Parallel.cs

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// Parallel.cs
//
// Copyright (c) 2008 Jérémie "Garuma" Laval
//
// 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, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// 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 FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
//

#if NET_4_0
using System;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Threading;
using System.Runtime.InteropServices;

namespace System.Threading.Tasks
{
    public static class Parallel
    {
        internal static int GetBestWorkerNumber ()
        {
            return GetBestWorkerNumber (TaskScheduler.Current);
        }

        internal static int GetBestWorkerNumber (TaskScheduler scheduler)
        {
            return Math.Min (Environment.ProcessorCount, (scheduler ?? TaskScheduler.Current).MaximumConcurrencyLevel);
        }

        static int GetBestWorkerNumber (int from, int to, ParallelOptions options, out int step)
        {
            int num = GetBestWorkerNumber(options.TaskScheduler);
            if (options != null && options.MaxDegreeOfParallelism != -1)
                num = Math.Min (options.MaxDegreeOfParallelism, num);
            // Integer range that each task process
            if ((step = (to - from) / num) < 5) {
                step = 5;
                num = (to - from) / 5;
                if (num < 1)
                    num = 1;
            }

            return num;
        }

        static void HandleExceptions (IEnumerable<Task> tasks)
        {
            HandleExceptions (tasks, null);
        }

        static void HandleExceptions (IEnumerable<Task> tasks, ParallelLoopState.ExternalInfos infos)
        {
            List<Exception> exs = new List<Exception> ();
            foreach (Task t in tasks) {
                if (t.Exception != null)
                    exs.Add (t.Exception);
            }

            if (exs.Count > 0) {
                if (infos != null)
                    infos.IsExceptional = true;

                throw new AggregateException (exs).Flatten ();
            }
        }

        static void InitTasks (Task[] tasks, int count, Action action, ParallelOptions options)
        {
            TaskCreationOptions creation = TaskCreationOptions.LongRunning | TaskCreationOptions.AttachedToParent;

            for (int i = 0; i < count; i++) {
                if (options == null)
                    tasks [i] = Task.Factory.StartNew (action, creation);
                else
                    tasks [i] = Task.Factory.StartNew (action, options.CancellationToken, creation, options.TaskScheduler);
            }
        }

#region For

        public static ParallelLoopResult For (int fromInclusive, int toExclusive, Action<int> body)
        {
            return For (fromInclusive, toExclusive, ParallelOptions.Default, body);
        }

        public static ParallelLoopResult For (int fromInclusive, int toExclusive, Action<int, ParallelLoopState> body)
        {
            return For (fromInclusive, toExclusive, ParallelOptions.Default, body);
        }

        public static ParallelLoopResult For (int fromInclusive, int toExclusive, ParallelOptions parallelOptions, Action<int> body)
        {
            return For (fromInclusive, toExclusive, parallelOptions, (index, state) => body (index));
        }

        public static ParallelLoopResult For (int fromInclusive, int toExclusive, ParallelOptions parallelOptions, Action<int, ParallelLoopState> body)
        {
            return For<object> (fromInclusive, toExclusive, parallelOptions, () => null, (i, s, l) => { body (i, s); return null; }, _ => {});
        }

        public static ParallelLoopResult For<TLocal> (int fromInclusive,
                                                      int toExclusive,
                                                      Func<TLocal> localInit,
                                                      Func<int, ParallelLoopState, TLocal, TLocal> body,
                                                      Action<TLocal> localFinally)
        {
            return For<TLocal> (fromInclusive, toExclusive, ParallelOptions.Default, localInit, body, localFinally);
        }

        public static ParallelLoopResult For<TLocal> (int fromInclusive,
                                                      int toExclusive,
                                                      ParallelOptions parallelOptions,
                                                      Func<TLocal> localInit,
                                                      Func<int, ParallelLoopState, TLocal, TLocal> body,
                                                      Action<TLocal> localFinally)
        {
            if (body == null)
                throw new ArgumentNullException ("body");
            if (localInit == null)
                throw new ArgumentNullException ("localInit");
            if (localFinally == null)
                throw new ArgumentNullException ("localFinally");
            if (parallelOptions == null)
                throw new ArgumentNullException ("options");
            if (fromInclusive >= toExclusive)
                return new ParallelLoopResult (null, true);

            // Number of task toExclusive be launched (normally == Env.ProcessorCount)
            int step;
            int num = GetBestWorkerNumber (fromInclusive, toExclusive, parallelOptions, out step);

            Task[] tasks = new Task [num];

            StealRange[] ranges = new StealRange[num];
            for (int i = 0; i < num; i++)
                ranges[i] = new StealRange (fromInclusive, i, step);

            ParallelLoopState.ExternalInfos infos = new ParallelLoopState.ExternalInfos ();

            int currentIndex = -1;

            Action workerMethod = delegate {
                int localWorker = Interlocked.Increment (ref currentIndex);
                StealRange range = ranges[localWorker];
                int index = range.V64.Actual;
                int stopIndex = localWorker + 1 == num ? toExclusive : Math.Min (toExclusive, index + step);
                TLocal local = localInit ();

                ParallelLoopState state = new ParallelLoopState (infos);
                CancellationToken token = parallelOptions.CancellationToken;

                try {
                    for (int i = index; i < stopIndex;) {
                        if (infos.IsStopped)
                            return;

                        token.ThrowIfCancellationRequested ();

                        if (i >= stopIndex - range.V64.Stolen)
                            break;

                        if (infos.LowestBreakIteration != null && infos.LowestBreakIteration > i)
                            return;

                        state.CurrentIteration = i;
                        local = body (i, state, local);

                        if (i + 1 >= stopIndex - range.V64.Stolen)
                            break;

                        range.V64.Actual = ++i;
                    }

                    bool sixtyfour = IntPtr.Size == 8; // Environment.Is64BitProcess;
                    
                    // Try toExclusive steal fromInclusive our right neighbor (cyclic)
                    int len = num + localWorker;
                    for (int sIndex = localWorker + 1; sIndex < len; ++sIndex) {
                        int extWorker = sIndex % num;
                        range = ranges[extWorker];

                        stopIndex = extWorker + 1 == num ? toExclusive : Math.Min (toExclusive, fromInclusive + (extWorker + 1) * step);
                        int stolen = -1;

                        do {
                            do {
                                long old;
                                StealValue64 val = new StealValue64 ();

                                old = sixtyfour ? range.V64.Value : Interlocked.CompareExchange (ref range.V64.Value, 0, 0);
                                val.Value = old;

                                if (val.Actual >= stopIndex - val.Stolen - 2)
                                    goto next;
                                stolen = (val.Stolen += 1);

                                if (Interlocked.CompareExchange (ref range.V64.Value, val.Value, old) == old)
                                    break;
                            } while (true);

                            stolen = stopIndex - stolen;

                            if (stolen > range.V64.Actual)
                                local = body (stolen, state, local);
                            else
                                break;
                        } while (true);

                        next:
                        continue;
                    }
                } finally {
                    localFinally (local);
                }
            };

            InitTasks (tasks, num, workerMethod, parallelOptions);

            try {
                Task.WaitAll (tasks);
            } catch {
                HandleExceptions (tasks, infos);
            }

            return new ParallelLoopResult (infos.LowestBreakIteration, !(infos.IsStopped || infos.IsExceptional));
        }

        [StructLayout(LayoutKind.Explicit)]
        struct StealValue64 {
            [FieldOffset(0)]
            public long Value;
            [FieldOffset(0)]
            public int Actual;
            [FieldOffset(4)]
            public int Stolen;
        }

        class StealRange
        {
            public StealValue64 V64 = new StealValue64 ();

            public StealRange (int fromInclusive, int i, int step)
            {
                V64.Actual = fromInclusive + i * step;
            }
        }

#endregion

#region For (long)

        [MonoTODO]
        public static ParallelLoopResult For (long fromInclusive, long toExclusive, Action<long> body)
        {
            return For (fromInclusive, toExclusive, ParallelOptions.Default, body);
        }

        [MonoTODO]
        public static ParallelLoopResult For (long fromInclusive, long toExclusive, Action<long, ParallelLoopState> body)
        {
            return For (fromInclusive, toExclusive, ParallelOptions.Default, body);
        }

        [MonoTODO]
        public static ParallelLoopResult For (long fromInclusive, long toExclusive, ParallelOptions parallelOptions, Action<long> body)
        {
            return For (fromInclusive, toExclusive, parallelOptions, (index, state) => body (index));
        }

        [MonoTODO]
        public static ParallelLoopResult For (long fromInclusive, long toExclusive, ParallelOptions parallelOptions, Action<long, ParallelLoopState> body)
        {
            return For<object> (fromInclusive, toExclusive, parallelOptions, () => null, (i, s, l) => { body (i, s); return null; }, _ => {});
        }

        [MonoTODO]
        public static ParallelLoopResult For<TLocal> (long fromInclusive,
                                                      long toExclusive,
                                                      Func<TLocal> localInit,
                                                      Func<long, ParallelLoopState, TLocal, TLocal> body,
                                                      Action<TLocal> localFinally)
        {
            return For<TLocal> (fromInclusive, toExclusive, ParallelOptions.Default, localInit, body, localFinally);
        }

        [MonoTODO ("See how this can be refactored with the above For implementation")]
        public static ParallelLoopResult For<TLocal> (long fromInclusive,
                                                      long toExclusive,
                                                      ParallelOptions parallelOptions,
                                                      Func<TLocal> localInit,
                                                      Func<long, ParallelLoopState, TLocal, TLocal> body,
                                                      Action<TLocal> localFinally)
        {
            if (body == null)
                throw new ArgumentNullException ("body");
            if (localInit == null)
                throw new ArgumentNullException ("localInit");
            if (localFinally == null)
                throw new ArgumentNullException ("localFinally");
            if (parallelOptions == null)
                throw new ArgumentNullException ("options");
            if (fromInclusive >= toExclusive)
                return new ParallelLoopResult (null, true);

            throw new NotImplementedException ();
        }

#endregion

#region Foreach
        static ParallelLoopResult ForEach<TSource, TLocal> (Func<int, IList<IEnumerator<TSource>>> enumerable, ParallelOptions options,
                                                            Func<TLocal> init, Func<TSource, ParallelLoopState, TLocal, TLocal> action,
                                                            Action<TLocal> destruct)
        {
            if (enumerable == null)
                throw new ArgumentNullException ("source");
            if (options == null)
                throw new ArgumentNullException ("options");
            if (action == null)
                throw new ArgumentNullException ("action");
            if (init == null)
                throw new ArgumentNullException ("init");
            if (destruct == null)
                throw new ArgumentNullException ("destruct");

            int num = Math.Min (GetBestWorkerNumber (options.TaskScheduler),
                                options != null && options.MaxDegreeOfParallelism != -1 ? options.MaxDegreeOfParallelism : int.MaxValue);

            Task[] tasks = new Task[num];
            ParallelLoopState.ExternalInfos infos = new ParallelLoopState.ExternalInfos ();

            SimpleConcurrentBag<TSource> bag = new SimpleConcurrentBag<TSource> (num);
            const int bagCount = 5;

            IList<IEnumerator<TSource>> slices = enumerable (num);

            int sliceIndex = -1;

            Action workerMethod = delegate {
                IEnumerator<TSource> slice = slices[Interlocked.Increment (ref sliceIndex)];

                TLocal local = init ();
                ParallelLoopState state = new ParallelLoopState (infos);
                int workIndex = bag.GetNextIndex ();
                CancellationToken token = options.CancellationToken;

                try {
                    bool cont = true;
                    TSource element;

                    while (cont) {
                        if (infos.IsStopped || infos.IsBroken.Value)
                            return;

                        token.ThrowIfCancellationRequested ();

                        for (int i = 0; i < bagCount && (cont = slice.MoveNext ()); i++) {
                            bag.Add (workIndex, slice.Current);
                        }

                        for (int i = 0; i < bagCount && bag.TryTake (workIndex, out element); i++) {
                            if (infos.IsStopped)
                                return;

                            token.ThrowIfCancellationRequested ();

                            local = action (element, state, local);
                        }
                    }

                    while (bag.TrySteal (workIndex, out element)) {
                        token.ThrowIfCancellationRequested ();

                        local = action (element, state, local);

                        if (infos.IsStopped || infos.IsBroken.Value)
                            return;
                    }
                } finally {
                    destruct (local);
                }
            };

            InitTasks (tasks, num, workerMethod, options);

            try {
                Task.WaitAll (tasks);
            } catch {
                HandleExceptions (tasks, infos);
            }

            return new ParallelLoopResult (infos.LowestBreakIteration, !(infos.IsStopped || infos.IsExceptional));
        }

        public static ParallelLoopResult ForEach<TSource> (IEnumerable<TSource> source, Action<TSource> body)
        {
            if (source == null)
                throw new ArgumentNullException ("source");
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, object> (Partitioner.Create (source),
                                             ParallelOptions.Default,
                                             () => null,
                                             (e, s, l) => { body (e); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (IEnumerable<TSource> source, Action<TSource, ParallelLoopState> body)
        {
            if (source == null)
                throw new ArgumentNullException ("source");
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, object> (Partitioner.Create (source),
                                             ParallelOptions.Default,
                                             () => null,
                                             (e, s, l) => { body (e, s); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (IEnumerable<TSource> source,
                                                           Action<TSource, ParallelLoopState, long> body)
        {
            if (source == null)
                throw new ArgumentNullException ("source");
            if (body == null)
                throw new ArgumentNullException ("body");


            return ForEach<TSource, object> (Partitioner.Create (source),
                                             ParallelOptions.Default,
                                             () => null,
                                             (e, s, l) => { body (e, s, -1); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (Partitioner<TSource> source,
                                                           Action<TSource, ParallelLoopState> body)
        {
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, object> (source,
                                             ParallelOptions.Default,
                                             () => null,
                                             (e, s, l) => { body (e, s); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (OrderablePartitioner<TSource> source,
                                                           Action<TSource, ParallelLoopState, long> body)

        {
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, object> (source,
                                             ParallelOptions.Default,
                                             () => null,
                                             (e, s, i, l) => { body (e, s, i); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (Partitioner<TSource> source,
                                                           Action<TSource> body)

        {
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, object> (source,
                                             ParallelOptions.Default,
                                             () => null,
                                             (e, s, l) => { body (e); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (IEnumerable<TSource> source,
                                                           ParallelOptions parallelOptions,
                                                           Action<TSource> body)
        {
            if (source == null)
                throw new ArgumentNullException ("source");
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, object> (Partitioner.Create (source),
                                             parallelOptions,
                                             () => null,
                                             (e, s, l) => { body (e); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (IEnumerable<TSource> source, ParallelOptions parallelOptions,
                                                           Action<TSource, ParallelLoopState> body)
        {
            if (source == null)
                throw new ArgumentNullException ("source");
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, object> (Partitioner.Create (source),
                                             parallelOptions,
                                             () => null,
                                             (e, s, l) => { body (e, s); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (IEnumerable<TSource> source, ParallelOptions parallelOptions,
                                                           Action<TSource, ParallelLoopState, long> body)
        {
            if (source == null)
                throw new ArgumentNullException ("source");
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, object> (Partitioner.Create (source),
                                             parallelOptions,
                                             () => null,
                                             (e, s, i, l) => { body (e, s, i); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (OrderablePartitioner<TSource> source, ParallelOptions parallelOptions,
                                                           Action<TSource, ParallelLoopState, long> body)

        {
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, object> (source,
                                             parallelOptions,
                                             () => null,
                                             (e, s, i, l) => { body (e, s, i); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (Partitioner<TSource> source, ParallelOptions parallelOptions,
                                                           Action<TSource> body)
        {
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, object> (source,
                                             parallelOptions,
                                             () => null,
                                             (e, s, l) => { body (e); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource> (Partitioner<TSource> source, ParallelOptions parallelOptions,
                                                           Action<TSource, ParallelLoopState> body)
        {
            return ForEach<TSource, object> (source,
                                             parallelOptions,
                                             () => null,
                                             (e, s, l) => { body (e, s); return null; },
                                             _ => {});
        }

        public static ParallelLoopResult ForEach<TSource, TLocal> (IEnumerable<TSource> source, Func<TLocal> localInit,
                                                                   Func<TSource, ParallelLoopState, TLocal, TLocal> body,
                                                                   Action<TLocal> localFinally)
        {
            if (source == null)
                throw new ArgumentNullException ("source");

            return ForEach<TSource, TLocal> ((Partitioner<TSource>)Partitioner.Create (source),
                                             ParallelOptions.Default,
                                             localInit,
                                             body,
                                             localFinally);
        }

        public static ParallelLoopResult ForEach<TSource, TLocal> (IEnumerable<TSource> source, Func<TLocal> localInit,
                                                                   Func<TSource, ParallelLoopState, long, TLocal, TLocal> body,
                                                                   Action<TLocal> localFinally)
        {
            return ForEach<TSource, TLocal> (Partitioner.Create (source),
                                             ParallelOptions.Default,
                                             localInit,
                                             body,
                                             localFinally);
        }

        public static ParallelLoopResult ForEach<TSource, TLocal> (OrderablePartitioner<TSource> source, Func<TLocal> localInit,
                                                                   Func<TSource, ParallelLoopState, long, TLocal, TLocal> body,
                                                                   Action<TLocal> localFinally)
        {
            return ForEach<TSource, TLocal> (source, ParallelOptions.Default, localInit, body, localFinally);
        }

        public static ParallelLoopResult ForEach<TSource, TLocal> (Partitioner<TSource> source, Func<TLocal> localInit,
                                                                   Func<TSource, ParallelLoopState, TLocal, TLocal> body,
                                                                   Action<TLocal> localFinally)
        {
            return ForEach<TSource, TLocal> (source, ParallelOptions.Default, localInit, body, localFinally);
        }

        public static ParallelLoopResult ForEach<TSource, TLocal> (IEnumerable<TSource> source, ParallelOptions parallelOptions,
                                                                   Func<TLocal> localInit,
                                                                   Func<TSource, ParallelLoopState, TLocal, TLocal> body,
                                                                   Action<TLocal> localFinally)
        {
            if (source == null)
                throw new ArgumentNullException ("source");

            return ForEach<TSource, TLocal> (Partitioner.Create (source), parallelOptions, localInit, body, localFinally);
        }

        public static ParallelLoopResult ForEach<TSource, TLocal> (IEnumerable<TSource> source, ParallelOptions parallelOptions,
                                                                   Func<TLocal> localInit,
                                                                   Func<TSource, ParallelLoopState, long, TLocal, TLocal> body,
                                                                   Action<TLocal> localFinally)
        {
            if (source == null)
                throw new ArgumentNullException ("source");

            return ForEach<TSource, TLocal> (Partitioner.Create (source), parallelOptions, localInit, body, localFinally);
        }

        public static ParallelLoopResult ForEach<TSource, TLocal> (Partitioner<TSource> source, ParallelOptions parallelOptions,
                                                                   Func<TLocal> localInit,
                                                                   Func<TSource, ParallelLoopState, TLocal, TLocal> body,
                                                                   Action<TLocal> localFinally)
        {
            if (source == null)
                throw new ArgumentNullException ("source");
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<TSource, TLocal> (source.GetPartitions, parallelOptions, localInit, body, localFinally);
        }

        public static ParallelLoopResult ForEach<TSource, TLocal> (OrderablePartitioner<TSource> source, ParallelOptions parallelOptions,
                                                                   Func<TLocal> localInit,
                                                                   Func<TSource, ParallelLoopState, long, TLocal, TLocal> body,
                                                                   Action<TLocal> localFinally)
        {
            if (source == null)
                throw new ArgumentNullException ("source");
            if (body == null)
                throw new ArgumentNullException ("body");

            return ForEach<KeyValuePair<long, TSource>, TLocal> (source.GetOrderablePartitions,
                                                                 parallelOptions,
                                                                 localInit,
                                                                 (e, s, l) => body (e.Value, s, e.Key, l),
                                                                 localFinally);
        }
        #endregion

        #region Invoke
        public static void Invoke (params Action[] actions)
        {
            if (actions == null)
                throw new ArgumentNullException ("actions");

            Invoke (ParallelOptions.Default, actions);
        }

        public static void Invoke (ParallelOptions parallelOptions, params Action[] actions)
        {
            if (parallelOptions == null)
                throw new ArgumentNullException ("parallelOptions");
            if (actions == null)
                throw new ArgumentNullException ("actions");
            if (actions.Length == 0)
                throw new ArgumentException ("actions is empty");
            foreach (var a in actions)
                if (a == null)
                    throw new ArgumentException ("One action in actions is null", "actions");
            if (actions.Length == 1) {
                actions[0] ();
                return;
            }

            Task[] ts = new Task[actions.Length];
            for (int i = 0; i < ts.Length; i++)
                ts[i] = Task.Factory.StartNew (actions[i],
                                               parallelOptions.CancellationToken,
                                               TaskCreationOptions.None,
                                               parallelOptions.TaskScheduler);

            try {
                Task.WaitAll (ts, parallelOptions.CancellationToken);
            } catch {
                HandleExceptions (ts);
            }
        }
        #endregion

        #region SpawnBestNumber, used by PLinq
        internal static Task[] SpawnBestNumber (Action action, Action callback)
        {
            return SpawnBestNumber (action, -1, callback);
        }

        internal static Task[] SpawnBestNumber (Action action, int dop, Action callback)
        {
            return SpawnBestNumber (action, dop, false, callback);
        }

        internal static Task[] SpawnBestNumber (Action action, int dop, bool wait, Action callback)
        {
            // Get the optimum amount of worker to create
            int num = dop == -1 ? (wait ? GetBestWorkerNumber () + 1 : GetBestWorkerNumber ()) : dop;

            // Initialize worker
            CountdownEvent evt = new CountdownEvent (num);
            Task[] tasks = new Task [num];
            for (int i = 0; i < num; i++) {
                tasks [i] = Task.Factory.StartNew (() => {
                    action ();
                    evt.Signal ();
                    if (callback != null && evt.IsSet)
                        callback ();
                });
            }

            // If explicitely told, wait for all workers to complete
            // and thus let main thread participate in the processing
            if (wait)
                Task.WaitAll (tasks);

            return tasks;
        }
#endregion
    }
}
#endif