[swift-evolution] Collection index complexity and data structures

David Waite david at alkaline-solutions.com
Fri Jun 24 16:05:17 CDT 2016

And never mind me. Not sure how I missed that in the docs that many times.


> On Jun 24, 2016, at 1:38 PM, David Waite via swift-evolution <swift-evolution at swift.org> wrote:
> I noticed that the new Collection index(_:offsetBy:) <http://swiftdoc.org/v3.0/protocol/Collection/#comment-func-index_offsetby_> does not define that negative offsets require a BidirectionalCollection. It also declares that negative offsets must complete in O( | offset | ) time. This differs from other methods such as distance(from:to:) <http://swiftdoc.org/v3.0/protocol/Collection/#comment-func-distance-from_to_>which indicates start <= end if not a BidirectionalCollection
> This would preclude some data structures from strictly implementing the Collection protocol, such as singly-linked lists.
> Would it be appropriate to indicate instead that BidirectionalCollection defines the negative offset behavior and negative offset performance constraint?
> -DW
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