Ascii,unicode and UTF-8

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1.ASCII

We know that inside the computer, all information is ultimately a binary value. Each bits (bit) has 0 and 12 states, so eight bits can combine 256 states, which is called a byte. In other words, a byte can be used to represent 256 different states, each of which corresponds to a symbol, which is 256 symbols, from 00000000 to 11111111.
In the 60 's, the United States developed a set of character encodings, which made a uniform provision for the relationship between English characters and bits. This is known as ASCII code and has been used so far.

The ASCII code specifies a total of 128 characters, such as the space is 32 (binary 00100000), the uppercase letter A is 65 (binary 01000001). These 128 symbols (including 32 control symbols that cannot be printed out) occupy only one byte of the latter 7 bits, and the first one is 0.

2. Non-ASCII encoding

It is enough to encode 128 symbols in English, but 128 symbols are not enough to represent other languages. For example, in French, where there is a phonetic symbol above the letter, it cannot be represented by an ASCII code. As a result, some European countries decided to use the highest bits of the bytes that were idle to incorporate new symbols. For example, the code for E in French is 130 (binary 10000010). In this way, the coding system used in these European countries can represent a maximum of 256 symbols.

However, there are new problems. Different countries have different letters, so even if they are encoded using 256 symbols, the letters are not the same. For example, 130 is represented in the French code, but in Hebrew it represents the letter Gimel (?), and in the Russian language, another symbol is represented in the code. But anyway, in all of these encodings, 0--127 represents the same symbol, not just the 128--255 section.

As for Asian countries, the use of symbols is more, the Chinese character is about 100,000. A byte can represent only 256 symbols, which is certainly not enough, and must be expressed using multiple bytes to express a symbol. For example, the common encoding method in Simplified Chinese is GB2312, which uses two bytes to represent a Chinese character, so it is theoretically possible to represent up to a maximum of 256 x 65,536 symbols

The issue of Chinese coding needs to be discussed in this article, which is not covered by this note. It is only pointed out that although a symbol is represented in multiple bytes, the Chinese character coding of the GB class is irrelevant to the Unicode and UTF-8.

3.Unicode

As mentioned in the previous section, there are many coding methods in the world, and the same binary numbers can be interpreted as different symbols. Therefore, if you want to open a text file, you must know its encoding, or in the wrong way to interpret the code, there will be garbled. Why do e-mails often appear garbled? It is because the sender and the recipient are using different encoding methods.

It can be imagined that if there is an encoding, all the symbols in the world are included. Each symbol is given a unique encoding, then the garbled problem disappears. This is Unicode, as its name indicates, which is an encoding of all symbols.

Unicode is of course a large collection, and now the scale can accommodate the 100多万个 symbol. Each symbol is encoded differently, for example, u+0639 means that the Arabic letter ain,u+0041 represents the English uppercase letter A,u+4e25 The Chinese character is strict. The specific Symbol correspondence table, may query unicode.org, or the specialized Chinese character correspondence table.

4.Unicode problem

It is important to note that Unicode is just a set of symbols, which only specifies the binary code of the symbol, but does not specify how the binary code should be stored.

For example, the strict Unicode character is hexadecimal number 4E25, the conversion to a binary number is a full 15 bits (100111000100101), that is to say, the symbol of at least 2 bytes. Representing other larger symbols, it may take 3 bytes or 4 bytes, or more.

There are two serious problems here, and the first question is, how can you differentiate between Unicode and ASCII? How does the computer know that three bytes represents a symbol instead of three symbols? The second problem is that we already know that the English alphabet is only one byte to express enough, if Unicode uniform rules, each symbol with three or four bytes, then each letter must have two to three bytes is 0, which is a great waste for storage, the size of the text file will be two or three times times larger , it's unacceptable.

They result in: 1) There is a variety of Unicode storage methods, which means that there are many different binary formats that can be used to represent Unicode. 2) Unicode cannot be promoted for a long period of time until the advent of the Internet.

5.utf-8

The popularization of the Internet has strongly demanded the emergence of a unified coding method. UTF-8 is the most widely used form of Unicode implementation on the Internet. Other implementations include UTF-16 (characters in two-byte or four-byte notation) and UTF-32 (characters in four-byte notation), but not on the Internet. Again, the relationship here is that UTF-8 is one of the ways Unicode is implemented.

One of the biggest features of UTF-8 is that it is a variable-length coding method. It can use 1~4 bytes to represent a symbol, varying the length of a byte depending on the symbol.
The coding rules for UTF-8 are simple, with only two lines:

1) for a single-byte symbol, the first bit of the byte is set to 0, and the next 7 bits are the Unicode code for the symbol. so for the English alphabet, the UTF-8 encoding and ASCII code are the same.

2) for n-byte symbols (n > 1), the first n bits are set to 1, the Nth + 1 bits are set to 0, and the first two bits of the subsequent bytes are set to 10. The rest of the bits are not mentioned, all of which are Unicode codes for this symbol.
The following table summarizes the encoding rules, and the letter x represents the bits that are available for encoding. ---------------------This article from deft_mkjing mi csdn blog, full-text address click: 79460485?utm_source=copy

According to the above table, it is very simple to interpret UTF-8 coding. If the first bit of a byte is 0, then the byte is a single character, and if the first bit is 1, how many consecutive 1 is the number of bytes that the current character occupies.

Below, or take the Chinese character strictly as an example, demonstrates how to implement UTF-8 encoding.

Strict Unicode is 4E25 (100111000100101), according to the table above, you can find 4E25 in the range of the third row (0000 0800-0000 FFFF), so strict UTF-8 encoding requires three bytes, that is, the format is 1110xxxx 10xxxxxx 10xxxxxx. Then, the strict last bits begins, and sequentially fills in the format of the x, the extra bits complement 0. This gets, strict UTF-8 code is 11100100 10111000 10100101, converted into 16 binary is E4B8A5

Ascii,unicode and UTF-8