///////////////////////////////////////////////////////////////////////////////
//
//  Microsoft Research Singularity
//
//  Copyright (c) Microsoft Corporation.  All rights reserved.
//
//  File:   BlockWriter.sg
//
//  Note:
//

using Microsoft.SingSharp;
using Microsoft.Singularity.Channels;

using System;
using System.Threading;

namespace Microsoft.Singularity.Services.Fat.Fs
{
    public interface IBlockWriterUser
    {
        /// <remarks>
        /// This method is called immediately before the
        /// BlockWriter attempts to write the data to disk.  The
        /// receiver must give up the associated buffer.
        /// </remarks>
        byte[]! in ExHeap GetDataForWrite(ulong sectorId,
                                          int   cookie);

        /// <remarks>
        /// This method is called by the BlockWriter when the data
        /// has been written to the disk.
        /// </remarks>
        void WriteComplete(ulong                      sectorId,
                           [Claims] byte[]! in ExHeap buffer,
                           int                        cookie);
    }

    internal sealed class RequestQueue
    {
        private struct Request
        {
            IBlockWriterUser! user;
            ulong             sectorId;
            int               userCookie;

            public Request(IBlockWriterUser! theUser,
                           ulong             theSectorId,
                           int               theUserCookie)
            {
                this.user       = theUser;
                this.sectorId   = theSectorId;
                this.userCookie = theUserCookie;
            }

            public IBlockWriterUser! User   { get { return user; } }
            public ulong SectorId           { get { return sectorId; } }
            public int UserCookie           { get { return userCookie; } }
        }

        ///////////////////////////////////////////////////////////////////////
        // Fields

        Request[] requests;
        int head;
        int length;

        ///////////////////////////////////////////////////////////////////////
        // Invariants

        // invariant head >= 0 && head <= requests.Length;
        // invariant length >= 0 && length <= requests.Length;

        ///////////////////////////////////////////////////////////////////////
        // Methods

        internal RequestQueue(uint maxLength)
            requires maxLength >= 0;
        {
            requests = new Request[maxLength];
            head     = 0;
            length   = 0;
        }

        internal int MaxLength
        {
            get { return requests.Length; }
        }

        internal int Length
        {
            get { return length; }
        }

        internal bool Full
        {
            get { return length == requests.Length; }
        }

        internal bool Empty
        {
            get { return length == 0; }
        }

        internal void Enqueue(IBlockWriterUser! user,
                              ulong             theSectorId,
                              int               userCookie)
            requires (Full == false);
            ensures (Length == old(Length) + 1);
        {
            int index = (head + length) % requests.Length;
            requests[index] = new Request(user, theSectorId, userCookie);
            length++;
        }

        internal void Dequeue(out IBlockWriterUser user,
                              out ulong            sectorId,
                              out int              userCookie)
            requires Length != 0;
            ensures  Length == old(Length) - 1;
        {
            user       = requests[head].User;
            sectorId   = requests[head].SectorId;
            userCookie = requests[head].UserCookie;
            head   = (head + 1) % MaxLength;
            length = length - 1;
        }
    }

    /// <summary>
    /// Block writer is responsible for writing blocks to disk.
    /// It may operate synchronously or asynchronously.  When
    /// synchronous, all writes occur immediately.  When
    /// asynchronous they are performed when the request queue
    /// over flows or when the periodic worker thread is woken
    /// up.
    /// </summary>
    internal sealed class BlockWriter
    {
        ///////////////////////////////////////////////////////////////////////
        // Constants

        private const int MinimumSleepSecs = 5;
        private const int MaximumSleepSecs = 15;
        private const int RangeSleepSecs = MaximumSleepSecs - MinimumSleepSecs;

        ///////////////////////////////////////////////////////////////////////
        // Fields

        AutoResetEvent   wakeEvent;
        volatile bool    shutdown;
        Disk             disk;
        RequestQueue     requestQueue;
        Thread           worker;

        bool             writesAllowed;     // Sanity check for RO mount
        bool             diskDirtied;       // Data written to disk

        ///////////////////////////////////////////////////////////////////////
        // Methods

        [ Microsoft.Contracts.NotDelayed ]
        internal BlockWriter(Disk theDisk,
                             uint maxQueueLength,   // Synchronous if zero
                             bool areWritesAllowed)
        {
            this.wakeEvent     = new AutoResetEvent(false);
            this.shutdown      = false;
            this.disk          = theDisk;
            this.diskDirtied   = false;
            this.requestQueue  = new RequestQueue(maxQueueLength);
            this.writesAllowed = areWritesAllowed;

            if (requestQueue.MaxLength > 0) {
                worker = new Thread(new ThreadStart(WorkerMain));
                worker.Start();
            }
        }

        internal void Shutdown()
        {
            lock (this) {
                shutdown = true;
            }

            if (worker != null) {
                wakeEvent.Set();
                worker.Join();
                worker = null;
            }

            if (this.diskDirtied) {
                assert FatVolume.Fat.CleanShutdown == false;

                if (FatVolume.FatVersion == FatVersion.Fat32) {
                    Fat32 fat32 = (Fat32)FatVolume.Fat;
                    fat32.UpdateFsInfo32();
                }

                // The following method sets the appropriate bit
                // in the FAT(s), which will ripple down as a
                // dirty blocks in the cache for FAT16 and FAT32
                // (FAT12 does not record clean shutdown)
                FatVolume.Fat.CleanShutdown = true;

                // Synchronously flush the queue as the worker
                // thread has exited.
                int done = this.FlushQueue();
                assert done >= 1 || FatVolume.FatVersion == FatVersion.Fat12;
            }
        }

        private void WorkerMain()
        {
            Random rng = new Random();

            while (shutdown == false) {
                int secs = MinimumSleepSecs + (rng.Next() % RangeSleepSecs);
                wakeEvent.WaitOne(TimeSpan.FromSeconds(secs));
                Tracing.Log(Tracing.Debug, "BlockWriter worker flushing.");

                if (this.diskDirtied &&
                    FatVolume.FatVersion == FatVersion.Fat32) {
                    // Update free space estimate.  This might be done
                    // more conditionally, ie check whether free space has
                    // changed.
                    Fat32 fat32 = (Fat32)FatVolume.Fat;
                    fat32.UpdateFsInfo32();
                }

                FlushQueue();
            }
        }

        private int FlushQueue()
        {
            int done = 0;
            for (;;) {
                IBlockWriterUser user = null;
                ulong            sectorId;
                int              userCookie;

                lock (this) {
                    // We might want to be
                    // smart and re-order write requests here or
                    // add ordering to requests made to disk.
                    //
                    // We also want to look at changing
                    // the disk driver interface to support scatter-gather
                    // writes so we can write consecutive blocks back-to-back
                    // and avoid unnecessary seeks.
                    if (requestQueue.Empty)
                        break;
                    requestQueue.Dequeue(out user, out sectorId, out userCookie);
                }
                if (user != null) {
                    WriteBlock(user, sectorId, userCookie);
                    done++;
                }
            }
            return done;
        }

        private void WriteBlock(IBlockWriterUser! user,
                                ulong             sectorId,
                                int               cookie)
        {
            assert writesAllowed == true;

            if (this.diskDirtied == false) {
                FatVolume.Fat.CleanShutdown = false;
                this.diskDirtied = true;
            }

            //            DebugStub.Print("Writing sector {0}\n",
            //              __arglist(sectorId));

            byte[]! in ExHeap data = user.GetDataForWrite(sectorId, cookie);

            byte[] in ExHeap doneData = disk.Write(sectorId, data);
            user.WriteComplete(sectorId, (!) doneData, cookie);
        }

        internal bool Enqueue(IBlockWriterUser! user,
                              ulong             sectorId,
                              int               userCookie)
        {
            assert writesAllowed == true;

            IBlockWriterUser pushUser       = null;
            ulong            pushSectorId   = 0;
            int              pushUserCookie = 0;

            if (requestQueue.MaxLength == 0) {
                // Queue has no capacity so we're synchronous and
                // will need to push this write to disk
                pushUser       = user;
                pushSectorId   = sectorId;
                pushUserCookie = userCookie;
            }
            else {
                lock (this) {
                    // We're async, and need to push head from queue if
                    // queue overflows.
                    if (requestQueue.Full) {
                        requestQueue.Dequeue(out pushUser,
                                             out pushSectorId,
                                             out pushUserCookie);
                    }
                    requestQueue.Enqueue(user, sectorId, userCookie);
                }
            }

            if (pushUser != null) {
                Tracing.Log(Tracing.Audit,
                            "Request enqueue pushed block to disk.");
                WriteBlock(pushUser, pushSectorId, pushUserCookie);
            }

            return true;
        }

        internal void WakeUp()
        {
            wakeEvent.Set();
        }
    }
}