Splitting strings by a separator string in Swift

StringSplit.swift

import Foundation

extension String {
    public func split(separator: String) -> [String] {
        if separator.isEmpty {
            return map(self) { String($0) }
        }
        if var pre = self.rangeOfString(separator) {
            var parts = [self.substringToIndex(pre.startIndex)]
            while let rng = self.rangeOfString(separator, range: pre.endIndex..<endIndex) {
                parts.append(self.substringWithRange(pre.endIndex..<rng.startIndex))
                pre = rng
            }
            parts.append(self.substringWithRange(pre.endIndex..<endIndex))
            return parts
        } else {
            return [self]
        }
    }
}

// Examples:
"".split(", ")             //=> [""]
"a".split(", ")            //=> ["a"]
"a, b, c, def".split(", ") //=> ["a", "b", "c", "def"]

"foobar".split("o") //=> ["f", "", "bar"]
"foo".split("foo")  //=> ["", ""]

// Special case: splitting by empty separator gets the list of glyphs as Strings
"".split("")             //=> [], notably different from "".split(s) with any other separator s
"a".split("")            //=> ["a"]
"abcd".split("")         //=> ["a", "b", "c", "d"]
"O\u{305}saka".split("") //=> ["O̅", "s", "a", "k", "a"], works with combining diacriticals

Interestingly, rangeOfString needs Foundation imported. The String struct is clearly still missing a lot of useful members in the standard library.

This seems to cause a runtime error with Swift 1.2 in Xcode 6.3 beta 1 on line 12, but I'm not sure why.