using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace RBG_Server
{
    /// <summary>
    /// Stores data in either memory or disk, depending on the required behaviour of the system, namely, if the
    /// byte array exceeds the size of the Memory Limit (objects can be required to stay in RAM)
    /// </summary>
    public struct CachedByteArray : IDisposable
    {
        // Static event, so that every instance of CachedByteArray is affected by a change in memory limit
        private static event EventHandler MemoryLimitModifiedEvent;
        // Properties to keep track of the actual allocated cache amounts
        private static long memoryAllocationTotal = 0;
        private static long memoryAllocationRAM = 0;
        // static fields
        private static int memoryLimit = 1024 * 1024 * 1; // Start with a memory limit of 1 MB

        private static readonly string folderPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData), "Remote Board Game\\Cache Objects");
        // static properties
        public static long MemoryAllocationRAM { get { return memoryAllocationRAM; } }
        public static long MemoryAllocationTotal { get { return memoryAllocationTotal; } }
        public static long MemoryAllocationDisk { get { return memoryAllocationTotal - memoryAllocationRAM; } }

        private SemaphoreSlim accessSem;

        public bool IsInRam { get; private set; }

        // struct data
        private bool useOnlyRAM;

        private Stream dataStream;

        private string dataHashCode;
        public bool UseOnlyRAM { get => useOnlyRAM; set => useOnlyRAM = value; }
        // Properties from the array itself
        public long Length => dataStream.Length;
        /// <summary>
        /// Set the memory limit for keeping data in RAM, as opposed to storing on disk.
        /// Will raise events if modified, so all instances will be refactored to disk if appropriate
        /// </summary>
        public static int MemoryLimit
        {
            get
            {
                return memoryLimit;
            }
            set
            {
                memoryLimit = value;
                MemoryLimitModifiedEvent?.Invoke(null, EventArgs.Empty);
            }
        }

        public CachedByteArray(byte[] b) : this(b, false)
        {
        }

        public CachedByteArray(byte[] b, bool useOnlyRAM)
        {
            accessSem = new SemaphoreSlim(0,1);
            this.useOnlyRAM = useOnlyRAM;
            dataHashCode = GenerateDataHash(b);
            if (!useOnlyRAM && b.Length > MemoryLimit)
            {
                // Save to a file; load the data
                string path = Path.Combine(folderPath, dataHashCode);
                dataStream = CopyIntoFile(b, path);
                IsInRam = false;
            }
            else
            {
                dataStream = new MemoryStream(b);
                memoryAllocationRAM += dataStream.Length;
                IsInRam = true;
            }
            dataStream.Position = 0;
            memoryAllocationTotal += dataStream.Length;
            // Signal the semaphore is ready
            accessSem.Release();
            // Listen to changes in the memory limit
            MemoryLimitModifiedEvent += CachedByteArray_memoryLimitModifiedEvent;
        }
        /// <summary>
        /// Generates a SHA256 hash of the data in the array, and returns it in a safe string format
        /// </summary>
        /// <param name="byteData"></param>
        /// <returns></returns>
        private static string GenerateDataHash(byte[] byteData)
        {
            using SHA256 dataHash = SHA256.Create();
            byte[] hashCode = dataHash.ComputeHash(byteData);
            string generatedHashCode = "";
            foreach (byte hashByte in hashCode)
            {
                generatedHashCode += $"{hashByte:X2}";
            }
            return generatedHashCode;
        }
        /// <summary>
        /// Copies the given <see cref="byte[]"/> into the file at <paramref name="filePath"/>, opening the stream, and returning it
        /// </summary>
        /// <param name="byteData"></param>
        /// <param name="filePath"></param>
        /// <returns></returns>
        private static Stream CopyIntoFile(byte[] byteData, string filePath)
        {
            FileStream s = File.OpenWrite(filePath);
            foreach (byte b in byteData)
            {
                s.WriteByte(b);
            }
            return s;
        }

        private void CachedByteArray_memoryLimitModifiedEvent(object sender, EventArgs e)
        {
            if (!useOnlyRAM)
            {
                if (dataStream.Length > MemoryLimit && IsInRam)
                {
                    // Memory stream exceeds the limit; create an FS and copy
                    accessSem.Wait();
                    // Get the underlying byte[]
                    byte[] dataBytes = ((MemoryStream)dataStream).ToArray();
                    // Generate the hashcode from the data
                    dataHashCode = GenerateDataHash(dataBytes);
                    // Create the file, and copy the data
                    dataStream = CopyIntoFile(dataBytes, Path.Combine(folderPath, dataHashCode));
                    // Disregard the allocation from RAM
                    memoryAllocationRAM -= dataStream.Length;
                    accessSem.Release();
                }
                else if (dataStream.Length <= MemoryLimit && !IsInRam)
                {
                    // Data stream is no larger than the limit, but the file is saved on disk; load to RAM instead
                    // Obtain a lock on the stream
                    accessSem.Wait();
                    // Copy data to memory
                    dataStream.Position = 0;
                    MemoryStream ms = new();
                    dataStream.CopyTo(ms);
                    // Increase the amount noted in RAM
                    memoryAllocationRAM += ms.Length;
                    // Close the FileStream
                    dataStream.Close();
                    // Delete the file
                    DeleteFile(Path.Combine(folderPath, dataHashCode));
                    // Set the datastream to the memory location
                    dataStream = ms;
                    accessSem.Release();
                    // Check the loaded data matches the hashcode we already have
                    string loadedHashCode = GenerateDataHash(ms.ToArray());
                    if (loadedHashCode != dataHashCode)
                    {
                        throw new UnauthorizedAccessException("Data was modified");
                    }
                }
                // Otherwise, the data is already in the correct spot, so we don't need to do anything else
            }
        }
        private static void DeleteFile(string filePath)
        {
            try
            {
                File.Delete(filePath);
            }
            catch (Exception ex)
            {
                Debug.WriteLine(ex);
            }
        }
        public static implicit operator byte[](CachedByteArray c)
        {
            byte[] dataBytes = new byte[c.dataStream.Length];
            c.accessSem.Wait();
            if (c.IsInRam)
            {
                // We can use the underlying array from the MemoryStream
                dataBytes = (c.dataStream as MemoryStream).ToArray();
            }
            else
            {
                // Ensure we read from the beginning of the stream
                c.dataStream.Position = 0;
                c.dataStream.Read(dataBytes, 0, dataBytes.Length);
            }
            c.accessSem.Release();
            return dataBytes;
        }

        public static explicit operator CachedByteArray(byte[] b) => new CachedByteArray(b);
        /// <summary>
        /// Array index operator, allows the CachedByteArray to act more completely as a byte[]
        /// </summary>
        /// <param name="index"></param>
        /// <returns></returns>
        public byte this[int index]
        {
            get
            {
                int b;
                // Aquire the semaphore, so that the read is effectively atomic (ensures the read is performed on the expected data)
                accessSem.Wait();
                dataStream.Position = index;
                b = dataStream.ReadByte();
                accessSem.Release();
                if (b == -1)
                {
                    throw new IndexOutOfRangeException(index.ToString());
                }
                return (byte)b;
            }
        }

        public void Dispose()
        {
            accessSem.Wait();
            // Remove the event listener
            MemoryLimitModifiedEvent -= CachedByteArray_memoryLimitModifiedEvent;
            // Remove the allocation from the counter
            memoryAllocationTotal -= dataStream.Length;
            memoryAllocationRAM -= dataStream.Length;
            // If the FileStream is opened, close and dispose it
            dataStream?.Dispose();
            // If the file is on disk, remove it
            DeleteFile(Path.Combine(folderPath, dataHashCode));
            // Release all resources obtained by the semaphore
            accessSem.Dispose();
        }
    }
}