swiftlang/swift

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

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#45,984 创建于 2016年12月12日

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affects ABIgood first issueimprovementstandard libraryswift evolution proposal needed

描述
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.

贡献者指南

技术栈
swift
领域
performance
议题类型
performance
难度面向新贡献者的预计实现难度，1 表示很小改动，5 表示专家级工作。
3
预计时间有经验贡献者完成调查、实现、测试并准备 pull request 的粗略时间范围。
half day
活动状态议题当前的可参与程度：新鲜、活跃、陈旧、阻塞或等待维护者输入。
stale
清晰度议题是否清楚说明期望改动、验收标准和下一步。
clear
前置要求
Swift collection protocolsunderstanding of reversed()
新手友好度1-100 的估计分数，表示该议题对首次贡献者的友好程度。
75
研究方向
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.