[swift-dev] String comparison improvements

[Michael Ilseman]

> On Jan 17, 2018, at 1:46 PM, Itai Ferber via swift-dev <swift-dev at swift.org> wrote:
> 
> Hi Lance,
> 
> I read Michael’s emails but I don’t remember at the moment — what is the new string comparison implementation going to be based on?
> 

It is the lexicographical ordering of NFC-normalized UTF-16 code units. We use ICU to normalize strings, and there are a number of fast paths for scenarios where portions of strings are already in this canonical form.

> Also, how will this affect bridged strings? If I compare two NSStrings, I may get a different result than if I compare the same two strings as bridged through String, correct?
> 

If they have type String (i.e. they were bridged), they will get the new ordering semantics. I’m not familiar with how the type NSString is ordered in corelibs-foundation, do you know? Either way, as a different type, NSString could independently choose different ordering semantics.

> — Itai
> 
> On 17 Jan 2018, at 13:19, Lance Parker via swift-dev wrote:
> 
> Hey Swift-Dev,
> 
> The swift standard library team  have been working on a new implementation for comparing Swift strings for Swift 5. Michael touched on the motivations in the State of String email but I’ll summarize here:
> 
> The Swift String comparison implementations on Apple platforms and Linux differ in results and performance. Apple platforms use CFStringCompare with no locale, while Linux uses ICU libraries. Unifying the algorithms that Swift strings use for comparison is reason alone for doing a new implementation. 
> We've come up with some great common fast paths that speed up comparisons for a lot of common cases. Our microbenchmarks show up to a 6.8x increase in performance and there is still some low hanging fruit in our implementation that would bring further speedups. 
> 
> Bare in mind this is not intended to be a replacement for sorting strings that will be presented to users, for that developers should stick to NSLocalizedString APIs. 
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