On Tue, Aug 8, 2017 at 11:51 PM, Taylor Swift via swift-users < swift-users@swift.org> wrote:
> > > On Wed, Aug 9, 2017 at 12:29 AM, Félix Cloutier via swift-evolution < > swift-evolut...@swift.org> wrote: > >> Yes, exactly. An Array<T> is a struct wrapper for a reference type >> representing storage. Mutating functions first check if they own the only >> reference to the storage using isKnownUniquelyReferenced >> <https://developer.apple.com/documentation/swift/2429905-isknownuniquelyreferenced>. >> If not, they make a fresh copy before applying the mutating operation. >> >> There's no difference for `let` arrays. Access control is enforced at >> compile-time through Array's design: the compiler will prevent you from >> calling `mutating` functions on `let` structs, and Array is careful to not >> expose functionality that could modify its storage outside of `mutating` >> functions. >> >> There is no secret. Anyone could implement the same thing only using >> publicly available and documented compiler features. In fact, it's been >> done already for some very powerful collections >> <https://github.com/lorentey/BTree>. >> > > This isn’t entirely true. That BTree module readme seems to contain a lot > of unsubstantiated hyperbole. It’s possible to implement a classic > red-black tree in Swift that performs better than a sorted Array, down to > about *n* = 1,500 items, not *n* = *100,000* items as it claims. > (Actually, heap allocators these days are good enough that performance is > on par with Array all the way down to *n* = 1.) Red-Black trees are slow > when *distributed* as packages because of the crossmodule optimization > boundary. (This also means the BTree module is much slower than Array for > most reasonable *n*.) It’s possible to write modules using compiler > attributes that mitigate this slowdown (reclaiming over 50% of lost > performance) but it’s hacky and forces you to design your libraries like > the standard library (meaning: ugly underscored properties everywhere and > everything is public). And these features aren’t “publicly available” or > documented at all. > This seems harsh. I didn't notice Félix making any claims about BTree's performance. The necessary API for implementing COW is indisputably public and documented: https://developer.apple.com/documentation/swift/2429905-isknownuniquelyreferenced
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