如何在Swift中将代理项​​对转换为Unicode标量

Question

The following example is taken from the Strings and Characters documentation :

The values 55357 ( U+D83D in hex) and 56374 ( U+DC36 in hex) are the surrogate pairs that form the Unicode scalar U+1F436 , which is the DOG FACE character. Is there any way to go the other direction? That is, can I convert a surrogate pair into a scalar?

I tried

let myChar: Character = "\u{D83D}\u{DC36}"

but I got an "Invalid Unicode scalar" error.

This Objective C answer and this project seem to be custom solutions, but is there anything built into Swift (especially Swift 2.0+) that does this?

Answer 1

There are formulas to calculate the original code point based on a surrogate pair and vice versa. From https://mathiasbynens.be/notes/javascript-encoding#surrogate-formulae :

Section 3.7 of The Unicode Standard 3.0 defines the algorithms for converting to and from surrogate pairs.

A code point C greater than 0xFFFF corresponds to a surrogate pair <H, L> as per the following formula:

 H = Math.floor((C - 0x10000) / 0x400) + 0xD800 L = (C - 0x10000) % 0x400 + 0xDC00 

The reverse mapping, ie from a surrogate pair <H, L> to a Unicode code point C , is given by:

 C = (H - 0xD800) * 0x400 + L - 0xDC00 + 0x10000 

Answer 2

Given an sequence of UTF-16 code units (ie 16-bit numbers, such as you get from String.utf16 or just an array of numbers), you can use the UTF16 type and its decode method to turn it into UnicodeScalars , which you can then convert into a String .

It's a bit of a grungy item, that takes a generator (as it does stateful processing) and returns an enum that indicates a result (with an associated type of the scalar), or an error or completion. Swift 2.0 pattern matching makes it a lot easier to use:

let u16data: [UInt16] = [0xD83D,0xDC36]
//or let u16data = "Hello, 🌍".utf16

var g = u16data.generate()
var s: String = ""
var utf16 = UTF16()
while case let .Result(scalar) = utf16.decode(&g) {
    print(scalar, &s)
}
print(s) // prints 🐶