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draft: Improve map performance through unsafe uninitialized capacity #83297

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draft: Improve map performance through unsafe uninitialized capacity #83297

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2e6a661
change map implementation to use unsafeUninitializedCapacity array in…
JaapWijnen Jul 24, 2025
de689fd
Update stdlib/public/core/Collection.swift
JaapWijnen Jul 25, 2025
36eddee
Update stdlib/public/core/Collection.swift
JaapWijnen Jul 25, 2025
3b759d2
Update Collection.swift
JaapWijnen Jul 26, 2025
4d2bb42
Apply suggestions from code review
glessard Jul 28, 2025
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24 changes: 12 additions & 12 deletions stdlib/public/core/Collection.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -1199,18 +1199,18 @@ extension Collection {
return []
}

var result = ContiguousArray<T>()
result.reserveCapacity(n)

var i = self.startIndex

for _ in 0..<n {
result.append(try transform(self[i]))
formIndex(after: &i)
}

_expectEnd(of: self, is: i)
return Array(result)
return try unsafe Array(ContiguousArray<T>(unsafeUninitializedCapacity: n) { (
storage: inout UnsafeMutableBufferPointer<T>,
initializedCount: inout Int
) throws(E) -> Void in
var collectionIndex = self.startIndex
for bufferIndex in storage.indices {
unsafe storage.initializeElement(at: bufferIndex, to: try transform(self[collectionIndex]))
formIndex(after: &collectionIndex)
initializedCount += 1
}
_expectEnd(of: self, is: collectionIndex)
})
}

// ABI-only entrypoint for the rethrows version of map, which has been
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