[swift-evolution] (Draft) Add last(where:) and lastIndex(where:) methods

[Xiaodi Wu]

If these are to be added, I'd advocate for renaming `index(of:)` and
`index(where:)` to `firstIndex(of:)` and `firstIndex(where:)`, respectively.

On Tue, May 10, 2016 at 13:54 Nate Cook via swift-evolution <
swift-evolution at swift.org> wrote:

> I've needed these in the past and used them in other languages—any
> feedback on this  idea?
> ------------------------------
> Add last(where:) and lastIndex(where:) Methods to Bidirectional
> Collections
>
> The standard library should include methods for finding the last element
> of a bidirectional collection that matches a predicate, along with the
> index of that element.
> Motivation
>
> The standard library currently has (or will soon have) methods that
> perform a linear search from the beginning of a collection to find an
> element that matches a predicate:
>
> let a = [20, 30, 10, 40, 20, 30, 10, 40, 20]
> a.first(where: { $0 > 25 })         // 30
> a.index(of: 10)                     // 2
> a.index(where: { $0 > 25 })         // 1
>
> Unfortunately, there is no such method that searches from the end of a
> bidirectional collection. Finding the last of particular kind of element
> has multiple applications, particularly with text, such as wrapping a long
> string into lines of a maximum length or trimming whitespace from the
> beginning and end of a string.
>
> This limitation can be worked around by using the methods above on the
> reversed collection, but the resulting code is truly dreadful. For example,
> to find the corresponding last index to a.index(where: { $0 > 25 }), this
> unholy incantation is required:
>
> (a.reversed().index(where: { $0 > 25 })?.base).flatMap({ a.index(before: $0) })
>
> Wat.
> Proposed solution
>
> Bidirectional collections should include three new methods for symmetry
> with the existing forward-searching APIs: last(where:), lastIndex(where:),
> and lastIndex(of:), specifically for collections of Equatable elements.
>
> These additions would remove the need for searching in a reversed
> collection and allow code like the following:
>
> a.last(where: { $0 > 25 })          // 40
> a.lastIndex(of: 10)                 // 6
> a.lastIndex(where: { $0 > 25 })     // 7
>
> Much better!
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