#define COSMOSDEBUG

using System;
using System.Collections.Generic;

using Cosmos.Common.Extensions;
using Cosmos.HAL.BlockDevice;
using Cosmos.System.FileSystem.FAT.Listing;
using Cosmos.System.FileSystem.Listing;

namespace Cosmos.System.FileSystem.FAT
{
    internal class FatFileSystem : FileSystem
    {
        internal class Fat
        {

            private readonly FatFileSystem mFileSystem;

            private readonly ulong mFirstSector;

            /// <summary>
            /// Initializes a new instance of the <see cref="Fat"/> class.
            /// </summary>
            /// <param name="aFileSystem">The file system.</param>
            /// <param name="aFirstSector">Tje first sector of the FAT table.</param>
            public Fat(FatFileSystem aFileSystem, ulong aFirstSector)
            {
                if (aFileSystem == null)
                {
                    throw new ArgumentNullException(nameof(aFileSystem));
                }

                mFileSystem = aFileSystem;
                mFirstSector = aFirstSector;
            }

            /// <summary>
            /// Gets the size of a fat entry in bytes.
            /// </summary>
            /// <returns>The size of a fat entry in bytes.</returns>
            /// <exception cref="NotSupportedException">Can not get the FAT entry size for an unknown FAT type.</exception>
            private uint GetFatEntrySizeInBytes()
            {
                switch (mFileSystem.mFatType)
                {
                    case FatTypeEnum.Fat32:
                        return 4;
                    case FatTypeEnum.Fat16:
                        return 2;
                    case FatTypeEnum.Fat12:
                        // TODO:
                        break;
                }

                throw new NotSupportedException("Can not get the FAT entry size for an unknown FAT type.");
            }

            /// <summary>
            /// Gets the fat chain.
            /// </summary>
            /// <param name="aFirstEntry">The first entry.</param>
            /// <param name="aDataSize">Size of a data to be stored in bytes.</param>
            /// <returns>An array of cluster numbers for the fat chain.</returns>
            /// <exception cref="ArgumentOutOfRangeException"></exception>
            public ulong[] GetFatChain(ulong aFirstEntry, uint aDataSize = 0)
            {
                Global.mFileSystemDebugger.SendInternal("-- Fat.GetFatChain --");
                Global.mFileSystemDebugger.SendInternal("aFirstEntry =");
                Global.mFileSystemDebugger.SendInternal(aFirstEntry);
                Global.mFileSystemDebugger.SendInternal("aDataSize =");
                Global.mFileSystemDebugger.SendInternal(aDataSize);

                var xReturn = new ulong[0];
                ulong xCurrentEntry = aFirstEntry;
                ulong xValue;

                uint xEntriesRequired = aDataSize / mFileSystem.BytesPerCluster;
                if (aDataSize % mFileSystem.BytesPerCluster != 0)
                {
                    xEntriesRequired++;
                }

                GetFatEntry(xCurrentEntry, out xValue);
                Array.Resize(ref xReturn, xReturn.Length + 1);
                xReturn[xReturn.Length - 1] = xCurrentEntry;

                while (!FatEntryIsEof(xValue))
                {
                    xCurrentEntry = xValue;
                    GetFatEntry(xCurrentEntry, out xValue);
                    Array.Resize(ref xReturn, xReturn.Length + 1);
                    if (!FatEntryIsEof(xValue))
                    {
                        xReturn[xReturn.Length - 1] = xValue;
                    }
                    else
                    {
                        xReturn[xReturn.Length - 1] = xCurrentEntry;
                    }
                }

                if (xEntriesRequired > xReturn.Length)
                {
                    ulong xNewClusters = (uint)xReturn.Length - xEntriesRequired;
                    for (ulong i = 0; i < xNewClusters; i++)
                    {
                        xCurrentEntry = GetNextUnallocatedFatEntry();
                        ulong xLastFatEntry = xReturn[xReturn.Length - 1];
                        SetFatEntry(xLastFatEntry, xCurrentEntry);
                        SetFatEntry(xCurrentEntry, FatEntryEofValue());
                        Array.Resize(ref xReturn, xReturn.Length + 1);
                        xReturn[xReturn.Length - 1] = xCurrentEntry;
                    }
                }

                return xReturn;
            }

            /// <summary>
            /// Gets the next unallocated fat entry.
            /// </summary>
            /// <returns>The index of the next unallocated FAT entry.</returns>
            /// <exception cref="Exception">Failed to find an unallocated FAT entry.</exception>
            public uint GetNextUnallocatedFatEntry()
            {
                Global.mFileSystemDebugger.SendInternal("-- Fat.GetNextUnallocatedFatEntry --");

                uint xEntryNumber = 0;
                uint xEntrySize = GetFatEntrySizeInBytes();

                for (uint i = 0; i < mFileSystem.FatSectorCount; i++)
                {
                    byte[] xSector;
                    ReadFatTableSector(i, out xSector);
                    for (uint j = 0; j < xSector.Length; j += xEntrySize)
                    {
                        uint xEntryValue = xSector.ToUInt32(j);
                        if (!FatEntryIsEof(xEntryValue))
                        {
                            Global.mFileSystemDebugger.SendInternal("xEntryNumber =");
                            Global.mFileSystemDebugger.SendInternal(xEntryNumber);
                            Global.mFileSystemDebugger.SendInternal("xEntryValue =");
                            Global.mFileSystemDebugger.SendInternal(xEntryValue);
                            Global.mFileSystemDebugger.SendInternal("Offset =");
                            Global.mFileSystemDebugger.SendInternal(j);
                            return xEntryNumber;
                        }
                        xEntryNumber++;
                    }
                }

                // TODO: What should we return if no available entry is found.
                throw new Exception("Failed to find an unallocated FAT entry.");
            }

            /// <summary>
            /// Reads the fat table sector.
            /// </summary>
            /// <param name="xSectorNum">The sector number.</param>
            /// <param name="aData">The sector data.</param>
            private void ReadFatTableSector(ulong xSectorNum, out byte[] aData)
            {
                Global.mFileSystemDebugger.SendInternal("-- Fat.ReadFatTableSector --");
                Global.mFileSystemDebugger.SendInternal("xSectorNum =");
                Global.mFileSystemDebugger.SendInternal(xSectorNum);

                ulong xSectorToRead = mFirstSector + xSectorNum;
                aData = mFileSystem.NewSectorArray();
                mFileSystem.mDevice.ReadBlock(xSectorToRead, 1, aData);
            }

            /// <summary>
            /// Writes the fat table sector.
            /// </summary>
            /// <param name="xSectorNum">The sector number.</param>
            /// <param name="aData">The data to write.</param>
            private void WriteFatTableSector(ulong xSectorNum, byte[] aData)
            {
                Global.mFileSystemDebugger.SendInternal("-- Fat.WriteFatTableSector --");
                Global.mFileSystemDebugger.SendInternal("xSectorNum =");
                Global.mFileSystemDebugger.SendInternal(xSectorNum);

                if (aData == null)
                {
                    throw new ArgumentNullException(nameof(aData));
                }

                ulong xSectorToWrite = mFirstSector + xSectorNum;
                mFileSystem.mDevice.WriteBlock(xSectorToWrite, 1, aData);
            }

            /// <summary>
            /// Gets a fat entry.
            /// </summary>
            /// <param name="aEntryNumber">The entry number.</param>
            /// <param name="aValue">The entry value.</param>
            /// <exception cref="Exception">Unknown file system type.</exception>
            private void GetFatEntry(ulong aEntryNumber, out ulong aValue)
            {
                Global.mFileSystemDebugger.SendInternal("-- Fat.GetFatEntry --");
                Global.mFileSystemDebugger.SendInternal("aEntryNumber =");
                Global.mFileSystemDebugger.SendInternal(aEntryNumber);

                uint xEntrySize = GetFatEntrySizeInBytes();
                ulong xTotalOffset = aEntryNumber * xEntrySize;
                ulong xSectorNumber = xTotalOffset / mFileSystem.BytesPerSector;
                ulong xSectorOffset = xTotalOffset - xSectorNumber * mFileSystem.BytesPerSector;
                byte[] xSector;

                ReadFatTableSector(xSectorNumber, out xSector);
                switch (mFileSystem.mFatType)
                {
                    case FatTypeEnum.Fat12:
                        // We now access the FAT entry as a WORD just as we do for FAT16, but if the cluster number is
                        // EVEN, we only want the low 12-bits of the 16-bits we fetch. If the cluster number is ODD
                        // we want the high 12-bits of the 16-bits we fetch.
                        uint xResult = xSector.ToUInt16(xSectorOffset);
                        if ((aEntryNumber & 0x01) == 0)
                        {
                            aValue = xResult & 0x0FFF; // Even
                        }
                        else
                        {
                            aValue = xResult >> 4; // Odd
                        }
                        break;
                    case FatTypeEnum.Fat16:
                        aValue = xSector.ToUInt16(xSectorOffset);
                        break;
                    case FatTypeEnum.Fat32:
                        aValue = xSector.ToUInt32(xSectorOffset) & 0x0FFFFFFF;
                        break;
                    default:
                        throw new Exception("Unknown file system type.");
                }
            }

            /// <summary>
            /// Sets a fat entry.
            /// </summary>
            /// <param name="aClusterNum">The cluster number.</param>
            /// <param name="aValue">The value.</param>
            /// <exception cref="Exception">TODO: Sector Span</exception>
            private void SetFatEntry(ulong aClusterNum, ulong aValue)
            {
                Global.mFileSystemDebugger.SendInternal("-- Fat.SetFatEntry --");
                Global.mFileSystemDebugger.SendInternal("aClusterNum =");
                Global.mFileSystemDebugger.SendInternal(aClusterNum);
                Global.mFileSystemDebugger.SendInternal("aValue =");
                Global.mFileSystemDebugger.SendInternal(aValue);

                ulong xOffset = aClusterNum * 8;
                ulong xSectorNumber = xOffset / mFileSystem.BytesPerSector;
                ulong xSectorOffset = xSectorNumber * mFileSystem.BytesPerSector - xOffset;
                byte[] xSector;

                ReadFatTableSector(xSectorNumber, out xSector);
                switch (mFileSystem.mFatType)
                {
                    case FatTypeEnum.Fat12:
                        if (xOffset == mFileSystem.BytesPerSector - 1)
                        {
#warning TODO: Sector Span
                            throw new Exception("TODO: Sector Span");
                            /* This cluster access spans a sector boundary in the FAT */
                            /* There are a number of strategies to handling this. The */
                            /* easiest is to always load FAT sectors into memory */
                            /* in pairs if the volume is FAT12 (if you want to load */
                            /* FAT sector N, you also load FAT sector N+1 immediately */
                            /* following it in memory unless sector N is the last FAT */
                            /* sector). It is assumed that this is the strategy used here */
                            /* which makes this if test for a sector boundary span */
                            /* unnecessary. */
                        }
                        // We now access the FAT entry as a WORD just as we do for FAT16, but if the cluster number is
                        // EVEN, we only want the low 12-bits of the 16-bits we fetch. If the cluster number is ODD
                        // we want the high 12-bits of the 16-bits we fetch.
                        xSector.SetUInt16(xSectorOffset, (ushort)aValue);
                        break;
                    case FatTypeEnum.Fat16:
                        xSector.SetUInt16(xSectorOffset, (ushort)aValue);
                        break;
                    default:
                        xSector.SetUInt32(xSectorOffset, (uint)aValue);
                        break;
                }

                WriteFatTableSector(xSectorNumber, xSector);
            }

            /// <summary>
            /// Is the FAT entry EOF?
            /// </summary>
            /// <param name="aValue">The entry index.</param>
            /// <returns></returns>
            /// <exception cref="Exception">Unknown file system type.</exception>
            private bool FatEntryIsEof(ulong aValue)
            {
                switch (mFileSystem.mFatType)
                {
                    case FatTypeEnum.Fat12:
                        return aValue >= 0xFF8;
                    case FatTypeEnum.Fat16:
                        return aValue >= 0xFFF8;
                    case FatTypeEnum.Fat32:
                        return aValue >= 0xFFFFFF8;
                    default:
                        throw new Exception("Unknown file system type.");
                }
            }

            /// <summary>
            /// The the EOF value for a specific FAT type.
            /// </summary>
            /// <returns>The EOF value.</returns>
            /// <exception cref="Exception">Unknown file system type.</exception>
            private ulong FatEntryEofValue()
            {
                switch (mFileSystem.mFatType)
                {
                    case FatTypeEnum.Fat12:
                        return 0x0FFF;
                    case FatTypeEnum.Fat16:
                        return 0xFFFF;
                    case FatTypeEnum.Fat32:
                        return 0x0FFFFFFF;
                    default:
                        throw new Exception("Unknown file system type.");
                }
            }
        }

        public readonly uint BytesPerCluster;

        public readonly uint BytesPerSector;

        public readonly uint ClusterCount;

        public readonly uint DataSector; // First Data Sector

        public readonly uint DataSectorCount;

        public readonly uint FatSectorCount;

        private readonly FatTypeEnum mFatType;

        public readonly uint NumberOfFATs;

        public readonly uint ReservedSectorCount;

        public readonly uint RootCluster; // FAT32

        public readonly uint RootEntryCount;

        public readonly uint RootSector; // FAT12/16

        public readonly uint RootSectorCount; // FAT12/16, FAT32 remains 0

        public readonly uint SectorsPerCluster;

        public readonly uint TotalSectorCount;

        private readonly Fat[] mFats;

        /// <summary>
        /// Initializes a new instance of the <see cref="FatFileSystem"/> class.
        /// </summary>
        /// <param name="aDevice">The partition.</param>
        /// <param name="aRootPath">The root path.</param>
        /// <exception cref="Exception">FAT signature not found.</exception>
        public FatFileSystem(Partition aDevice, string aRootPath)
            : base(aDevice, aRootPath)
        {
            if (aDevice == null)
            {
                throw new ArgumentNullException(nameof(aDevice));
            }

            if (string.IsNullOrEmpty(aRootPath))
            {
                throw new ArgumentException("Argument is null or empty", nameof(aRootPath));
            }

            var xBPB = mDevice.NewBlockArray(1);

            mDevice.ReadBlock(0UL, 1U, xBPB);

            ushort xSig = xBPB.ToUInt16(510);
            if (xSig != 0xAA55)
            {
                throw new Exception("FAT signature not found.");
            }

            BytesPerSector = xBPB.ToUInt16(11);
            SectorsPerCluster = xBPB[13];
            BytesPerCluster = BytesPerSector * SectorsPerCluster;
            ReservedSectorCount = xBPB.ToUInt16(14);
            NumberOfFATs = xBPB[16];
            RootEntryCount = xBPB.ToUInt16(17);

            TotalSectorCount = xBPB.ToUInt16(19);
            if (TotalSectorCount == 0)
            {
                TotalSectorCount = xBPB.ToUInt32(32);
            }

            // FATSz
            FatSectorCount = xBPB.ToUInt16(22);
            if (FatSectorCount == 0)
            {
                FatSectorCount = xBPB.ToUInt32(36);
            }

            DataSectorCount = TotalSectorCount -
                              (ReservedSectorCount + NumberOfFATs * FatSectorCount + ReservedSectorCount);

            // Computation rounds down.
            ClusterCount = DataSectorCount / SectorsPerCluster;
            // Determine the FAT type. Do not use another method - this IS the official and
            // proper way to determine FAT type.
            // Comparisons are purposefully < and not <=
            // FAT16 starts at 4085, FAT32 starts at 65525
            if (ClusterCount < 4085)
            {
                mFatType = FatTypeEnum.Fat12;
            }
            else if (ClusterCount < 65525)
            {
                mFatType = FatTypeEnum.Fat16;
            }
            else
            {
                mFatType = FatTypeEnum.Fat32;
            }

            if (mFatType == FatTypeEnum.Fat32)
            {
                RootCluster = xBPB.ToUInt32(44);
            }
            else
            {
                RootSector = ReservedSectorCount + NumberOfFATs * FatSectorCount;
                RootSectorCount = (RootEntryCount * 32 + (BytesPerSector - 1)) / BytesPerSector;
            }
            DataSector = ReservedSectorCount + NumberOfFATs * FatSectorCount + RootSectorCount;

            mFats = new Fat[NumberOfFATs];
            for (ulong i = 0; i < NumberOfFATs; i++)
            {
                mFats[i] = new Fat(this, (ReservedSectorCount + i * FatSectorCount));
            }
        }

        internal Fat GetFat(int aTableNumber)
        {
            if (mFats.Length > aTableNumber)
            {
                return mFats[aTableNumber];
            }

            throw new Exception("The fat table number doesn't exist.");
        }

        internal byte[] NewClusterArray()
        {
            return new byte[BytesPerCluster];
        }

        internal byte[] NewSectorArray()
        {
            return new byte[BytesPerSector];
        }

        private void ReadInternal(ulong aFirstCluster, out byte[] aData)
        {
            Global.mFileSystemDebugger.SendInternal("-- FatFileSystem.ReadInternal --");
            Global.mFileSystemDebugger.SendInternal("aFirstCluster =");
            Global.mFileSystemDebugger.SendInternal(aFirstCluster);

            if (mFatType == FatTypeEnum.Fat32)
            {
                aData = NewClusterArray();
                ulong xSector = DataSector + (aFirstCluster - 2) * SectorsPerCluster;
                mDevice.ReadBlock(xSector, SectorsPerCluster, aData);
            }
            else
            {
                aData = mDevice.NewBlockArray(1);
                mDevice.ReadBlock(aFirstCluster, RootSectorCount, aData);
            }
        }

        private void WriteInternal(ulong aFirstCluster, byte[] aData)
        {
            Global.mFileSystemDebugger.SendInternal("-- FatFileSystem.WriteInternal --");
            Global.mFileSystemDebugger.SendInternal("aFirstCluster =");
            Global.mFileSystemDebugger.SendInternal(aFirstCluster);

            if (aData == null)
            {
                throw new ArgumentNullException(nameof(aData));
            }

            if (mFatType == FatTypeEnum.Fat32)
            {
                ulong xSector = DataSector + (aFirstCluster - 2) * SectorsPerCluster;
                mDevice.WriteBlock(xSector, SectorsPerCluster, aData);
            }
            else
            {
                mDevice.WriteBlock(aFirstCluster, RootSectorCount, aData);
            }
        }

        internal void Read(ulong aFirstCluster, out byte[] aData, ulong aSize = 0, ulong aOffset = 0)
        {
            Global.mFileSystemDebugger.SendInternal("-- FatFileSystem.Read --");
            Global.mFileSystemDebugger.SendInternal("aFirstCluster =");
            Global.mFileSystemDebugger.SendInternal(aFirstCluster);

            if (aSize == 0)
            {
                aSize = BytesPerCluster;
            }

            if (aSize > BytesPerCluster - aOffset)
            {
                throw new NotImplementedException("TODO: Add cluster spanning read.");
            }

            aData = new byte[aSize];
            byte[] xTempData;
            ReadInternal(aFirstCluster, out xTempData);
            Array.Copy(xTempData, (long)aOffset, aData, 0, (long)aSize);
        }

        internal void Write(ulong aFirstCluster, byte[] aData, ulong aSize = 0, ulong aOffset = 0)
        {
            Global.mFileSystemDebugger.SendInternal("-- FatFileSystem.Write --");
            Global.mFileSystemDebugger.SendInternal("aFirstCluster =");
            Global.mFileSystemDebugger.SendInternal(aFirstCluster);

            if (aSize == 0)
            {
                aSize = BytesPerCluster;
            }

            if (aSize > BytesPerCluster - aOffset)
            {
                throw new NotImplementedException("TODO: Add cluster spanning write.");
            }

            byte[] xTempData;
            ReadInternal(aFirstCluster, out xTempData);
            Array.Copy(aData, (long)aOffset, xTempData, 0, (long)aSize);
            WriteInternal(aFirstCluster, aData);
        }

        public static bool IsDeviceFat(Partition aDevice)
        {
            if (aDevice == null)
            {
                throw new ArgumentNullException(nameof(aDevice));
            }

            var xBPB = aDevice.NewBlockArray(1);
            aDevice.ReadBlock(0UL, 1U, xBPB);
            ushort xSig = xBPB.ToUInt16(510);
            if (xSig != 0xAA55)
            {
                return false;
            }
            return true;
        }

        public override void DisplayFileSystemInfo()
        {
            global::System.Console.WriteLine("-------File System--------");
            global::System.Console.WriteLine("Bytes per Cluster     = " + BytesPerCluster);
            global::System.Console.WriteLine("Bytes per Sector      = " + BytesPerSector);
            global::System.Console.WriteLine("Cluster Count         = " + ClusterCount);
            global::System.Console.WriteLine("Data Sector           = " + DataSector);
            global::System.Console.WriteLine("Data Sector Count     = " + DataSectorCount);
            global::System.Console.WriteLine("FAT Sector Count      = " + FatSectorCount);
            global::System.Console.WriteLine("FAT Type              = " + (uint)mFatType);
            global::System.Console.WriteLine("Number of FATS        = " + NumberOfFATs);
            global::System.Console.WriteLine("Reserved Sector Count = " + ReservedSectorCount);
            global::System.Console.WriteLine("Root Cluster          = " + RootCluster);
            global::System.Console.WriteLine("Root Entry Count      = " + RootEntryCount);
            global::System.Console.WriteLine("Root Sector           = " + RootSector);
            global::System.Console.WriteLine("Root Sector Count     = " + RootSectorCount);
            global::System.Console.WriteLine("Sectors per Cluster   = " + SectorsPerCluster);
            global::System.Console.WriteLine("Total Sector Count    = " + TotalSectorCount);

            Global.mFileSystemDebugger.SendInternal("Bytes per Cluster =");
            Global.mFileSystemDebugger.SendInternal(BytesPerCluster);
            Global.mFileSystemDebugger.SendInternal("Bytes per Sector =");
            Global.mFileSystemDebugger.SendInternal(BytesPerSector);
            Global.mFileSystemDebugger.SendInternal("Cluster Count =");
            Global.mFileSystemDebugger.SendInternal(ClusterCount);
            Global.mFileSystemDebugger.SendInternal("Data Sector =");
            Global.mFileSystemDebugger.SendInternal(DataSector);
            Global.mFileSystemDebugger.SendInternal("Data Sector Count =");
            Global.mFileSystemDebugger.SendInternal(DataSectorCount);
            Global.mFileSystemDebugger.SendInternal("FAT Sector Count =");
            Global.mFileSystemDebugger.SendInternal(FatSectorCount);
            Global.mFileSystemDebugger.SendInternal("FAT Type =");
            Global.mFileSystemDebugger.SendInternal((uint)mFatType);
            Global.mFileSystemDebugger.SendInternal("Number of FATS =");
            Global.mFileSystemDebugger.SendInternal(NumberOfFATs);
            Global.mFileSystemDebugger.SendInternal("Reserved Sector Count =");
            Global.mFileSystemDebugger.SendInternal(ReservedSectorCount);
            Global.mFileSystemDebugger.SendInternal("Root Cluster =");
            Global.mFileSystemDebugger.SendInternal(RootCluster);
            Global.mFileSystemDebugger.SendInternal("Root Entry Count =");
            Global.mFileSystemDebugger.SendInternal(RootEntryCount);
            Global.mFileSystemDebugger.SendInternal("Root Sector =");
            Global.mFileSystemDebugger.SendInternal(RootSector);
            Global.mFileSystemDebugger.SendInternal("Root Sector Count =");
            Global.mFileSystemDebugger.SendInternal(RootSectorCount);
            Global.mFileSystemDebugger.SendInternal("Sectors per Cluster =");
            Global.mFileSystemDebugger.SendInternal(SectorsPerCluster);
            Global.mFileSystemDebugger.SendInternal("Total Sector Count =");
            Global.mFileSystemDebugger.SendInternal(TotalSectorCount);
        }

        public override List<DirectoryEntry> GetDirectoryListing(DirectoryEntry baseDirectory)
        {
            Global.mFileSystemDebugger.SendInternal("-- FatFileSystem.GetDirectoryListing --");

            if (baseDirectory == null)
            {
                throw new ArgumentNullException(nameof(baseDirectory));
            }

            var result = new List<DirectoryEntry>();
            var xEntry = (FatDirectoryEntry)baseDirectory;
            var fatListing = xEntry.ReadDirectoryContents();

            for (int i = 0; i < fatListing.Count; i++)
            {
                result.Add(fatListing[i]);
            }
            return result;
        }

        public override DirectoryEntry GetRootDirectory()
        {
            Global.mFileSystemDebugger.SendInternal("-- FatFileSystem.GetRootDirectory --");

            var xRootEntry = new FatDirectoryEntry(this, null, mRootPath, mRootPath, RootCluster);
            return xRootEntry;
        }

        public override DirectoryEntry CreateDirectory(DirectoryEntry aParentDirectory, string aNewDirectory)
        {
            Global.mFileSystemDebugger.SendInternal("-- FatFileSystem.CreateDirectory --");
            Global.mFileSystemDebugger.SendInternal("aParentDirectory.Name =");
            Global.mFileSystemDebugger.SendInternal(aParentDirectory?.mName);
            Global.mFileSystemDebugger.SendInternal("aNewDirectory =");
            Global.mFileSystemDebugger.SendInternal(aNewDirectory);

            if (aParentDirectory == null)
            {
                throw new ArgumentNullException(nameof(aParentDirectory));
            }

            if (string.IsNullOrEmpty(aNewDirectory))
            {
                throw new ArgumentNullException(nameof(aNewDirectory));
            }

            var xParentDirectory = (FatDirectoryEntry)aParentDirectory;
            var xDirectoryEntryToAdd = xParentDirectory.AddDirectoryEntry(aNewDirectory, DirectoryEntryTypeEnum.Directory);
            return xDirectoryEntryToAdd;
        }

        public override DirectoryEntry CreateFile(DirectoryEntry aParentDirectory, string aNewFile)
        {
            Global.mFileSystemDebugger.SendInternal("-- FatFileSystem.CreateFile --");
            Global.mFileSystemDebugger.SendInternal("aParentDirectory.Name =");
            Global.mFileSystemDebugger.SendInternal(aParentDirectory?.mName);
            Global.mFileSystemDebugger.SendInternal("aNewFile =");
            Global.mFileSystemDebugger.SendInternal(aNewFile);

            if (aParentDirectory == null)
            {
                throw new ArgumentNullException(nameof(aParentDirectory));
            }

            if (string.IsNullOrEmpty(aNewFile))
            {
                throw new ArgumentNullException(nameof(aNewFile));
            }

            var xParentDirectory = (FatDirectoryEntry)aParentDirectory;
            var xDirectoryEntryToAdd = xParentDirectory.AddDirectoryEntry(aNewFile, DirectoryEntryTypeEnum.File);
            return xDirectoryEntryToAdd;
        }

        private enum FatTypeEnum
        {
            Unknown,

            Fat12,

            Fat16,

            Fat32
        }
    }
}