///////////////////////////////////////////////////////////////////////////////
//
//  Microsoft Research Singularity
//
//  Copyright (c) Microsoft Corporation.  All rights reserved.
//
//  File:   Fat12.sg
//
//  This code bangs hopelessly on the buffer cache.  12 bit
//  pointers equates to pointers that span byte and sector
//  boundaries.  These are painful to work with.  To date no
//  effort has been made to optimize the chain allocation, grow,
//  free, and truncate routines.
//

//#define DEBUG_FAT12 // Turn on sanity checks such as they are

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

using System;

namespace Microsoft.Singularity.Services.Fat.Fs
{
    internal sealed class Fat12 : Fat
    {
        Fat12Internal fatInternal;

        internal Fat12(BlockCache! theBlockCache,
                       BpbSummary! theBpbSummary)
        {
            fatInternal = new Fat12Internal(theBlockCache, theBpbSummary);
        }

        internal override bool AllocateChain(int hintClusterArea,
                                             int targetLength,
                                             out int allocStart,
                                             out int allocLength)
        {
            return fatInternal.AllocateChain(hintClusterArea, targetLength,
                                             out allocStart, out allocLength);
        }

        internal override void FreeChain(int startCluster)
        {
            fatInternal.FreeChain(startCluster);
        }

        internal override bool GrowChain(BlockIndex! index,
                                         int         requestedExtensionLength,
                                         out int     actualExtensionLength)
        {
            return fatInternal.GrowChain(index, requestedExtensionLength,
                                         out actualExtensionLength);
        }

        internal override void TruncateChain(BlockIndex! index,
                                             int         lengthInClusters)
        {
            fatInternal.TruncateChain(index, lengthInClusters);
        }

        internal override void PopulateIndex(BlockIndex! index,
                                             int         firstCluster)
        {
            fatInternal.PopulateIndex(index, firstCluster);
        }

        internal override bool CleanShutdown
        {
            get { return fatInternal.CleanShutdown; }
            set { fatInternal.CleanShutdown = value; }
        }

        internal override bool HardError
        {
            get { return fatInternal.HardError; }
            set { fatInternal.HardError = value; }
        }

        internal override int TotalClusters
        {
            get { return fatInternal.TotalClusters; }
        }

        internal override int EndOfChain
        {
            get { return fatInternal.EndOfChain; }
        }

        internal override int BadCluster
        {
            get { return fatInternal.BadCluster; }
        }

        // --------------------------------------------------------------------
        // Internal representation of FAT12 structure

        private sealed class Fat12Internal
        {
            private const int UnallocatedMarker = 0;
            private const int EndOfChainMarker  = 0xfff;
            private const int EndOfChainMinimum = 0xff8;
            private const int BadClusterMarker  = 0xff7;
            public  const int MaxClusters       = 0xff7;
            public  const int ReservedClusters  = 2;

            private const ushort Visited = 0xffff;      // Used by DEBUG_FAT12

            private BlockCache! blockCache;
            private BpbSummary! bpbSummary;
            private uint        preferredFat;
            private Bitmap!     bitmap;
            private bool        cleanShutdown;
            private bool        hardError;

            [ Microsoft.Contracts.NotDelayed ]
            internal Fat12Internal(BlockCache! theBlockCache,
                                   BpbSummary! theBpbSummary)
                requires theBpbSummary.Version       == FatVersion.Fat12;
                requires theBpbSummary.SectorsPerFat >= 1;
                requires theBpbSummary.NumberOfFats  >= 1;
                requires theBpbSummary.ClusterCount  >= 1;
            {
                this.blockCache    = theBlockCache;
                this.bpbSummary    = theBpbSummary;
                this.preferredFat  = 0;
                this.bitmap        =
                    new Bitmap((int)theBpbSummary.ClusterCount + ReservedClusters);
                this.cleanShutdown = true;
                this.hardError     = false;
                base();
                InitializeBitmap(this.preferredFat);
            }

            internal bool CleanShutdown
            {
                get { return this.cleanShutdown; }
                set { this.cleanShutdown = value; }
            }

            internal bool HardError
            {
                get { return this.hardError; }
                set { this.hardError = value; }
            }

            internal int TotalClusters
            {
                get {
                    return (int)bpbSummary.ClusterCount + ReservedClusters;
                }
            }

            internal int EndOfChain
            {
                get { return EndOfChainMarker; }
            }

            internal int BadCluster
            {
                get { return BadClusterMarker; }
            }

            private void InitializeBitmap(uint fat)
            {
                // Mark the two reserved clusters as used
                int allocStart, allocLength;
                bitmap.Allocate(0, ReservedClusters,
                                out allocStart, out allocLength);
                DebugStub.Assert(allocStart  == 0);
                DebugStub.Assert(allocLength == ReservedClusters);

                // Scan clusters to find those free.
                for (int i = ReservedClusters; i < bitmap.Length; i++) {
                    int next;
                    GetNextInFat(fat, i, out next);
                    if (next != UnallocatedMarker) {
                        bitmap.Allocate(i, 1, out allocStart, out allocLength);
                        DebugStub.Assert(allocStart == i);
                        DebugStub.Assert(allocLength == 1);
                    }
                }
                DumpFat("Initial FAT");
            }

            // ---------------------------------------------------------------
            // Debugging routines

            [ System.Diagnostics.Conditional("DEBUG_FAT12") ]
            private void DumpFat(string! title)
            {
                DebugStub.Print("{0}\n", __arglist(title));
                for (int i = 0; i < TotalClusters; i++) {
                    int next;
                    GetNextInFat(0, i, out next);
                    if (next != 0) {
                        DebugStub.Print("Fat[{0:x8}] = {1:x8}\n",
                                        __arglist(i, next));
                    }

                    for (uint fat = 1; fat < bpbSummary.NumberOfFats; fat++) {
                        int onext;
                        GetNextInFat(fat, i, out onext);
                        DebugStub.Assert(onext == next);
                    }
                }
                FollowFat();
            }

            [ System.Diagnostics.Conditional("DEBUG_FAT12") ]
            private void FollowFat()
            {
                ushort [] clusters = new ushort [TotalClusters];
                for (int i = 0; i < TotalClusters; i++) {
                    int next;
                    GetNextInFat(0, i, out next);
                    clusters[i] = (ushort)next;
                }

                for (int i = ReservedClusters; i < TotalClusters; i++) {
                    if ((int)clusters[i] == Visited ||
                        (int)clusters[i] == UnallocatedMarker) {
                        continue;
                    }
                    VisitChain(clusters, i);
                }
            }

            [ System.Diagnostics.Conditional("DEBUG_FAT12") ]
            private void VisitChain(ushort []! clusters, int start)
            {
                while (start < EndOfChainMinimum) {
                    if (start < ReservedClusters || start == Visited) {
                        DebugStub.Break();
                    }
                    int next = clusters[start];
                    clusters[start] = Visited;
                    start = next;
                }
            }

            // ----------------------------------------------------------------
            // Chain growth and creation related methods

            internal bool GrowChain(BlockIndex! index,
                                    int         clustersToAdd,
                                    out int     clustersAdded)
                requires index.Count > 0;
                requires clustersToAdd > 0;
            {
                if (clustersToAdd > Bitmap.MaxAllocationLength) {
                    clustersToAdd = Bitmap.MaxAllocationLength;
                }

                lock (this) {
                    int tail;
                    if (index.Lookup(index.Count - 1, out tail) == false) {
                        assert false;
                    }

                    int newTailStart;
                    if (LockedAllocateChain(tail, clustersToAdd, index,
                                            out newTailStart,
                                            out clustersAdded)) {
                        SetNext(tail, newTailStart);
                        return true;
                    }
                    return false;
                }
            }

            internal bool AllocateChain(int hintStart,
                                        int targetLength,
                                        out int allocStart,
                                        out int allocLength)
                requires hintStart < Fat12Internal.MaxClusters;
                requires targetLength > 0;
            {
                if (targetLength > Bitmap.MaxAllocationLength) {
                    targetLength = Bitmap.MaxAllocationLength;
                }
                lock (this) {
                    return LockedAllocateChain(hintStart, targetLength, null,
                                               out allocStart,
                                               out allocLength);
                }
            }

            private void LockedBlockIndexAppend(BlockIndex index,
                                              int        tail,
                                              int        length)
            {
                if (index != null) {
                    index.Append(tail, length);
                }
            }

            private bool LockedAllocateChain(int        tail,
                                             int        clustersToAdd,
                                             BlockIndex index,
                                             out int    newTailStart,
                                             out int    newTailAllocated)
            {
                DebugStub.Assert(clustersToAdd >= 1);
                if (!bitmap.Allocate(tail, 1,
                                     out newTailStart,
                                     out newTailAllocated)) {
                    return false;
                }
                LockedBlockIndexAppend(index, newTailStart, newTailAllocated);

                int current = newTailStart;
                while (newTailAllocated != clustersToAdd) {
                    int delta, next;
                    if (!bitmap.Allocate(current, 1, out next, out delta)) {
                        break;
                    }
                    DebugStub.Assert(delta == 1);
                    LockedBlockIndexAppend(index, next, delta);
                    newTailAllocated++;
                    SetNext(current, next);
                    current = next;
                }
                SetNext(current, EndOfChainMarker);

                return true;
            }

            // ----------------------------------------------------------------
            // Chain truncation and deletion related methods

            internal void TruncateChain(BlockIndex! index,
                                        int clusterLength)
                requires clusterLength >= 0 && clusterLength < index.Count;
            {
                if (clusterLength == index.Count) {
                    return;
                }

                if (clusterLength == 0) {
                    int headCluster;
                    if (index.Lookup(0, out headCluster) == false) {
                        assert false;
                    }
                    FreeChain(headCluster);
                    index.TruncateToLength(clusterLength);
                    return;
                }

                int newTail;
                if (index.Lookup(clusterLength - 1, out newTail) == false) {
                    assert false;
                }

                int zap;
                if (index.Lookup(clusterLength, out zap) == false) {
                    assert false;
                }

                lock (this) {
                    SetNext(newTail, EndOfChainMarker);
                    LockedFreeChain(zap);
                }
                index.TruncateToLength(clusterLength);
            }

            internal void FreeChain(int startCluster)
                requires (startCluster >= ReservedClusters &&
                          startCluster < MaxClusters);
            {
                lock (this) {
                    LockedFreeChain(startCluster);
                }
            }

            private void LockedFreeChain(int startCluster)
            {
                int nextCluster;
                do {
                    if (GetNext(startCluster, out nextCluster) == false) {
                        break;
                    }
                    bitmap.Free(startCluster, 1);
                    SetNext(startCluster, UnallocatedMarker);
                    startCluster = nextCluster;
                    DebugStub.Assert(startCluster >= ReservedClusters);
                } while (startCluster < MaxClusters);
            }

            // ----------------------------------------------------------------
            // Populate

            internal void PopulateIndex(BlockIndex! index,
                                        int firstCluster)
                requires firstCluster >= Fat12Internal.ReservedClusters;
                requires index.Count == 0;
            {
                int totalClusters = this.TotalClusters;
                int cluster = firstCluster;
                do {
                    index.Append(cluster);
                } while (GetNext(cluster, out cluster) != false &&
                         cluster >= ReservedClusters &&
                         cluster < totalClusters);

#if DEBUG_FAT12
                int lastCluster;
                index.Lookup(index.Count - 1, out lastCluster);
                int nextCluster;
                GetNextInFat(0, lastCluster, out nextCluster);
                DebugStub.Assert(nextCluster == EndOfChainMarker);
#endif // DEBUG_FAT12
            }

            // ----------------------------------------------------------------
            // Single link traversal and modification methods

            private void GetNextInFat(uint fat, int cluster, out int next)
            {
                int fatOffset    = cluster + cluster / 2;
                int sectorOffset = fatOffset / (int)bpbSummary.BytesPerSector;
                int entryOffset  = fatOffset % (int)bpbSummary.BytesPerSector;
                int sector       = (int)(bpbSummary.FirstFatSector +
                                         fat * bpbSummary.SectorsPerFat +
                                         sectorOffset);

                byte b0, b1;
                if (entryOffset == bpbSummary.BytesPerSector - 1) {
                    byte[]! in ExHeap blockData0 =
                        blockCache.BeginQuickBlockOperation((uint)sector);
                    b0 = blockData0[entryOffset];
                    blockCache.EndQuickBlockOperation((uint)sector,
                                                      blockData0, false);

                    byte[]! in ExHeap blockData1 =
                        blockCache.BeginQuickBlockOperation((uint)sector + 1);
                    b1 = blockData1[0];
                    blockCache.EndQuickBlockOperation((uint)sector + 1,
                                                      blockData1, false);
                }
                else {
                    byte[]! in ExHeap blockData =
                        blockCache.BeginQuickBlockOperation((uint)sector);
                    b0 = blockData[entryOffset];
                    b1 = blockData[entryOffset + 1];
                    blockCache.EndQuickBlockOperation((uint)sector,
                                                      blockData, false);
                }

                GetClusterValue(cluster, b0, b1, out next);

                // Fix the out value so even
                // non-MS formatted filesystems appear to show the
                // same EOC marker.  NB First two clusters are not used
                // and their next pointers may have special purposes.
                if (cluster >= ReservedClusters &&
                    next >= EndOfChainMinimum) {
                    next = EndOfChainMarker;
                }
            }

            private static void GetClusterValue(int cluster,
                                                byte b0,
                                                byte b1,
                                                out int next)
            {
                int tmp = (((int)b1) << 8) + (int)b0;
                if ((cluster & 0x1) == 0x1) {
                    tmp >>= 4;
                }
                else {
                    tmp &= 0xfff;
                }
                next = tmp;
            }

            private static void SetClusterValue(int     cluster,
                                                int     next,
                                                ref byte b0,
                                                ref byte b1)
            {
                DebugStub.Assert(cluster >= ReservedClusters &&
                                 cluster <= EndOfChainMarker &&
                                 next <= EndOfChainMarker);

                if ((cluster & 0x1) == 0x1) {
                    b0 = (byte)((b0 & 0xf) + ((next << 4) & 0xff));
                    b1 = (byte)((next >> 4) & 0xff);
                }
                else {
                    b0 = (byte)(next & 0xff);
                    b1 = (byte)((b1 & 0xf0) + (next >> 8));
                }
            }

            private void SetNextInFat(uint fat, int cluster, int next)
            {
                int fatOffset    = cluster + cluster / 2;
                int sectorOffset = fatOffset / (int)bpbSummary.BytesPerSector;
                int entryOffset  = fatOffset % (int)bpbSummary.BytesPerSector;
                int sector       = (int)(bpbSummary.FirstFatSector +
                                         fat * bpbSummary.SectorsPerFat +
                                         sectorOffset);

                if (entryOffset == bpbSummary.BytesPerSector - 1) {
                    byte dummy = 0;

                    byte[]! in ExHeap blockData0 =
                        blockCache.BeginQuickBlockOperation((uint)sector);

                    SetClusterValue(cluster, next,
                                    ref blockData0[entryOffset],
                                    ref dummy);

                    blockCache.EndQuickBlockOperation((uint)sector + 0,
                                                      blockData0, true);

                    byte[]! in ExHeap blockData1 =
                        blockCache.BeginQuickBlockOperation((uint)sector + 1);

                    SetClusterValue(cluster, next,
                                    ref dummy,
                                    ref blockData1[0]);

                    blockCache.EndQuickBlockOperation((uint)sector + 1,
                                                      blockData1, true);
                }
                else {
                    byte[]! in ExHeap blockData =
                        blockCache.BeginQuickBlockOperation((uint)sector);

                    SetClusterValue(cluster, next,
                                    ref blockData[entryOffset],
                                    ref blockData[entryOffset + 1]);

                    blockCache.EndQuickBlockOperation((uint)sector,
                                                      blockData, true);
                }

#if DEBUG_FAT12
                int got;
                GetNextInFat(fat, cluster, out got);
                DebugStub.Assert(got == next);
#endif // DEBUG_FAT12
            }

            private bool GetNext(int cluster, out int next)
            {
                DebugStub.Assert(cluster < TotalClusters);

                for (uint i = 0; i < bpbSummary.NumberOfFats; i++) {
                    try {
                        GetNextInFat(preferredFat, cluster, out next);
                        return true;
                    }
                    catch (Exception e) {
                        DebugStub.WriteLine("Caught {0}", __arglist(e.ToString()));
                    }
                    preferredFat = (preferredFat + 1) %
                        bpbSummary.NumberOfFats;
                }

                DebugStub.Break();
                next = 0;
                return false;
                // XXX: Fatal error could not read Fat entry
            }

            private void SetNext(int cluster, int next)
            {
                DebugStub.Assert(cluster < TotalClusters);
                DebugStub.Assert(
                    ((next < TotalClusters) ||
                     (next >= BadClusterMarker && next <= EndOfChainMarker) ||
                     (cluster < ReservedClusters))
                    );

                for (uint i = 0; i < bpbSummary.NumberOfFats; i++) {
                    SetNextInFat(i, cluster, next);
                }
            }
        } // class Fat12Internal
    } // class Fat12
}