using System.Text;
namespace CYQ.Data.Tool
{
    /// <summary>
    /// 字节文本编码检测
    /// </summary>
    internal class TextEncodingDetect
    {
        private readonly byte[] _UTF8Bom =
        {
            0xEF,
            0xBB,
            0xBF
        };
        //utf16le _UnicodeBom
        private readonly byte[] _UTF16LeBom =
        {
            0xFF,
            0xFE
        };

        //utf16be _BigUnicodeBom
        private readonly byte[] _UTF16BeBom =
        {
            0xFE,
            0xFF
        };

        //utf-32le
        private readonly byte[] _UTF32LeBom =
        {
            0xFF,
            0xFE,
            0x00,
            0x00
        };
        //utf-32Be
        //private readonly byte[] _UTF32BeBom =
        //{
        //    0x00,
        //    0x00,
        //    0xFE,
        //    0xFF
        //};
        /// <summary>
        /// 是否中文
        /// </summary>
        public bool IsChinese = false;
        /// <summary>
        /// 是否拥有Bom头
        /// </summary>
        public bool hasBom = false;
        public enum TextEncode
        {
            None, // Unknown or binary
            Ansi, // 0-255
            Ascii, // 0-127
            Utf8Bom, // UTF8 with BOM
            Utf8Nobom, // UTF8 without BOM
            UnicodeBom, // UTF16 LE with BOM
            UnicodeNoBom, // UTF16 LE without BOM
            BigEndianUnicodeBom, // UTF16-BE with BOM
            BigEndianUnicodeNoBom, // UTF16-BE without BOM

            Utf32Bom,//UTF-32LE with BOM
            Utf32NoBom //UTF-32 without BOM

        }
        private bool IsChineseEncoding(Encoding encoding)
        {
            return encoding == Encoding.GetEncoding("gb2312") || encoding == Encoding.GetEncoding("gbk") || encoding == Encoding.GetEncoding("big5");
        }
        /// <summary>
        /// 获取文件编码
        /// </summary>
        /// <returns></returns>
        public Encoding GetEncoding(byte[] buff)
        {
            return GetEncoding(buff, IOHelper.DefaultEncoding);
        }
        public Encoding GetEncoding(byte[] buff, Encoding defaultEncoding)
        {
            hasBom = true;
            //检测Bom
            switch (DetectWithBom(buff))
            {
                case TextEncodingDetect.TextEncode.Utf8Bom:
                    return Encoding.UTF8;
                case TextEncodingDetect.TextEncode.UnicodeBom:
                    return Encoding.Unicode;
                case TextEncodingDetect.TextEncode.BigEndianUnicodeBom:
                    return Encoding.BigEndianUnicode;
                case TextEncodingDetect.TextEncode.Utf32Bom:
                    return Encoding.UTF32;
            }

            hasBom = false;
            if (defaultEncoding != IOHelper.DefaultEncoding && defaultEncoding != Encoding.ASCII)//自定义设置编码，优先处理。
            {
                return defaultEncoding;
            }
            switch (DetectWithoutBom(buff, buff.Length))//自动检测。
            {

                case TextEncodingDetect.TextEncode.Utf8Nobom:
                    return Encoding.UTF8;

                case TextEncodingDetect.TextEncode.UnicodeNoBom:
                    return Encoding.Unicode;

                case TextEncodingDetect.TextEncode.BigEndianUnicodeNoBom:
                    return Encoding.BigEndianUnicode;

                case TextEncodingDetect.TextEncode.Utf32NoBom:
                    return Encoding.UTF32;

                case TextEncodingDetect.TextEncode.Ansi:
                    if (IsChineseEncoding(IOHelper.DefaultEncoding) && !IsChineseEncoding(defaultEncoding))
                    {
                        if (IsChinese)
                        {
                            return Encoding.GetEncoding("gbk");
                        }
                        else//非中文时，默认选一个。
                        {
                            return Encoding.Unicode;
                        }
                    }
                    else
                    {
                        return defaultEncoding;
                    }

                case TextEncodingDetect.TextEncode.Ascii:
                    return Encoding.ASCII;

                default:
                    return defaultEncoding;
            }

        }
        public TextEncode DetectWithBom(byte[] buffer)
        {
            if (buffer != null)
            {
                int size = buffer.Length;
                // Check for BOM
                if (size >= 2 && buffer[0] == _UTF16LeBom[0] && buffer[1] == _UTF16LeBom[1])
                {
                    return TextEncode.UnicodeBom;
                }

                if (size >= 2 && buffer[0] == _UTF16BeBom[0] && buffer[1] == _UTF16BeBom[1])
                {
                    if (size >= 4 && buffer[2] == _UTF32LeBom[2] && buffer[3] == _UTF32LeBom[3])
                    {
                        return TextEncode.Utf32Bom;
                    }
                    return TextEncode.BigEndianUnicodeBom;
                }

                if (size >= 3 && buffer[0] == _UTF8Bom[0] && buffer[1] == _UTF8Bom[1] && buffer[2] == _UTF8Bom[2])
                {
                    return TextEncode.Utf8Bom;
                }
            }
            return TextEncode.None;
        }

        /// <summary>
        ///     Automatically detects the Encoding type of a given byte buffer.
        /// </summary>
        /// <param name="buffer">The byte buffer.</param>
        /// <param name="size">The size of the byte buffer.</param>
        /// <returns>The Encoding type or Encoding.None if unknown.</returns>
        public TextEncode DetectWithoutBom(byte[] buffer, int size)
        {
            // Now check for valid UTF8
            TextEncode encoding = CheckUtf8(buffer, size);
            if (encoding == TextEncode.Utf8Nobom)
            {
                return encoding;
            }

            // ANSI or None (binary) then 一个零都没有情况。
            if (!ContainsZero(buffer, size))
            {
                CheckChinese(buffer, size);
                if (IsChinese)
                {
                    return TextEncode.Ansi;
                }
            }

            // Now try UTF16  按寻找换行字符先进行判断
            encoding = CheckByNewLineChar(buffer, size);
            if (encoding != TextEncode.None)
            {
                return encoding;
            }

            // 没办法了，只能按0出现的次数比率，做大体的预判
            encoding = CheckByZeroNumPercent(buffer, size);
            if (encoding != TextEncode.None)
            {
                return encoding;
            }

            // Found a null, return based on the preference in null_suggests_binary_
            return TextEncode.None;
        }

        /// <summary>
        ///     Checks if a buffer contains text that looks like utf16 by scanning for
        ///     newline chars that would be present even in non-english text.
        ///     以检测换行符标识来判断。
        /// </summary>
        /// <param name="buffer">The byte buffer.</param>
        /// <param name="size">The size of the byte buffer.</param>
        /// <returns>Encoding.none, Encoding.Utf16LeNoBom or Encoding.Utf16BeNoBom.</returns>
        private static TextEncode CheckByNewLineChar(byte[] buffer, int size)
        {
            if (size < 2)
            {
                return TextEncode.None;
            }

            // Reduce size by 1 so we don't need to worry about bounds checking for pairs of bytes
            size--;

            int le16 = 0;
            int be16 = 0;
            int le32 = 0;//检测是否utf32le。
            int zeroCount = 0;//utf32le 每4位后面多数是0
            uint pos = 0;
            while (pos < size)
            {
                byte ch1 = buffer[pos++];
                byte ch2 = buffer[pos++];

                if (ch1 == 0)
                {
                    if (ch2 == 0x0a || ch2 == 0x0d)//\r \t 换行检测。
                    {
                        ++be16;
                    }
                }
                if (ch2 == 0)
                {
                    zeroCount++;
                    if (ch1 == 0x0a || ch1 == 0x0d)
                    {
                        ++le16;
                        if (pos + 1 <= size && buffer[pos] == 0 && buffer[pos + 1] == 0)
                        {
                            ++le32;
                        }

                    }
                }

                // If we are getting both LE and BE control chars then this file is not utf16
                if (le16 > 0 && be16 > 0)
                {
                    return TextEncode.None;
                }
            }

            if (le16 > 0)
            {
                if (le16 == le32 && buffer.Length % 4 == 0)
                {
                    return TextEncode.Utf32NoBom;
                }
                return TextEncode.UnicodeNoBom;
            }
            else if (be16 > 0)
            {
                return TextEncode.BigEndianUnicodeNoBom;
            }
            else if (buffer.Length % 4 == 0 && zeroCount >= buffer.Length / 4)
            {
                return TextEncode.Utf32NoBom;
            }
            return TextEncode.None;
        }

        /// <summary>
        /// Checks if a buffer contains any nulls. Used to check for binary vs text data.
        /// </summary>
        /// <param name="buffer">The byte buffer.</param>
        /// <param name="size">The size of the byte buffer.</param>
        private static bool ContainsZero(byte[] buffer, int size)
        {
            uint pos = 0;
            while (pos < size)
            {
                if (buffer[pos++] == 0)
                {
                    return true;
                }
            }

            return false;
        }

        /// <summary>
        ///     Checks if a buffer contains text that looks like utf16. This is done based
        ///     on the use of nulls which in ASCII/script like text can be useful to identify.
        ///     按照一定的空0数的概率来预测。
        /// </summary>
        /// <param name="buffer">The byte buffer.</param>
        /// <param name="size">The size of the byte buffer.</param>
        /// <returns>Encoding.none, Encoding.Utf16LeNoBom or Encoding.Utf16BeNoBom.</returns>
        private TextEncode CheckByZeroNumPercent(byte[] buffer, int size)
        {
            //单数
            int oddZeroCount = 0;
            //双数
            int evenZeroCount = 0;

            // Get even nulls
            uint pos = 0;
            while (pos < size)
            {
                if (buffer[pos] == 0)
                {
                    evenZeroCount++;
                }

                pos += 2;
            }

            // Get odd nulls
            pos = 1;
            while (pos < size)
            {
                if (buffer[pos] == 0)
                {
                    oddZeroCount++;
                }

                pos += 2;
            }

            double evenZeroPercent = evenZeroCount * 2.0 / size;
            double oddZeroPercent = oddZeroCount * 2.0 / size;

            // Lots of odd nulls, low number of even nulls 这里的条件做了修改
            if (evenZeroPercent < 0.1 && oddZeroPercent > 0)
            {
                return TextEncode.UnicodeNoBom;
            }

            // Lots of even nulls, low number of odd nulls 这里的条件也做了修改
            if (oddZeroPercent < 0.1 && evenZeroPercent > 0)
            {
                return TextEncode.BigEndianUnicodeNoBom;
            }

            // Don't know
            return TextEncode.None;
        }

        /// <summary>
        ///     Checks if a buffer contains valid utf8.
        ///     以UTF8 的字节范围来检测。
        /// </summary>
        /// <param name="buffer">The byte buffer.</param>
        /// <param name="size">The size of the byte buffer.</param>
        /// <returns>
        ///     Encoding type of Encoding.None (invalid UTF8), Encoding.Utf8NoBom (valid utf8 multibyte strings) or
        ///     Encoding.ASCII (data in 0.127 range).
        /// </returns>
        /// <returns>2</returns>
        private TextEncode CheckUtf8(byte[] buffer, int size)
        {
            // UTF8 Valid sequences
            // 0xxxxxxx  ASCII
            // 110xxxxx 10xxxxxx  2-byte
            // 1110xxxx 10xxxxxx 10xxxxxx  3-byte
            // 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx  4-byte
            //
            // Width in UTF8
            // Decimal      Width
            // 0-127        1 byte
            // 194-223      2 bytes
            // 224-239      3 bytes
            // 240-244      4 bytes
            //
            // Subsequent chars are in the range 128-191
            bool onlySawAsciiRange = true;
            uint pos = 0;

            while (pos < size)
            {
                byte ch = buffer[pos++];

                if (ch == 0)
                {
                    return TextEncode.None;
                }

                int moreChars;
                if (ch <= 127)
                {
                    // 1 byte
                    moreChars = 0;
                }
                else if (ch >= 194 && ch <= 223)
                {
                    // 2 Byte
                    moreChars = 1;
                }
                else if (ch >= 224 && ch <= 239)
                {
                    // 3 Byte
                    moreChars = 2;
                }
                else if (ch >= 240 && ch <= 244)
                {
                    // 4 Byte
                    moreChars = 3;
                }
                else
                {
                    return TextEncode.None; // Not utf8
                }

                // Check secondary chars are in range if we are expecting any
                while (moreChars > 0 && pos < size)
                {
                    onlySawAsciiRange = false; // Seen non-ascii chars now

                    ch = buffer[pos++];
                    if (ch < 128 || ch > 191)
                    {
                        return TextEncode.None; // Not utf8
                    }

                    --moreChars;
                }
            }

            // If we get to here then only valid UTF-8 sequences have been processed

            // If we only saw chars in the range 0-127 then we can't assume UTF8 (the caller will need to decide)
            return onlySawAsciiRange ? TextEncode.Ascii : TextEncode.Utf8Nobom;
        }
        /// <summary>
        /// 是否中文编码（GB2312、GBK、Big5）
        /// </summary>
        private void CheckChinese(byte[] buffer, int size)
        {
            IsChinese = false;
            if (size < 2)
            {
                return;
            }

            // Reduce size by 1 so we don't need to worry about bounds checking for pairs of bytes
            size--;
            uint pos = 0;
            bool isCN = false;
            while (pos < size)
            {
                //GB2312
                //0xB0-0xF7(176-247)
                //0xA0-0xFE（160-254）

                //GBK
                //0x81-0xFE（129-254）
                //0x40-0xFE（64-254）

                //Big5
                //0x81-0xFE（129-255）
                //0x40-0x7E（64-126）  OR 0xA1－0xFE（161-254）
                byte ch1 = buffer[pos++];
                byte ch2 = buffer[pos++];
                isCN = (ch1 >= 176 && ch1 <= 247 && ch2 >= 160 && ch2 <= 254)
                    || (ch1 >= 129 && ch1 <= 254 && ch2 >= 64 && ch2 <= 254)
                    || (ch1 >= 129 && ((ch2 >= 64 && ch2 <= 126) || (ch2 >= 161 && ch2 <= 254)));
                if (isCN)
                {
                    IsChinese = true;
                    return;
                }

            }

        }
    }
}