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For more reading on testing, check out Unit Tests Don't Prevent Bugs .

Of course, that particular map just covers the characters that might map to "e", and we'd have to do this before we applied the ASCII rule. But application would look something like this:

Of course, that particular map just covers the characters that might map to "e", and we'd have to do this before we applied the ASCII rule. But implementation would look something like this:

As an addendum, one thing that might be neat to add to the StringFormatter struct would be a means for mapping certain characters to be replaced with specified characters. So, for example, you're not going to allow ASCII, but you could potentially replace è with a "regular" e.

So we might add a property to the struct that looks like this:

var replacementMap = [Character: Character]()

So it's a dictionary where the key is the character we want to remove and the value is the character we want to replace it with.

stringFormatter.replacementMap = [
 "è" : "e",
 "é" : "e",
 "ê" : "e",
 "ë" : "e",
 "ē" : "e",
 "ė" : "e",
 "ę" : "e"
]

Of course, that particular map just covers the characters that might map to "e", and we'd have to do this before we applied the ASCII rule. But application would look something like this:

private func stringByApplyingReplacementMap(string: String) -> String {
 return String(string.characters.map { return self.replacementMap[0ドル] ?? 0ドル })
}