判断是否utf8编码的算法

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

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：

/// <summary> ///   Determines whether the given <paramref name="inputStream"/>is UTF8 encoding bytes. /// </summary> /// <param name="inputStream"> ///    The input stream. ///  </param> /// <returns> ///   <see langword="true"/> if given bystes stream is in UTF8 encoding; otherwise, <see langword="false"/>. /// </returns> /// <remarks> ///   All ASCII chars will regards not UTF8 encoding. /// </remarks> 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：

 

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

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