Why can't we store Unicode directly?

Question

I read some article about Unicode and UTF-8.

The Unicode standard describes how characters are represented by code points. A code point is an integer value, usually denoted in base 16. In the standard, a code point is written using the notation U+12CA to mean the character with value 0x12ca (4,810 decimal). The Unicode standard contains a lot of tables listing characters and their corresponding code points:

Strictly, these definitions imply that it’s meaningless to say ‘this is character U+12CA‘. U+12CA is a code point, which represents some particular character; in this case, it represents the character ‘ETHIOPIC SYLLABLE WI’. In informal contexts, this distinction between code points and characters will sometimes be forgotten.

To summarize the previous section: a Unicode string is a sequence of code points, which are numbers from 0 through 0x10FFFF (1,114,111 decimal). This sequence needs to be represented as a set of bytes (meaning, values from 0 through 255) in memory. The rules for translating a Unicode string into a sequence of bytes are called an encoding.

I wonder why we have to encode U+12CA to UTF-8 or UTF-16 instead of saving the binary of 12CA in the disk directly. I think the reason is:

1. Unicode is not Self-synchronizing code, so if

   10 represent A
   110 represent B
   10110 represent C
   

When I see 10110 in the disk we can't tell it's A and B or just C.

2. Unicode uses much more space instead of UTF-8 or UTF-16.

Am I right?

Answer 1

Consider one situation, even if you can directly save unicode binary into disk and close the file, what happens when you open the file again? It's just a bunch of binary, you don't know how many bytes a char occupied right, which means, if '🥶'(U+129398) and 'A' are the content of your file, then if you take it 1 byte for a char, then '🥶' can't be decoded correctly, which takes 2 bytes, then instead 1 emoji you see, you get two, which is U+63862 and U+65536 unicode char.

Answer 2

I wonder why we have to encode U+12CA to UTF-8 or UTF-16 instead of saving the binary of 12CA in the disk directly.

How do you write 12CA to a disk directly? It is a bigger value than a byte can hold, so you need to write at least two bytes. Do you write 12 followed by CA? You just encoded it in UTF-16BE. That's what an encoding is...a definition of how to write an abstract number as bytes.

Other reading:

The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)

Pragmatic Unicode

Answer 3

Read about Unicode, UTF-8 and the UTF-8 everywhere website.

There are more than a million Unicode code-points (you mentionned 1,114,111...). So you need at least 21 bits to be able to separate all of them (since 2²¹ > 1114111).

So you can store Unicode characters directly, if you represent each of them by a wide enough integral type. In practice, that type would be some 32 bits integer (because it is not convenient to handle 3-bytes i.e. 24 bits integers). This is called UCS-4 and some systems or software do already handle their Unicode string in such a format.

Notice also that displaying Unicode strings is quite difficult, because of the variety of human languages (and also since Unicode has combining characters). Some need to be displayed right to left (Arabic, Hebrew, ....), others left to right (English, French, Spanish, German, Russian ...), and some top to down (Chinese, ...). A library displaying Unicode strings should be capable of displaying a string containing English, Chinese and Arabic words.... Then you see that decoding UTF-8 is the easy part of Unicode string displaying (and storing UCS-4 strings won't help much).

But, since English is the dominant language in IT technology (for economical reasons), it is very often cheaper to keep strings in UTF8 form. If most of the strings handled by your system are English (or in some other European language using the Latin alphabet), it is cheaper and it takes less space to keep them in UTF-8.

I guess than when China will become a dominant power in IT, things might change (or maybe not).

^{(I have no idea of the most common encoding used today on Chinese supercomputers or smartphones; I guess it is still UTF-8)}

In practice, use a library (perhaps libunistring or Glib in C), to process UTF-8 strings and another one (e.g. pango and GTK in C) to display them. You'll find many Unicode related libraries in various programming languages.

Answer 4

For good and specific reasons, Unicode doesn't specify any particular encoding. If it makes sense for your scenario, you can specify your own.

Because Unicode doesn't specify any serialization, there is no way to "directly" store Unicode, just like you can't "directly" store a mathematical number or a flow chart to implement a program you designed. The question isn't really well-defined.

There are a number of existing serialization formats (encodings) so it is very likely that it makes the most sense to use an existing one unless your requirements are significantly different than what any existing encoding provides; even then, is it really worth the cost?

A stream of bits is just a stream of bits. Conventionally, we chop them up into groups of 8 and call that a "byte" and the latter half of your question is really "if it's not a byte, how can you tell which bits belong to which symbol?" There are many ways to do that, but the common ones generally define a sequence of some particular length (8, 16, and 32 are often convenient for reasons of compatibility with bus width on modern computers etc) but again, if you really wanted to, you could come up with something different. Huffman trees come to mind as one way to implement a way to communicate a structure of variable length (and is used for precisely that in many compression algorithms).