Validate Unicode code point in Swift

Question

I'm writing a routine in Swift that needs to try to convert an arbitrary integer to a UnicodeScalar or return an error. The constructor UnicodeScalar(_:Int) does the job for valid code points, but it crashes when passed integers that are not valid code points.

Is there a Swift (or Foundation) function I can call to pre-flight that an integer i is a valid Unicode code point and won't cause UnicodeScalar(i) to crash?

Answer 1

Update for Swift 3:

UnicodeScalar has a failable initializer now which verifies if the given number is a valid Unicode code point or not:

if let unicode = UnicodeScalar(0xD800) {
    print(unicode)
} else {
    print("invalid")
}

---

(Previous answer:) You can use the built-in UTF32() codec to check if a given integer is a valid Unicode scalar:

extension UnicodeScalar {
    init?(code: UInt32) {
        var codegen = GeneratorOfOne(code) // As suggested by @rintaro
        var utf32 = UTF32()
        guard case let .Result(scalar) = utf32.decode(&codegen) else {
            return nil
        }
        self = scalar
    }
}

(Using ideas from https://stackoverflow.com/a/24757284/1187415 and https://stackoverflow.com/a/31285671/1187415.)

Answer 2

The Swift documentation states

A Unicode scalar is any Unicode code point in the range U+0000 to U+D7FF inclusive or U+E000 to U+10FFFF inclusive.

The UnicodeScalar constructor does not crash for all values in those ranges in Swift 2.0b4. I use this convenience constructor:

extension UnicodeScalar {
    init?(code: Int) {
        guard (code >= 0 && code <= 0xD7FF) || (code >= 0xE000 && code <= 0x10FFFF) else {
            return nil
        }
        self.init(code)
    }
}