[swift-evolution] [Proposal] Formalized Ordering, take 2

[David Sweeris]

> On Jul 25, 2016, at 2:28 PM, Pyry Jahkola via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On 25 Jul 2016, at 21:23, Nevin Brackett-Rozinsky <nevin.brackettrozinsky at gmail.com <mailto:nevin.brackettrozinsky at gmail.com>> wrote:
>> 
>> My one question is, will I be able to write `someCollection.sort(.ascending)` and get the expected result? (This could be an additive future direction.)
> 
> 
> To sort to ascending order, you'd still just call `.sort()` with no arguments.
> 
> To limit the scope of this proposal somewhat, I moved the introduction of new sorting functions into the Future directions section. All of those changes are additive in a way or another:
> 
> 1. The default .sort() function would use `<=>` instead of `<`.
> 
> (a) On the one hand, we'd get improved performance in cases like Array<String>.sort() where doing another `<` comparison would be more costly.
> (b) On the other hand, we'd get well-defined behaviour when sorting Double arrays containing NaNs, because all non-NaN values would be sorted into one contiguous subrange of the result.

Two quick questions… 
1) What about adding an `isInvalid` case? That way types (such as Double/Float) which have “unordered” values can check for an invalid comparison.
2) What about making `Ordering` a tuple instead of an enum? “typealias Ordering = (isLessThan: Bool, isEqual: Bool, isGreaterThan: Bool, isInvalid: Bool)”? It’d take up more space on the stack, but I think this:
func < (lhs: Int, rhs: Int) -> Bool {
    return (lhs <=> rhs).isLessThan
}
or even this:
func < (lhs: Int, rhs: Int) -> Bool {
    let ord =  (lhs <=> rhs).isLessThan
    return ord.isLessThan && !ord.isInvalid
}

would execute faster than this:
func < (lhs: Int, rhs: Int) -> Bool {
    switch (lhs <=> rhs) {
    case lessThan: return true
    default: return false
}

because there wouldn’t be any branching beyond what’s necessary to do the actual comparison.

- Dave Sweeris
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