using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace AMWD.Common.Packing.Ar
{
	/// <summary>
	/// Reads UNIX ar (archive) files in the GNU format.
	/// </summary>
	public class ArReader
	{
		// Source: http://en.wikipedia.org/wiki/Ar_%28Unix%29

		private readonly Stream inStream;
		private readonly List<ArFileInfoExtended> files = new();
		private readonly long streamStartPosition;

		private static readonly DateTime unixEpoch = new(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);

		/// <summary>
		/// Initializes a new instance of the <see cref="ArReader"/> class.
		/// </summary>
		/// <param name="inStream">The stream to read the archive from.</param>
		public ArReader(Stream inStream)
		{
			if (!inStream.CanRead || !inStream.CanSeek)
				throw new ArgumentException("Stream not readable or seekable", nameof(inStream));

			streamStartPosition = inStream.Position;
			this.inStream = inStream;

			Initialize();
		}

		/// <summary>
		/// Returns a list with all filenames of the archive.
		/// </summary>
		public IEnumerable<string> GetFileList()
		{
			return files.Select(fi => fi.FileName).ToList();
		}

		/// <summary>
		/// Returns the file info of a specific file in the archive.
		/// </summary>
		/// <param name="fileName">The name of the specific file.</param>
		public ArFileInfo GetFileInfo(string fileName)
		{
			return files
				.Where(fi => fi.FileName == fileName)
				.Select(fi => new ArFileInfo
				{
					FileName = fi.FileName,
					FileSize = fi.FileSize,
					GroupId = fi.GroupId,
					Mode = fi.Mode,
					ModifyTime = fi.ModifyTime,
					UserId = fi.UserId
				})
				.FirstOrDefault();
		}

		/// <summary>
		/// Reads a file from the archive into a stream.
		/// </summary>
		/// <param name="fileName">The file name in the archive.</param>
		/// <param name="outStream">The output stream.</param>
		public void ReadFile(string fileName, Stream outStream)
		{
			if (!outStream.CanWrite)
				throw new ArgumentException("Stream not writable", nameof(outStream));

			var info = files.Where(fi => fi.FileName == fileName).FirstOrDefault();
			if (info == null)
				return;

			long bytesToRead = info.FileSize;
			byte[] buffer = new byte[1024 * 1024];

			inStream.Seek(info.DataPosition, SeekOrigin.Begin);
			while (bytesToRead > 0)
			{
				int readCount = (int)Math.Min(bytesToRead, buffer.Length);
				inStream.Read(buffer, 0, readCount);
				outStream.Write(buffer, 0, readCount);

				bytesToRead -= readCount;
			}
			inStream.Seek(streamStartPosition, SeekOrigin.Begin);
		}

		/// <summary>
		/// Reads a fie from the archive and saves it to disk.
		/// </summary>
		/// <param name="fileName">The file name in the archive.</param>
		/// <param name="destinationPath">The destination path on disk.</param>
		public void ReadFile(string fileName, string destinationPath)
		{
			var info = files.Where(fi => fi.FileName == fileName).FirstOrDefault();
			if (info == null)
				return;

			using (var fs = File.OpenWrite(destinationPath))
			{
				ReadFile(fileName, fs);
			}
			File.SetLastWriteTimeUtc(destinationPath, info.ModifyTime);
		}

		private void Initialize()
		{
			// Reset stream
			inStream.Seek(streamStartPosition, SeekOrigin.Begin);

			// Read header
			string header = ReadAsciiString(8);
			if (header != "!<arch>\n")
				throw new FormatException("The file stream is no archive");

			// Create file list
			while (inStream.Position < inStream.Length)
			{
				var info = ReadFileHeader();
				files.Add(info);

				// Move stream behind file content
				inStream.Seek(info.FileSize, SeekOrigin.Current);

				// Align to even offsets (padded with LF bytes)
				if (inStream.Position % 2 != 0)
					inStream.Seek(1, SeekOrigin.Current);
			}

			// Reset stream
			inStream.Seek(streamStartPosition, SeekOrigin.Begin);
		}

		private string ReadAsciiString(int byteCount)
		{
			byte[] buffer = new byte[byteCount];
			inStream.Read(buffer, 0, byteCount);
			return Encoding.ASCII.GetString(buffer);
		}

		private ArFileInfoExtended ReadFileHeader()
		{
			long startPosition = inStream.Position;

			string fileName = ReadAsciiString(16).Trim();

			int.TryParse(ReadAsciiString(12).Trim(), out int unixTimestamp);
			int.TryParse(ReadAsciiString(6).Trim(), out int userId);
			int.TryParse(ReadAsciiString(6).Trim(), out int groupId);
			int mode = Convert.ToInt32(ReadAsciiString(8).Trim(), 8);

			long.TryParse(ReadAsciiString(10).Trim(), out long fileSize);

			// file magic
			byte[] magic = new byte[2];
			inStream.Read(magic, 0, magic.Length);

			if (magic[0] != 0x60 || magic[1] != 0x0A) // `\n
				throw new FormatException("Invalid file magic");

			return new ArFileInfoExtended
			{
				HeaderPosition = startPosition,
				DataPosition = inStream.Position,
				FileName = fileName,
				ModifyTime = unixEpoch.AddSeconds(unixTimestamp),
				UserId = userId,
				GroupId = groupId,
				Mode = mode,
				FileSize = fileSize
			};
		}
	}
}