[抄录]Unicode & UTF

Unicode简介

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Unicode provides a unique number for every character, no matter what the platform, program, or language is.

Fundamentally, computers just deal with numbers. They store letters and other characters by assigning a number for each one. Before the Unicode standard was developed, there were many different systems, called character encodings, for assigning these numbers. These earlier character encodings were limited and did not cover characters for all the world’s languages. Even for a single language like English, no single encoding covered all the letters, punctuation, and technical symbols in common use. Pictographic languages, such as Japanese, were a challenge to support with these earlier encoding standards.

Early character encodings also conflicted with one another. That is, two encodings could use the same number for two different characters, or use different numbers for the same character. Any given computer might have to support many different encodings. However, when data is passed between computers and different encodings it increased the risk of data corruption or errors.

Character encodings existed for a handful of “large” languages. But many languages lacked character support altogether.

UTF

What is a UTF?

A Unicode transformation format (UTF) is an algorithmic mapping from every Unicode code point (except surrogate code points) to a unique byte sequence. The ISO/IEC 10646 standard uses the term “UCS transformation format” for UTF; the two terms are merely synonyms for the same concept.

Unicode Encoding Forms

The Unicode Standard supports three character encoding forms: UTF-32, UTF-16, and UTF-8. Each encoding form maps the Unicode code points U+0000..U+D7FF and U+E000..U+10FFFF to unique code unit sequences. The size of the code unit is specified for each encoding form.

Which of the UTFs do I need to support?

UTF-8 is most common on the web. UTF-16 is used by Java and Windows. UTF-8 and UTF-32 are used by Linux and various Unix systems. The conversions between all of them are algorithmically based, fast and lossless. This makes it easy to support data input or output in multiple formats, while using a particular UTF for internal storage or processing.

What are some of the differences between the UTFs?

The following table summarizes some of the properties of each of the UTFs.

UTFs