检拆字节流是否是UTF8编码

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
UTF8 Encoding Rule
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

?

其实java的操作也类似,有时间再写个