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[Stdlib] Optimize CollectionType.first #825

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Jan 1, 2016
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[Stdlib] Optimize CollectionType.first #825

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7 changes: 6 additions & 1 deletion stdlib/public/core/Collection.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -262,7 +262,12 @@ extension CollectionType {
///
/// - Complexity: O(1)
public var first: Generator.Element? {
return isEmpty ? nil : self[startIndex]
// NB: Accessing `startIndex` may not be O(1) for some lazy collections,
// so instead of testing `isEmpty` and then returning the first element,
// we'll just rely on the fact that the generator always yields the
// first element first.
var gen = generate()
return gen.next()
}

/// Returns a value less than or equal to the number of elements in
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