O(n) replacement in NSString.replaceOccurrances

Author: gmilos

Foundation/NSString.swift

@@ -1423,10 +1423,20 @@ extension NSMutableString {
 
         if let findResults = CFStringCreateArrayWithFindResults(kCFAllocatorSystemDefault, _cfObject, target._cfObject, CFRange(searchRange), options._cfValue(true)) {
             let numOccurrences = CFArrayGetCount(findResults)
+            let start = _storage.utf16.startIndex
+            var newStorage = Substring()
+            var sourceStringCurrentIndex = _storage.startIndex
             for cnt in 0..<numOccurrences {
-                let rangePtr = CFArrayGetValueAtIndex(findResults, backwards ? cnt : numOccurrences - cnt - 1)
-                replaceCharacters(in: NSRange(rangePtr!.load(as: CFRange.self)), with: replacement)
+                let rangePtr = CFArrayGetValueAtIndex(findResults, backwards ? numOccurrences - cnt - 1 : cnt)
+                let range = NSRange(rangePtr!.load(as: CFRange.self))
+                let matchStartIndex = _storage.utf16.index(start, offsetBy: range.location).samePosition(in: _storage)!
+                let matchEndIndex = _storage.utf16.index(start, offsetBy: range.location + range.length).samePosition(in: _storage)!
+                newStorage += _storage[sourceStringCurrentIndex..<matchStartIndex]
+                newStorage += replacement
+                sourceStringCurrentIndex = matchEndIndex
             }
+            newStorage += _storage[sourceStringCurrentIndex..<_storage.endIndex]
+            _storage = String(newStorage)
             return numOccurrences
         } else {
             return 0