A function to find the offsets of newlines ('\n') in UTF-16 encoded string. Try as I might, I cannot get a Swift version within an order of magnitude of the C++ version. Both routines must return arrays of the same size and with equal elements.

LineEndsFind.mm

#import <Foundation/Foundation.h>
#import <vector> // Needed for gist to compile.
#pragma mark - Pure Implementation Functions
const static unichar kUTF16Newline = (unichar)'\n'; // old naming habits die hard!
/**
 * Calculates an array of line end "positions" for a given string.
 * The equivalent Swift function was `(String) -> [Int]` or `(NSString) -> [Int]`
 *
 * In this context a "position" is the zero-based index of a newline
 * character in the string as if it were an array of UTF-16 codepoints.
 *
 * @param s the string.
 * @return: an array of newline positions.
 */
std::vector<size_t> LineEndsFind(NSString* s) {
    assert(s);

    std::vector<size_t> lineEnds;
    unichar *const start = (unichar *)[s cStringUsingEncoding:NSUTF16StringEncoding];
    unichar *current = start;
    while (*current != 0) {
        unichar c = *current++;
        if (c == kUTF16Newline) {
            lineEnds.push_back(current - start);
        }
    }
    return lineEnds;
}

LineEndsFind.swift

    /// This is a first, very Objective-C like version of
    /// `makeLineEndArray`.
    /// 
    public class func makeLineEndArray(string: NSString) -> [Int] {
        precondition(string.length != 0)
        var lineEnds = [Int]()
        for i in 0..<string.length {
            let c = string.character(at: i)
            if c == 10 {
                lineEnds.append(i)
            }
        }
        print("From swift: (returning line ends) lineEndsCount = \(lineEnds.count)")
        return lineEnds
    }

LineEndsFind2.swift

    /// 2nd more Swifty attempt UTF16 view.
    /// Didn't return just for testiing
    public class func makeLineEndArray2(string: String) {
        precondition(!string.isEmpty)
        var lineEnds = [Int]()

        for i in 0..<string.utf16.count {
            if string.utf16[String.UTF16View.Index(i)] == 10 {
                lineEnds.append(i)
            }
        }
        print("From swift: lineEndsCount = \(lineEnds.count)")
    }