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CS253 Unicode

Unicode (ISO-10646)

• First published 1991
• Version 13.0, published March 2020, has 143,924 characters.
• Incorporates ASCII as code points 0–127, no change.
• Incorporates ISO-8859-1 (Latin-1) as code points 0–255, no change.
• Code points, not encoding (patience)
  • U+0041 A LATIN CAPITAL LETTER A
  • U+2FC2 ⿂ KANGXI RADICAL FISH
  • U+1F355 🍕 SLICE OF PIZZA
• Meaning, not pictures

U+ notation

By convention, Unicode code points are represented as U+ followed by at four (more only if needed) upper-case hexadecimal digits.

RIGHT SQUARE BRACKET is written U+005D, not U+5D. You could also call it Unicode character #93, but don’t.

What’s in Unicode

All Unicode “blocks”: https://unicode.org/Public/UNIDATA/Blocks.txt

Code Points

Initially, Unicode is all about mapping integers to characters:

Now, do that for 143,000+ more characters.

Encoding

Fine, so we’ve defined this mapping. How do we actually represent those in a computer, in memory, or in a file on a disk? That’s the job of an encoding. An encoding is a mapping of the bits in an integer code point to bytes.

The difference is essential:

code point

mapping integers to characters, without caring about how big the integers get. This is not a computer problem. Bits & bytes have nothing to do with it. This is strictly a bookkeeping task, to be handled by a bureaucracy. It’s just a big list.

encoding

how to take those integer code points to bytes in a computer. For ASCII (0–127), the mapping is simple, since an 8-bit byte can contain 2⁸=256 different values.

16-bit Encodings

• UCS-2:
  • Fixed-length 16-bit.
  • Each character is two 8-bit bytes, whether in memory, or on a disk.
  • Certainly is straightforward.
  • Inadequate for modern Unicode, which has many more than 2¹⁶ characters. Can’t even represent U+1F554 🕔 CLOCK FACE FIVE OCLOCK.
  • Unicode originally had a much more modest scope, only living languages, so that might have worked.

     ····J···· ····a···· ····c···· ····k····
    ┌────┬────┬────┬────┬────┬────┬────┬────┐
    │ 00 │ 4A │ 00 │ 61 │ 00 │ 63 │ 00 │ 6B │
    └────┴────┴────┴────┴────┴────┴────┴────┘
      0    1    2    3    4    5    6    7

Endian

int n = * (int *) "\x11\x22\x33\x44";
switch (n) {
  case 0x11223344: cout << "Big";     break;
  case 0x44332211: cout << "Little";  break;
  case 0x22114433: cout << "Middle";  break;
  default:         cout << "Unknown"; break;
}
cout << "-endian\n"
     << "11 22 33 44 makes " << hex << n;

Little-endian
11 22 33 44 makes 44332211

16-bit Encodings

UTF-16:

• Slightly variable-length: values ≤ U+FFFF take two bytes, other values take four bytes.
• Consider U+203D ‽ INTERROBANG:
  • UTF-16BE (big-endian): bytes are 20 3D
  • UTF-16LE (little-endian): bytes are 3D 20
• For values ≥ U+10000 and < U+10FFFF:
  • Subtract out 0x10000, yielding a 20-bit number
  • Emit U+D800 plus the top ten bits.
  • Emit U+DC00 plus the lower ten bits.
  • There are no valid code points U+D800…U+DFFF.
• 100% overhead for ASCII text.

32-bit Encodings

UTF-32:

• straightforward rendering of the code point in binary, with the same problems about byte order:
  • UTF-32BE: big-endian version
  • UTF-32LE: little-endian version
• 300% overhead for ASCII text.
  • Sure, disk space is cheap, but, c’mon.

  ········J········   ········a········   ········c········   ········k········
┌────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┐
│ 00 │ 00 │ 00 │ 4A │ 00 │ 00 │ 00 │ 61 │ 00 │ 00 │ 00 │ 63 │ 00 │ 00 │ 00 │ 6B │
└────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┘
   0    1    2    3    4    5    6    7    8    9   10   11   12   13   14   15

False Positives

Hey, there’s a slash in this string! No, wait, there isn’t.

• U+002F / SOLIDUS
• U+262F ☯ YIN YANG

When using UTF-16 or UTF-32 encoding, a naïve algorithm will falsely detect a slash (oh, excuse me, a solidus) in one of the bytes of U+262F.

Similarly, a C-string cannot hold a UTF-16 or UTF-32 string, because of the embedded zero bytes.

UTF-8 Variable-Length Encoding

• ASCII only appears when intended; NUL and slash aren’t in other byte sequences.
• Therefore, most programs never notice.
• Find previous/next char are fast operations.
• Self-synchronizing: if bytes are damaged, finding next/previous character is easy.
• 0% overhead for ASCII text.

    J    a    c    k
  ┌────┬────┬────┬────┐
  │ 4A │ 61 │ 63 │ 6B │
  └────┴────┴────┴────┘
    0    1    2    3
  

Illustration of Various Encodings

Example

Byte Order Mark

Often, files contain a “magic number”—initial bytes that indicate file type.

The character U+FEFF ZERO WIDTH NO BREAK SPACE, is also used as a Byte Order Mark, or BOM. When used as the first bytes of a data file, indicates the encoding (assuming that you’re limited to Unicode).

If not processed as a BOM, then ZERO WIDTH NO BREAK SPACE is mostly harmless.

Programming

It’s all about bytes vs. characters. Too many languages have no byte type, so programmers use char instead. Trouble! The language has no idea whether you’re processing text, which should be treated as Unicode, or bytes of data, which would happen if a program were parsing a JPEG file.

cout << hex << 'jack';

c.cc:1: warning: multi-character character constant
6a61636b

cout << hex << "ñ" << 'ñ';

c.cc:1: warning: multi-character character constant
ñc3b1

for (unsigned char c : "abcñ")
    cout << hex << int(c) << ' ';

61 62 63 c3 b1 0 

Linux Commands

echo \u: up to four digits; \U: up to eight digits

    % echo -e '\uf1'
    ñ
    % echo -e '\U1f435'
    🐵

wc -c counts bytes; wc -m counts characters

    % echo -e '\U1f435' | wc -c
    5
    % echo -e '\U1f435' | wc -m
    2

Viewing Files

View with xxd or od:

    % echo -e 'ABC' | xxd
    00000000: 4142 430a                                ABC.
    % echo -e '\U1f435' | xxd
    00000000: f09f 90b5 0a                             .....

    % echo -e 'ABC' | od -t x1
    0000000 41 42 43 0a
    0000004
    % echo -e '\U1f435' | od -t x1
    0000000 f0 9f 90 b5 0a
    0000005