swiftlang/swift

[SR-3396] `reversed` for sequences and forward collections and should return a `ReversedRandomAccessCollection<[Iterator.Element]>`

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#45,984 opened on Dec 12, 2016

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affects ABIgood first issueimprovementstandard libraryswift evolution proposal needed

Description
Previous ID SR-3396
Radar rdar://problem/21098134
Original Reporter @airspeedswift
Type Improvement

Attachment: Download
Votes 0
Component/s Standard Library
Labels Improvement, AffectsABI, StarterProposal
Assignee None
Priority Medium

md5: 34c0ebf025eb58406a81d44f15afed48

Issue Description:

Currently calling reversed() on a bi-directional and random-access collections returns a ReversedCollection and ReversedRandomAccessCollection respectively. This is because the view can be placed over the underlying collection at no cost – it's just a question of transforming the index appropriately and then accessing the underlying base collection.

For sequences and forward-only collections, however, this can't be done. So for these, the contents are read into an array and then that array is in-place reversed. But the second step is unnecessary – the array, once created, could just be returned wrapped in a ReversedRandomAccessCollection. As well as efficiency, this would have the benefit of making these methods more consistent across the board in what they return.

Contributor guide

Tech stack
swift
Domain
performance
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Swift collection protocolsunderstanding of reversed()
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75
Research direction
Examine the current implementation of `reversed()` on Sequence and Collection in the Swift standard library (likely in Collection.swift or similar). The goal is to change the return type for sequences and forward only collections to `ReversedRandomAccessCollection` instead of an array. Ensure to handle the case where the sequence is already an array by directly reversing it. Write tests to verify the new behavior and ensure performance improvement. Consult the Swift evolution process for proposals if needed.
[SR-3396] `reversed` for sequences and forward collections and should return a `ReversedRandomAccessCollection<[Iterator.Element]>` · swiftlang/swift#45984 | Good First Issue