This chapter is from the book
This chapter is from the book
This chapter is from the book
Surrogate Characters
Surrogate characters are typically referred to as surrogate pairs. They are the combination of two characters, containing a single code point. To make the detection of surrogate pairs easy, the Unicode standard has reserved the range from U+D800 to U+DFFF for the use of UTF-16. No characters are assigned to code point values in this range. When programs see a bit sequence that falls in this range, they immediately—zip! zip!—know that they have encountered a surrogate pair.
This reserved range is composed of two parts:
- High surrogates—U+D800 to U+DBFF (total of 1,024 code points)
- Low surrogates—U+DC00 to U+DFFF (total of 1,024 code points)
A lone surrogate is invalid in UTF-16; surrogates are always written in pairs, with the high surrogate followed by the low.
With UTF-16 encoding, characters with code points in ranges U+0000 through U+D7FF and U+E000 through U+FFFD are stored as single 16-bit units.
Table 2.6 contains examples of surrogate pairs.
Table 2.6 Examples of Surrogate Pairs
|
Character |
Code Point |
Surrogate Pair |
|
|
U+10000 |
{U+D800, U+DC00} |
|
|
U+10E6D |
{U+D803, U+DE6D} |
|
|
U+1D11E |
{U+D834, U+DD1E} |
|
|
U+10FFFF |
{U+DBFF, U+DFFF} |
The following code snippet shows you how to get a printout of a surrogate pair when you are given its code point:
uniChar characterArray[2]; CFStringGetSurrogatePairForLongCharacter(0x10FFFF, characterArray); NSString *surrogate = [[NSString alloc] initWithCharacters:characterArray length:2]; NSLog(@"Surrogate: %@", surrogate);
Note that this is taking advantage of the CFStringGetSurrogatePairForLongCharacter function, which maps a UTF-32 character to a pair of UTF-16 surrogate characters. We need an array to plug the resulting UTF-16 pair into—that’s what the characterArray is for—and then the initWithCharacters:length: method of NSString does the rest.
Emoji
I’m making a special callout on the Emoji characters because they are extremely popular, and Apple both uses a special font to represent them and provides a keyboard to input just Emoji characters.
Introduced in the late 1990s from a Japanese mobile phone provider, Emoji is the Japanese term for picture characters. Created by Shigetaka Kurita as an effort to retain his company’s customer base, the smiley-faced icons gave their text messages more cuteness. The other supporting factor of the Emoji characters was the ability to give contextual information with a single character. What’s the weather going to be like today? That’s easily presented with a sun or umbrella or cloud Emoji character.
Figure 2.2 shows the first page of the Emoji keyboard.
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Figure 2.2 The Emoji keyboard.
Apple Color Emoji is a font available on both iOS and OS X to provide support for the Unicode Emoji characters. Instead of this font having glyphs with black and white outlines, it has full-color, higher-resolution images for each of the nearly 900 glyphs it supports.
Strong support of Emoji has been a hard target to hit because it has historically occupied a private use area of Unicode with a range of code points from U+1F604 to U+1F539.