using System;
using System.IO;
using System.Runtime.InteropServices;

using Microsoft.Win32.SafeHandles;

namespace Server
{
	public static class FileOperations
	{
		public const int KB = 1024;
		public const int MB = 1024 * KB;

		private const FileOptions NoBuffering = (FileOptions)0x20000000;

		[DllImport("Kernel32", CharSet = CharSet.Auto, SetLastError = true)]
		private static extern SafeFileHandle CreateFile(string lpFileName, int dwDesiredAccess, FileShare dwShareMode, IntPtr securityAttrs, FileMode dwCreationDisposition, int dwFlagsAndAttributes, IntPtr hTemplateFile);

		private static int bufferSize = 1 * MB;
		private static int concurrency = 1;

		private static bool unbuffered = true;

		public static int BufferSize
		{
			get => bufferSize;
			set => bufferSize = value;
		}

		public static int Concurrency
		{
			get => concurrency;
			set => concurrency = value;
		}

		public static bool Unbuffered
		{
			get => unbuffered;
			set => unbuffered = value;
		}

		public static bool AreSynchronous => concurrency < 1;

		public static bool AreAsynchronous => concurrency > 0;

		public static FileStream OpenSequentialStream(string path, FileMode mode, FileAccess access, FileShare share)
		{
			var options = FileOptions.SequentialScan;

			if (concurrency > 0)
			{
				options |= FileOptions.Asynchronous;
			}

			if (unbuffered)
			{
				options |= NoBuffering;
			}
			else
			{
				return new FileStream(path, mode, access, share, bufferSize, options);
			}

			var fileHandle = CreateFile(path, (int)access, share, IntPtr.Zero, mode, (int)options, IntPtr.Zero);

			if (fileHandle.IsInvalid)
			{
				throw new IOException();
			}

			return new UnbufferedFileStream(fileHandle, access, bufferSize, concurrency > 0);
		}

		private class UnbufferedFileStream : FileStream
		{
			private readonly SafeFileHandle fileHandle;

			public UnbufferedFileStream(SafeFileHandle fileHandle, FileAccess access, int bufferSize, bool isAsync)
				: base(fileHandle, access, bufferSize, isAsync)
			{
				this.fileHandle = fileHandle;
			}

			public override void Write(byte[] array, int offset, int count)
			{
				base.Write(array, offset, bufferSize);
			}

			public override IAsyncResult BeginWrite(byte[] array, int offset, int numBytes, AsyncCallback userCallback, object stateObject)
			{
				return base.BeginWrite(array, offset, bufferSize, userCallback, stateObject);
			}

			protected override void Dispose(bool disposing)
			{
				if (!fileHandle.IsClosed)
				{
					fileHandle.Close();
				}

				base.Dispose(disposing);
			}
		}
	}
}