gitextensions-ICSharpCode.TextEditor/Project/Src/Util/FileReader.cs

// <file>
//     <copyright see="prj:///doc/copyright.txt"/>
//     <license see="prj:///doc/license.txt"/>
//     <owner name="Daniel Grunwald" email="daniel@danielgrunwald.de"/>
//     <version>$Revision$</version>
// </file>

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

namespace ICSharpCode.TextEditor.Util
{
    /// <summary>
    ///     Class that can open text files with auto-detection of the encoding.
    /// </summary>
    public static class FileReader
    {
        public static bool IsUnicode(Encoding encoding)
        {
            var codepage = encoding.CodePage;
            // return true if codepage is any UTF codepage
            return codepage == 65001 || codepage == 65000 || codepage == 1200 || codepage == 1201;
        }

        public static string ReadFileContent(Stream fs, ref Encoding encoding)
        {
            using (var reader = OpenStream(fs, encoding))
            {
                reader.Peek();
                encoding = reader.CurrentEncoding;
                return reader.ReadToEnd();
            }
        }

        public static string ReadFileContent(string fileName, Encoding encoding)
        {
            using (var fs = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
            {
                return ReadFileContent(fs, ref encoding);
            }
        }

        public static StreamReader OpenStream(Stream fs, Encoding defaultEncoding)
        {
            if (fs == null)
                throw new ArgumentNullException(nameof(fs));

            if (fs.Length >= 2)
            {
                // the autodetection of StreamReader is not capable of detecting the difference
                // between ISO-8859-1 and UTF-8 without BOM.
                var firstByte = fs.ReadByte();
                var secondByte = fs.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
                        fs.Position = 0;
                        return new StreamReader(fs);
                    default:
                        return AutoDetect(fs, (byte)firstByte, (byte)secondByte, defaultEncoding);
                }
            }

            if (defaultEncoding != null)
                return new StreamReader(fs, defaultEncoding);

            return new StreamReader(fs);
        }

        private static StreamReader AutoDetect(Stream fs, byte firstByte, byte secondByte, Encoding defaultEncoding)
        {
            var max = (int)Math.Min(fs.Length, val2: 500000); // look at max. 500 KB
            const int ASCII = 0;
            const int Error = 1;
            const int UTF8 = 2;
            const int UTF8Sequence = 3;
            var state = ASCII;
            var sequenceLength = 0;
            for (var i = 0; i < max; i++)
            {
                byte b;
                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;
                        }

                        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))
                        defaultEncoding = Encoding.Default; // use system encoding instead
                    return new StreamReader(fs, defaultEncoding);
                default:
                    return new StreamReader(fs);
            }
        }
    }
}