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

namespace ryCommon
{
    /// <summary>   
    /// 用于取得一个文本文件的编码方式(Encoding)。   
    /// </summary>   
    public class TxtFileEncoder
    {
        /// <summary>
        /// 用于取得一个文本文件的编码方式(Encoding)。   
        /// </summary>
        public TxtFileEncoder()
        {
            //   
            // TODO: 在此处添加构造函数逻辑   
            //   
        }
        /// <summary>   
        /// 取得一个文本文件的编码方式。如果无法在文件头部找到有效的前导符，Encoding.Default将被返回。   
        /// </summary>   
        /// <param name="fileName">文件名。</param>   
        /// <returns></returns>   
        public static Encoding GetEncoding(string fileName)
        {
            return GetEncoding(fileName, Encoding.Default);
        }
        /// <summary>   
        /// 取得一个文本文件流的编码方式。   
        /// </summary>   
        /// <param name="stream">文本文件流。</param>   
        /// <returns></returns>   
        public static Encoding GetEncoding(FileStream stream)
        {
            return GetEncoding(stream, Encoding.Default);
        }
        /// <summary>   
        /// 取得一个文本文件的编码方式。   
        /// </summary>   
        /// <param name="fileName">文件名。</param>   
        /// <param name="defaultEncoding">默认编码方式。当该方法无法从文件的头部取得有效的前导符时，将返回该编码方式。</param>   
        /// <returns></returns>   
        public static Encoding GetEncoding(string fileName, Encoding defaultEncoding)
        {
            Encoding targetEncoding = defaultEncoding;
            try
            {
                FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
                try
                {
                    targetEncoding = GetEncoding(fs, defaultEncoding);
                }
                catch { }
                fs.Close();
            }
            catch
            {
                return defaultEncoding;
            }
            return targetEncoding;
        }
        /// <summary>   
        /// 取得一个文本文件流的编码方式。   
        /// </summary>   
        /// <param name="stream">文本文件流。</param>   
        /// <param name="defaultEncoding">默认编码方式。当该方法无法从文件的头部取得有效的前导符时，将返回该编码方式。</param>   
        /// <returns></returns>   
        public static Encoding GetEncoding(FileStream stream, Encoding defaultEncoding)
        {
            Encoding targetEncoding = defaultEncoding;
            if (stream != null && stream.Length >= 2)
            {
                //保存文件流的前4个字节   
                byte byte1;
                byte byte2;
                byte byte3 = 0;
                byte byte4;
                BinaryReader r = new BinaryReader(stream, System.Text.Encoding.Default);
                byte[] ss;
                ss = r.ReadBytes(stream.Length.ToInt());
                byte1 = ss.Length>=1?ss[0]:byte.MinValue;
                byte2 = ss.Length >= 2 ? ss[1] : byte.MinValue;
                if (stream.Length >= 3)
                {
                    byte3 = ss.Length >= 3 ? ss[2] : byte.MinValue;
                }
                if (stream.Length >= 4)
                {
                    byte4 = ss.Length >= 4 ? ss[3] : byte.MinValue;
                }
                //根据文件流的前4个字节判断Encoding   
                //Unicode {0xFF, 0xFE};   
                //BE-Unicode {0xFE, 0xFF};   
                //UTF8 = {0xEF, 0xBB, 0xBF};   
                if (byte1 == 0xFE && byte2 == 0xFF)//UnicodeBe   
                {
                    targetEncoding = Encoding.BigEndianUnicode;
                }
                else if (byte1 == 0xFF && byte2 == 0xFE && byte3 != 0xFF)//Unicode   
                {
                    targetEncoding = Encoding.Unicode;
                }
                else if (byte1 == 0xEF && byte2 == 0xBB && byte3 == 0xBF)//UTF8   
                {
                    targetEncoding = Encoding.UTF8;
                }
                else if (IsUTF8Bytes(ss))//UTF8   
                {
                    targetEncoding =new UTF8Encoding(false);
                }
                //恢复Seek位置         
                r.Close();
            }
            return targetEncoding;
        }



        // 新增加一个方法，解决了不带BOM的 UTF8 编码问题   

        /// <summary>   
        /// 通过给定的文件流，判断文件的编码类型   
        /// </summary>   
        /// <param name="fs">文件流</param>   
        /// <returns>文件的编码类型</returns>   
        public static System.Text.Encoding GetEncoding(Stream fs)
        {
            byte[] Unicode = new byte[] { 0xFF, 0xFE, 0x41 };
            byte[] UnicodeBIG = new byte[] { 0xFE, 0xFF, 0x00 };
            byte[] UTF8 = new byte[] { 0xEF, 0xBB, 0xBF }; //带BOM   
            Encoding reVal = Encoding.Default;

            BinaryReader r = new BinaryReader(fs, System.Text.Encoding.Default);
            byte[] ss = r.ReadBytes(4);
            if (ss[0] == 0xFE && ss[1] == 0xFF && ss[2] == 0x00)
            {
                reVal = Encoding.BigEndianUnicode;
            }
            else if (ss[0] == 0xFF && ss[1] == 0xFE && ss[2] == 0x41)
            {
                reVal = Encoding.Unicode;
            }
            else
            {
                if (ss[0] == 0xEF && ss[1] == 0xBB && ss[2] == 0xBF)
                {
                    reVal = Encoding.UTF8;
                }
                else
                {
                    int.TryParse(fs.Length.ToString(), out int i);
                    ss = r.ReadBytes(i);

                    if (GetBytesEncoding(ss)==Encoding.UTF8)
                        reVal = Encoding.UTF8;
                }
            }
            r.Close();
            return reVal;

        }
        private static Encoding GetBytesEncoding(byte[] bs)
        {
            int len = bs.Length;
            if (len >= 3 && bs[0] == 0xEF && bs[1] == 0xBB && bs[2] == 0xBF)
            {
                return Encoding.UTF8;
            }
            int[] cs = { 7, 5, 4, 3, 2, 1, 0, 6, 14, 30, 62, 126 };
            for (int i = 0; i < len; i++)
            {
                int bits = -1;
                for (int j = 0; j < 6; j++)
                {
                    if (bs[i] >> cs[j] == cs[j + 6])
                    {
                        bits = j;
                        break;
                    }
                }
                if (bits == -1)
                {
                    return Encoding.Default;
                }
                while (bits-- > 0)
                {
                    i++;
                    if (i == len || bs[i] >> 6 != 2)
                    {
                        return Encoding.Default;
                    }
                }
            }
            return Encoding.UTF8;
        }
        private static byte UTF8CharacterMask1Byte = 0b1000_0000;
        private static byte Valid1Byte = 0b0000_0000;//0b0xxx_xxxx

        private static byte UTF8CharacterMask2Byte = 0b1110_0000;
        private static byte Valid2Byte = 0b1100_0000;//0b110x_xxxx

        private static byte UTF8CharacterMask3Byte = 0b1111_0000;
        private static byte Valid3Byte = 0b1110_0000;//0b1110_xxxx

        private static byte UTF8CharacterMask4Byte = 0b1111_1000;
        private static byte Valid4Byte = 0b1111_0000;//0b1111_0xxx

        private static byte UTF8CharacterMaskForExtraByte = 0b1100_0000;
        private static byte ValidExtraByte = 0b1000_0000;//0b10xx_xxxx
        /// <summary>   
        /// 判断是否是不带 BOM 的 UTF8 格式   
        /// </summary>   
        /// <param name="bytes"></param>   
        /// <returns></returns>   
        public static bool IsUTF8Bytes(byte[] bytes)
        {
            short extraByteCount = 0;

            foreach (byte bt in bytes)
            {

                if (extraByteCount > 0)
                {
                    extraByteCount--;

                    // Extra Byte Pattern.
                    if ((bt & UTF8CharacterMaskForExtraByte) != ValidExtraByte)
                        return false;
                    continue;
                }
                else
                {
                    // 1 Byte Pattern.
                    if ((bt & UTF8CharacterMask1Byte) == Valid1Byte)
                    {
                        continue;
                    }

                    // 2 Bytes Pattern.
                    if ((bt & UTF8CharacterMask2Byte) == Valid2Byte)
                    {
                        extraByteCount = 1;
                        continue;
                    }

                    // 3 Bytes Pattern.
                    if ((bt & UTF8CharacterMask3Byte) == Valid3Byte)
                    {
                        extraByteCount = 2;
                        continue;
                    }

                    // 4 Bytes Pattern.
                    if ((bt & UTF8CharacterMask4Byte) == Valid4Byte)
                    {
                        extraByteCount = 3;
                        continue;
                    }

                    // invalid UTF8-Bytes.
                    return false;
                }
            }

            return extraByteCount >= 0;
        }
    }
}