to resume.
                }
            }
            suspendPolicy = JDWP.SuspendPolicy.NONE;

        }

    }

    
Filter out internal events
/**
     * Filter out internal events
     */
    EventSet userFilter() {
        return this;
    }

    
Filter out user events.
/**
     * Filter out user events.
     */
    EventSet internalFilter() {
        return this.internalEventSet;
    }

    EventImpl createEvent(JDWP.Event.Composite.Events evt) {
        JDWP.Event.Composite.Events.EventsCommon comm = evt.aEventsCommon;
        switch (evt.eventKind) {
            case JDWP.EventKind.THREAD_START:
                return new ThreadStartEventImpl(
                      (JDWP.Event.Composite.Events.ThreadStart)comm);

            case JDWP.EventKind.THREAD_END:
                return new ThreadDeathEventImpl(
                      (JDWP.Event.Composite.Events.ThreadDeath)comm);

            case JDWP.EventKind.EXCEPTION:
                return new ExceptionEventImpl(
                      (JDWP.Event.Composite.Events.Exception)comm);

            case JDWP.EventKind.BREAKPOINT:
                return new BreakpointEventImpl(
                      (JDWP.Event.Composite.Events.Breakpoint)comm);

            case JDWP.EventKind.METHOD_ENTRY:
                return new MethodEntryEventImpl(
                      (JDWP.Event.Composite.Events.MethodEntry)comm);

            case JDWP.EventKind.METHOD_EXIT:
                return new MethodExitEventImpl(
                      (JDWP.Event.Composite.Events.MethodExit)comm);

            case JDWP.EventKind.METHOD_EXIT_WITH_RETURN_VALUE:
                return new MethodExitEventImpl(
                      (JDWP.Event.Composite.Events.MethodExitWithReturnValue)comm);

            case JDWP.EventKind.FIELD_ACCESS:
                return new AccessWatchpointEventImpl(
                      (JDWP.Event.Composite.Events.FieldAccess)comm);

            case JDWP.EventKind.FIELD_MODIFICATION:
                return new ModificationWatchpointEventImpl(
                      (JDWP.Event.Composite.Events.FieldModification)comm);

            case JDWP.EventKind.SINGLE_STEP:
                return new StepEventImpl(
                      (JDWP.Event.Composite.Events.SingleStep)comm);

            case JDWP.EventKind.CLASS_PREPARE:
                return new ClassPrepareEventImpl(
                      (JDWP.Event.Composite.Events.ClassPrepare)comm);

            case JDWP.EventKind.CLASS_UNLOAD:
                return new ClassUnloadEventImpl(
                      (JDWP.Event.Composite.Events.ClassUnload)comm);

            case JDWP.EventKind.MONITOR_CONTENDED_ENTER:
                return new MonitorContendedEnterEventImpl(
                      (JDWP.Event.Composite.Events.MonitorContendedEnter)comm);

            case JDWP.EventKind.MONITOR_CONTENDED_ENTERED:
                return new MonitorContendedEnteredEventImpl(
                      (JDWP.Event.Composite.Events.MonitorContendedEntered)comm);

            case JDWP.EventKind.MONITOR_WAIT:
                return new MonitorWaitEventImpl(
                      (JDWP.Event.Composite.Events.MonitorWait)comm);

            case JDWP.EventKind.MONITOR_WAITED:
                return new MonitorWaitedEventImpl(
                      (JDWP.Event.Composite.Events.MonitorWaited)comm);

            case JDWP.EventKind.VM_START:
                return new VMStartEventImpl(
                      (JDWP.Event.Composite.Events.VMStart)comm);

            case JDWP.EventKind.VM_DEATH:
                return new VMDeathEventImpl(
                      (JDWP.Event.Composite.Events.VMDeath)comm);

            default:
                // Ignore unknown event types
                System.err.println("Ignoring event cmd " +
                                   evt.eventKind + " from the VM");
                return null;
        }
    }

    public VirtualMachine virtualMachine() {
        return vm;
    }

    public int suspendPolicy() {
        return EventRequestManagerImpl.JDWPtoJDISuspendPolicy(suspendPolicy);
    }

    private ThreadReference eventThread() {
        for (Event event : this) {
            if (event instanceof ThreadedEventImpl) {
                return ((ThreadedEventImpl)event).thread();
            }
        }
        return null;
    }

    public void resume() {
        switch (suspendPolicy()) {
            case EventRequest.SUSPEND_ALL:
                vm.resume();
                break;
            case EventRequest.SUSPEND_EVENT_THREAD:
                ThreadReference thread = eventThread();
                if (thread == null) {
                    throw new InternalException("Inconsistent suspend policy");
                }
                thread.resume();
                break;
            case EventRequest.SUSPEND_NONE:
                // Do nothing
                break;
            default:
                throw new InternalException("Invalid suspend policy");
        }
    }

    public Iterator<Event> iterator() {
        return new Itr();
    }

    public EventIterator eventIterator() {
        return new Itr();
    }

    public class Itr implements EventIterator {
        
Index of element to be returned by subsequent call to next.
/**
         * Index of element to be returned by subsequent call to next.
         */
        int cursor = 0;

        public boolean hasNext() {
            return cursor != size();
        }

        public Event next() {
            try {
                Event nxt = get(cursor);
                ++cursor;
                return nxt;
            } catch(IndexOutOfBoundsException e) {
                throw new NoSuchElementException();
            }
        }

        public Event nextEvent() {
            return next();
        }

        public void remove() {
            throw new UnsupportedOperationException();
        }
    }

    @Override
    public Spliterator<Event> spliterator() {
        return Spliterators.spliterator(this, Spliterator.DISTINCT);
    }

    /* below make this unmodifiable */

    public boolean add(Event o){
        throw new UnsupportedOperationException();
    }
    public boolean remove(Object o) {
        throw new UnsupportedOperationException();
    }
    public boolean addAll(Collection<? extends Event> coll) {
        throw new UnsupportedOperationException();
    }
    public boolean removeAll(Collection<?> coll) {
        throw new UnsupportedOperationException();
    }
    public boolean retainAll(Collection<?> coll) {
        throw new UnsupportedOperationException();
    }
    public void clear() {
        throw new UnsupportedOperationException();
    }
}