swiftlang/swift

[SR-10858] Bridging NSString-keyed Dictionaries should only set allowsDuplicates if there are non-ASCII (non-NFC?) Strings

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#53,248 opened on Jun 7, 2019

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Dictionarygood first issueimprovementstandard library

Description
Previous ID SR-10858
Radar None
Original Reporter @Catfish-Man
Type Improvement
Status In Progress
Resolution
Votes 1
Component/s Standard Library
Labels Improvement, StarterBug
Assignee valeriyvan (JIRA)
Priority Medium

md5: d18df56f9579713f39cb8ff948cf30ca

Issue Description:

Currently we have code like this:

// String and NSString have different concepts of equality, so
// string-keyed NSDictionaries may generate key collisions when bridged
// over to Swift. See rdar://problem/35995647
let handleDuplicates = (Key.self == String.self)
      
result = Dictionary(_unsafeUninitializedCapacity: numElems,
   allowingDuplicates: handleDuplicates) { keys, vals in

This is overly conservative. We only actually need to set allowingDuplicates if there are non-NFC NSStrings!

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.
1-3 hours
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 programmingUnicode normalizationSwift standard library internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
The optimization involves modifying the bridging logic in the Swift standard library where `allowingDuplicates` is set. Search for the code snippet containing ` unsafeUninitializedCapacity` and `allowingDuplicates` in the repository. Understand how NSString equality differs from Swift String equality and how NFC normalization affects key collisions. The issue suggests that `allowingDuplicates` should only be set when there are non ASCII or non NFC strings. Review related comments in the issue for further guidance.