Appending to an Array stored in an enum case payload: O(1) or O(n)?

[lukasa (Cory Benfield)]

@taylorswift

what’s the preferred way of appending to the stored Array s? it seems like the naïve way of switch ing, appending, and writing back to self would be O(n), not O(1) due to copy-on-write, which would make populating the arrays O(n2) overall.

This should optimize fine. Using Godbolt.org as an example, you can see that while there is an ARC operation, that retain is on the String argument, not on the array of entries. As this doesn't +1 the array of entries, it doesn't inherently cause a CoW.

@taylorswift

the best idea i’ve been able to come up with is overwriting self beforehand with an “empty” case to deinitialize the original storage and bring the Array reference count back down to 1. but this would cause more ARC traffic, and only works if the data model has a logical “empty” value.

In older versions of Swift the compiler was not so smart, and so we (SwiftNIO) did use exactly this pattern by having a deliberately uninhabited case called _modifying or similar. This is largely unnecessary now.