判断是否utf8编码的算法

参考：http://www.cnblogs.com/powertoolsteam/archive/2010/09/20/1831638.html

问题：由于utf8编码分为有bom头和无bom头，而有bom头的编码很好判断，如果前三个字节是0xE 0xBB 0xBF，那么就是utf8带bom的编码，如果没有bom头，则不能好快的判断到底是utf8还是ansi编码，因为他们都没有头，且全英文的编码是兼容的，者就是问题所在。

解决方案：

几天前偶尔看到有人发帖子问“如何自动识别判断url中的中文参数是GB2312还是Utf-8编码”

也拜读了wcwtitxu使用巨牛的正则表达式检测UTF8编码的算法。

使用无数或条件的正则表达式用起来却是性能不高。

刚好曾经在项目中有类似的需求，这里把处理思路和整理后的源代码贴出来供大家参考

先聊聊原理：

UTF8的编码规则如下表

看起来很复杂，总结起来如下：

ASCII码（U+0000 - U+007F），不编码

其余编码规则为

•第一个Byte二进制以形式为n个1紧跟个0 (n >= 2), 0后面的位数用来存储真正的字符编码，n的个数说明了这个多Byte字节组字节数（包括第一个Byte） 
•结下来会有n个以10开头的Byte，后6个bit存储真正的字符编码。 
因此对整个编码byte流进行分析可以得出是否是UTF8编码的判断。

根据这个规则，我给出的C#代码如下：

///
///   Determines whether the given is UTF8 encoding bytes.
///
///
///    The input stream.
///
///
///   if given bystes stream is in UTF8 encoding; otherwise, .
///
///
///   All ASCII chars will regards not UTF8 encoding.
///
public  static  bool  IsTextUTF8( ref  byte [] inputStream)
{
    int  encodingBytesCount = 0;
    bool  allTextsAreASCIIChars = true ;

    for  ( int  i = 0; i < inputStream.Length; i++)
    {
        byte  current = inputStream[i];

        if  ((current & 0x80) == 0x80)
        {
            allTextsAreASCIIChars = false ;
        }
        // First byte
        if  (encodingBytesCount == 0)
        {
            if  ((current & 0x80) == 0)
            {
                // ASCII chars, from 0x00-0x7F
                continue ;
            }

            if  ((current & 0xC0) == 0xC0)
            {
                encodingBytesCount = 1;
                current <<= 2;

                // More than two bytes used to encoding a unicode char.
                // Calculate the real length.
                while  ((current & 0x80) == 0x80)
                {
                    current <<= 1;
                    encodingBytesCount++;
                }
            }
            else
            {
                // Invalid bits structure for UTF8 encoding rule.
                return  false ;
            }
        }
        else
        {
            // Following bytes, must start with 10.
            if  ((current & 0xC0) == 0x80)
            {
                encodingBytesCount--;
            }
            else
            {
                // Invalid bits structure for UTF8 encoding rule.
                return  false ;
            }
        }
    }

    if  (encodingBytesCount != 0)
    {
        // Invalid bits structure for UTF8 encoding rule.
        // Wrong following bytes count.
        return  false ;
    }

    // Although UTF8 supports encoding for ASCII chars, we regard as a input stream, whose contents are all ASCII as default encoding.
    return  !allTextsAreASCIIChars;
}

 

 

再附上单元测试代码：

 

///
///This is a test class for EncodingHelperTest and is intended
///to contain all EncodingHelperTest Unit Tests
///
[TestClass()]
public  class  EncodingHelperTest
{
    ///
    ///  Normal test for this method.
    ///
    [TestMethod()]
    public  void  IsTextUTF8Test()
    {
        for  ( int  i = 0; i < 1000; i++)
        {
            List chars = new  List< char >();
            chars.Add( '中' );

            List temp = new  List();
            Random rd = new  Random(( int )(DateTime.Now.Ticks & 0x7FFFFFFF));

            for  ( int  j = 0; j < 255; j++)
            {
                char  ch = ( char )rd.Next(0xFFFF);
                UnicodeCategory uc = System.Globalization.CharUnicodeInfo.GetUnicodeCategory(ch);
                if  (uc == UnicodeCategory.Surrogate || // Single surrogate could not be encoding correctly.
                    uc == UnicodeCategory.PrivateUse || // Private use blocks should be excluded.
                    uc == UnicodeCategory.OtherNotAssigned
                    )
                {
                    j--;
                }
                else
                {
                    chars.Add(ch);
                    temp.Add(uc);
                }
            }

            string  str = new  string (chars.ToArray());

            byte [] inputStream = Encoding.UTF8.GetBytes(str);
            bool  expected = true ;
            bool  actual;
            actual = EncodingHelper.IsTextUTF8( ref  inputStream);
            Assert.AreEqual(expected, actual, string .Format( "UTF8_Assert Fails at:{0}" , str));

            inputStream = Encoding.GetEncoding(932).GetBytes(str);
            expected = false ;

            actual = EncodingHelper.IsTextUTF8( ref  inputStream);
            Assert.AreEqual(expected, actual, string .Format( "ShiftJIS_Assert Fails at:{0}" , str));
        }
    }

    ///
    ///   Check with All ASCII chars
    ///
    [TestMethod]
    public  void  IsTextUTF8Test_AllASCII()
    {
        string  str = "ABCDEFGHKLHSJKLDFHJKLHAJKLSHJKLHAJKLSHDJKLAHSDJKLHAJKLSDHJKLASHDJKLHASJKLDHJKLASD" ;

        byte [] inputStream = Encoding.UTF8.GetBytes(str);
        bool  expected = false ;
        bool  actual;
        actual = EncodingHelper.IsTextUTF8( ref  inputStream);
        Assert.AreEqual(expected, actual, string .Format( "UTF8_Assert Fails at:{0}" , str));


    }
}

 

另：

如果是判断一个文件是否使用了UTF8编码，不一定非用这种方法，因为通常以UTF8格式保存的文件最初两个字符是BOM头，标示该文件使用了UTF8编码。

参考：

维基百科：http://en.wikipedia.org/wiki/UTF-8