軟件測(cè)試+JAVA

UTF-8 is a variant-length character encoding for Unicode, created by Ken Thompson in 1992, in a New Jersey diner, where he designed it in the presence of Rob Pike on a placemat. It is currently standardized as RFC 3629. UTF-8 uses 1 to 6 bytes to encode one UNICODE character. (If the UNICODE char is represented on 2 bytes, there is a need for mostly 3 bytes; if the UNICODE char is represented as 4 bytes, 6 bytes may be needed.) 4 or 6 bytes to encode a single char may seem too much, but the UNICODE chars that need that are rarely used.

The transformation table for UTF-8 is presented below:

The UNICODE characters that actually represent ASCII chars are encoded in a single byte, and the UTF-8 representation is actually the ASCII representation. All other UNICODE characters require at least 2 bytes. Each of these bytes starts with an escape sequence. The first byte has a unique sequence, composed on N bits on 1 followed by 1 bit of 0. The N number of bits of 1 indicates the number of bytes on which the character is encoded.

Examples

UNICODE \uCA (11001010) requires 2 bytes for UTF-8 encoding:

\uCA -> C3 8A

UNICODE \uF03F (11110000 0011111) requires 3 bytes for UTF-8 encoding:

\u F03F -> EF 80 BF

Advantages

Here are several advantages of UTF-8:

• UTF-8 can be read and written quickly just with bit-mask and bit-shift operations.
• Comparing two char strings in C/C++ with strcmp() gives the same result as wcscmp(), so that legicographic sorting and tree-search order are preserved.
• Bytes FF and FE never appear in an UTF-8 output, so they can be used to indicate an UTF-16 or UTF-32 text (see BOM).
• UTF-8 is byte order independent. The bytes order is the same on all systems, so that it doesn't actually require a BOM.

Disadvantages

UTF-8 has several disadvantages:

• You cannot determine the number of bytes of the UTF-8 text from the number of UNICODE characters because UTF-8 uses a variable length encoding.
• It needs 2 bytes for those non-Latin characters that are encoded in just 1 byte with extended ASCII char sets.
• ISO Latin-1, a subset of UNICODE, is not a subset of UTF-8.
• The 8-bit chars of UTF-8 are stripped by many mail gateways because Internet messages were originally designed as 7-bit ASCII. The problem led to the creation of UTF-7.
• UTF-8 uses the values 100xxxxx in more than 50% of its representation, but existing implementation of ISO 2022, 4873, 6429, and 8859 systems mistake these as C1 control codes. The problem led to the creation of UTF-7,5.

Modified UTF-8

Java uses UTF-16 for the internal text representation and supports a non-standard modification of UTF-8 for string serialization. There are two differences between the standard and modified UTF-8:

• In modified UTF-8, the null character (U+0000) is encoded with two bytes (11000000 10000000) instead of just one (00000000), which ensures that there are no embedded nulls in the encoded string (so that if the string is processed with a C-like language, the text is not truncated to the first null character).
• In standard UTF-8, characters outside the BMP (Basic Multilingual Plain) are encoded using the 4-byte format, but in modified UTF-8 they are represented as surrogate pairs and then the surrogate pairs are encoded individually in sequence. As a result, characters that require 4 bytes in standard UTF-8 require 6 bytes in modified UTF-8.

Byte Order Mark

BOM is a character that indicates the endianness of a UNICODE text encoded in UTF-16, UTF-32 and in the same time a marker to indicate that text is encoded in UTF-8, UTF-16, UTF-32 (UTF-8 is byte-order independent).

UTF-8 C++ Encoding Sample

Here are four functions written in C++ that encode and decode 2 and 4 bytes UNICODE text in/from UTF-8.

#define         MASKBITS                0x3F
#define         MASKBYTE                0x80
#define         MASK2BYTES              0xC0
#define         MASK3BYTES              0xE0
#define         MASK4BYTES              0xF0
#define         MASK5BYTES              0xF8
#define         MASK6BYTES              0xFC

typedefunsignedshort   Unicode2Bytes;
typedefunsignedint     Unicode4Bytes;

void UTF8Encode2BytesUnicode(std::vector< Unicode2Bytes > input,
                             std::vector< byte >& output)
{
   for(int i=0; i < input.size(); i++)
   {
      // 0xxxxxxxif(input[i] < 0x80)
      {
         output.push_back((byte)input[i]);
      }
      // 110xxxxx 10xxxxxxelseif(input[i] < 0x800)
      {
         output.push_back((byte)(MASK2BYTES | input[i] >> 6));
         output.push_back((byte)(MASKBYTE | input[i] & MASKBITS));
      }
      // 1110xxxx 10xxxxxx 10xxxxxxelseif(input[i] < 0x10000)
      {
         output.push_back((byte)(MASK3BYTES | input[i] >> 12));
         output.push_back((byte)(MASKBYTE | input[i] >> 6 & MASKBITS));
         output.push_back((byte)(MASKBYTE | input[i] & MASKBITS));
      }
   }
}

void UTF8Decode2BytesUnicode(std::vector< byte > input,
                             std::vector< Unicode2Bytes >& output)
{
   for(int i=0; i < input.size();)
   {
      Unicode2Bytes ch;

      // 1110xxxx 10xxxxxx 10xxxxxxif((input[i] & MASK3BYTES) == MASK3BYTES)
      {
         ch = ((input[i] & 0x0F) << 12) | (
               (input[i+1] & MASKBITS) << 6)
              | (input[i+2] & MASKBITS);
         i += 3;
      }
      // 110xxxxx 10xxxxxxelseif((input[i] & MASK2BYTES) == MASK2BYTES)
      {
         ch = ((input[i] & 0x1F) << 6) | (input[i+1] & MASKBITS);
         i += 2;
      }
      // 0xxxxxxxelseif(input[i] < MASKBYTE)
      {
         ch = input[i];
         i += 1;
      }

      output.push_back(ch);
   }
}

void UTF8Encode4BytesUnicode(std::vector< Unicode4Bytes > input,
                             std::vector< byte >& output)
{
   for(int i=0; i < input.size(); i++)
   {
      // 0xxxxxxxif(input[i] < 0x80)
      {
         output.push_back((byte)input[i]);
      }
      // 110xxxxx 10xxxxxxelseif(input[i] < 0x800)
      {
         output.push_back((byte)(MASK2BYTES | input[i] > 6));
         output.push_back((byte)(MASKBYTE | input[i] & MASKBITS));
      }
      // 1110xxxx 10xxxxxx 10xxxxxxelseif(input[i] < 0x10000)
      {
         output.push_back((byte)(MASK3BYTES | input[i] >> 12));
         output.push_back((byte)(MASKBYTE | input[i] >> 6 & MASKBITS));
         output.push_back((byte)(MASKBYTE | input[i] & MASKBITS));
      }
      // 11110xxx 10xxxxxx 10xxxxxx 10xxxxxxelseif(input[i] < 0x200000)
      {
         output.push_back((byte)(MASK4BYTES | input[i] >> 18));
         output.push_back((byte)(MASKBYTE | input[i] >> 12 & MASKBITS));
         output.push_back((byte)(MASKBYTE | input[i] >> 6 & MASKBITS));
         output.push_back((byte)(MASKBYTE | input[i] & MASKBITS));
      }
      // 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxxelseif(input[i] < 0x4000000)
      {
         output.push_back((byte)(MASK5BYTES | input[i] >> 24));
         output.push_back((byte)(MASKBYTE | input[i] >> 18 & MASKBITS));
         output.push_back((byte)(MASKBYTE | input[i] >> 12 & MASKBITS));
         output.push_back((byte)(MASKBYTE | input[i] >> 6 & MASKBITS));
         output.push_back((byte)(MASKBYTE | input[i] & MASKBITS));
      }
      // 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxxelseif(input[i] < 0x8000000)
      {
         output.push_back((byte)(MASK6BYTES | input[i] >> 30));
         output.push_back((byte)(MASKBYTE | input[i] >> 18 & MASKBITS));
         output.push_back((byte)(MASKBYTE | input[i] >> 12 & MASKBITS));
         output.push_back((byte)(MASKBYTE | input[i] >> 6 & MASKBITS));
         output.push_back((byte)(MASKBYTE | input[i] & MASKBITS));
      }
   }
}

void UTF8Decode4BytesUnicode(std::vector< byte > input,
                             std::vector< Unicode4Bytes >& output)
{
   for(int i=0; i < input.size();)
   {
      Unicode4Bytes ch;

      // 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxxif((input[i] & MASK6BYTES) == MASK6BYTES)
      {
         ch = ((input[i] & 0x01) << 30) | ((input[i+1] & MASKBITS) << 24)
              | ((input[i+2] & MASKBITS) << 18) | ((input[i+3]
                        & MASKBITS) << 12)
              | ((input[i+4] & MASKBITS) << 6) | (input[i+5] & MASKBITS);
         i += 6;
      }
      // 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxxelseif((input[i] & MASK5BYTES) == MASK5BYTES)
      {
         ch = ((input[i] & 0x03) << 24) | ((input[i+1]
                & MASKBITS) << 18)
              | ((input[i+2] & MASKBITS) << 12) | ((input[i+3]
                  & MASKBITS) << 6)
              | (input[i+4] & MASKBITS);
         i += 5;
      }
      // 11110xxx 10xxxxxx 10xxxxxx 10xxxxxxelseif((input[i] & MASK4BYTES) == MASK4BYTES)
      {
         ch = ((input[i] & 0x07) << 18) | ((input[i+1]
                & MASKBITS) << 12)
              | ((input[i+2] & MASKBITS) << 6) | (input[i+3] & MASKBITS);
         i += 4;
      }
      // 1110xxxx 10xxxxxx 10xxxxxxelseif((input[i] & MASK3BYTES) == MASK3BYTES)
      {
         ch = ((input[i] & 0x0F) << 12) | ((input[i+1] & MASKBITS) << 6)
              | (input[i+2] & MASKBITS);
         i += 3;
      }
      // 110xxxxx 10xxxxxxelseif((input[i] & MASK2BYTES) == MASK2BYTES)
      {
         ch = ((input[i] & 0x1F) << 6) | (input[i+1] & MASKBITS);
         i += 2;
      }
      // 0xxxxxxxelseif(input[i] < MASKBYTE)
      {
         ch = input[i];
         i += 1;
      }
      output.push_back(ch);
   }
}

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