// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)

using System;
using System.IO;
using System.Text;

namespace ICSharpCode.AvalonEdit.Utils
{
	/// <summary>
	/// Class that can open text files with auto-detection of the encoding.
	/// </summary>
	public static class FileReader
	{
		/// <summary>
		/// Gets if the given encoding is a Unicode encoding (UTF).
		/// </summary>
		/// <remarks>
		/// Returns true for UTF-7, UTF-8, UTF-16 LE, UTF-16 BE, UTF-32 LE and UTF-32 BE.
		/// Returns false for all other encodings.
		/// </remarks>
		public static bool IsUnicode(Encoding encoding)
		{
			if (encoding == null)
				throw new ArgumentNullException("encoding");
			switch (encoding.CodePage) {
				case 65000: // UTF-7
				case 65001: // UTF-8
				case 1200: // UTF-16 LE
				case 1201: // UTF-16 BE
				case 12000: // UTF-32 LE
				case 12001: // UTF-32 BE
					return true;
				default:
					return false;
			}
		}
		
		/// <summary>
		/// Reads the content of the given stream.
		/// </summary>
		/// <param name="stream">The stream to read.
		/// The stream must support seeking and must be positioned at its beginning.</param>
		/// <param name="defaultEncoding">The encoding to use if the encoding cannot be auto-detected.</param>
		/// <returns>The file content as string.</returns>
		public static string ReadFileContent(Stream stream, Encoding defaultEncoding)
		{
			using (StreamReader reader = OpenStream(stream, defaultEncoding)) {
				return reader.ReadToEnd();
			}
		}
		
		/// <summary>
		/// Reads the content of the file.
		/// </summary>
		/// <param name="fileName">The file name.</param>
		/// <param name="defaultEncoding">The encoding to use if the encoding cannot be auto-detected.</param>
		/// <returns>The file content as string.</returns>
		public static string ReadFileContent(string fileName, Encoding defaultEncoding)
		{
			using (FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read)) {
				return ReadFileContent(fs, defaultEncoding);
			}
		}
		
		/// <summary>
		/// Opens the specified file for reading.
		/// </summary>
		/// <param name="fileName">The file to open.</param>
		/// <param name="defaultEncoding">The encoding to use if the encoding cannot be auto-detected.</param>
		/// <returns>Returns a StreamReader that reads from the stream. Use
		/// <see cref="StreamReader.CurrentEncoding"/> to get the encoding that was used.</returns>
		public static StreamReader OpenFile(string fileName, Encoding defaultEncoding)
		{
			if (fileName == null)
				throw new ArgumentNullException("fileName");
			FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read);
			try {
				return OpenStream(fs, defaultEncoding);
				// don't use finally: the stream must be kept open until the StreamReader closes it
			} catch {
				fs.Dispose();
				throw;
			}
		}
		
		/// <summary>
		/// Opens the specified stream for reading.
		/// </summary>
		/// <param name="stream">The stream to open.</param>
		/// <param name="defaultEncoding">The encoding to use if the encoding cannot be auto-detected.</param>
		/// <returns>Returns a StreamReader that reads from the stream. Use
		/// <see cref="StreamReader.CurrentEncoding"/> to get the encoding that was used.</returns>
		public static StreamReader OpenStream(Stream stream, Encoding defaultEncoding)
		{
			if (stream == null)
				throw new ArgumentNullException("stream");
			if (stream.Position != 0)
				throw new ArgumentException("stream is not positioned at beginning.", "stream");
			if (defaultEncoding == null)
				throw new ArgumentNullException("defaultEncoding");
			
			if (stream.Length >= 2) {
				// the autodetection of StreamReader is not capable of detecting the difference
				// between ISO-8859-1 and UTF-8 without BOM.
				int firstByte = stream.ReadByte();
				int secondByte = stream.ReadByte();
				switch ((firstByte << 8) | secondByte) {
					case 0x0000: // either UTF-32 Big Endian or a binary file; use StreamReader
					case 0xfffe: // Unicode BOM (UTF-16 LE or UTF-32 LE)
					case 0xfeff: // UTF-16 BE BOM
					case 0xefbb: // start of UTF-8 BOM
						// StreamReader autodetection works
						stream.Position = 0;
						return new StreamReader(stream);
					default:
						return AutoDetect(stream, (byte)firstByte, (byte)secondByte, defaultEncoding);
				}
			} else {
				if (defaultEncoding != null) {
					return new StreamReader(stream, defaultEncoding);
				} else {
					return new StreamReader(stream);
				}
			}
		}
		
		static StreamReader AutoDetect(Stream fs, byte firstByte, byte secondByte, Encoding defaultEncoding)
		{
			int max = (int)Math.Min(fs.Length, 500000); // look at max. 500 KB
			const int ASCII = 0;
			const int Error = 1;
			const int UTF8  = 2;
			const int UTF8Sequence = 3;
			int state = ASCII;
			int sequenceLength = 0;
			byte b;
			for (int i = 0; i < max; i++) {
				if (i == 0) {
					b = firstByte;
				} else if (i == 1) {
					b = secondByte;
				} else {
					b = (byte)fs.ReadByte();
				}
				if (b < 0x80) {
					// normal ASCII character
					if (state == UTF8Sequence) {
						state = Error;
						break;
					}
				} else if (b < 0xc0) {
					// 10xxxxxx : continues UTF8 byte sequence
					if (state == UTF8Sequence) {
						--sequenceLength;
						if (sequenceLength < 0) {
							state = Error;
							break;
						} else if (sequenceLength == 0) {
							state = UTF8;
						}
					} else {
						state = Error;
						break;
					}
				} else if (b >= 0xc2 && b < 0xf5) {
					// beginning of byte sequence
					if (state == UTF8 || state == ASCII) {
						state = UTF8Sequence;
						if (b < 0xe0) {
							sequenceLength = 1; // one more byte following
						} else if (b < 0xf0) {
							sequenceLength = 2; // two more bytes following
						} else {
							sequenceLength = 3; // three more bytes following
						}
					} else {
						state = Error;
						break;
					}
				} else {
					// 0xc0, 0xc1, 0xf5 to 0xff are invalid in UTF-8 (see RFC 3629)
					state = Error;
					break;
				}
			}
			fs.Position = 0;
			switch (state) {
				case ASCII:
				case Error:
					// when the file seems to be ASCII or non-UTF8,
					// we read it using the user-specified encoding so it is saved again
					// using that encoding.
					if (IsUnicode(defaultEncoding)) {
						// the file is not Unicode, so don't read it using Unicode even if the
						// user has choosen Unicode as the default encoding.
						
						// If we don't do this, SD will end up always adding a Byte Order Mark
						// to ASCII files.
						defaultEncoding = Encoding.Default; // use system encoding instead
					}
					return new StreamReader(fs, defaultEncoding);
				default:
					return new StreamReader(fs);
			}
		}
	}
}