[swift-users] Optimising Set<String> comparisons

[Tim Vermeulen]

guard a.isSubset(of: b) else { return false }

This should be the most efficient way to do what you’re trying to do.

> Using Swift 3 I have a function that's called extremely frequently and is appearing regularly in Profiler runs. The function takes two Set<String>instances and simply attempts to determine whether all items from Set A are present in Set B (it's important to note that Set B may well contain additional items).
> 
> I've attempted to approach the problem in two ways:
> 
> let diff = a.subtracting(b)
> guard diff.count == 0 else { return false }
> 
> 
> And also by simply iterating over the contents of Set A, like so:
> 
> for item in a {
> if !b.contains(item) {
> return false
> }
> }
> 
> 
> Both ultimately end up spending the majority of their time in String._compareDeterministicUnicodeCollaton(String) ->Int. Which makes sense, given what I'm doing - but ideally I'd like to come up with a more efficient way of performing the above check. Swift's String representation is incredibly robust, but for my needs the strings could be adequately represented in ASCII encoding. I've also considered storing and comparing the hashValue of the strings, to speed up comparisons...
> 
> Hopefully this is an acceptable question for this mailing list. I'm aware that this may not be a Swift-specific question and could well be solved with a more efficient data structure or approach, but I'd really appreciate some input from more experienced Swift developers :-)_______________________________________________
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