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duplicates:

• SR-9323 KeyPaths are quite slow

Issue Description:

Let's say we have a struct Thing, which has a member number of type Int

I performed two tests designed to convert an array [Thing] to a dictionary [Int : Thing], where the keys are number values for the corresponding struct. Basically, indexing things by number.

The first method performs the indexing by accessing struct members with a closure passed as an argument.

The second method performs the indexing by accessing struct members with a keyPath passed as an argument.

extension Sequence {
   func indexed<V: Hashable>(by callback: (Element) -> V) -> [V: Element] {...}       
   func indexed<V: Hashable>(by keyPath: KeyPath<Element, V>) -> [V : Element] {...}
}

Turns out the keypath method takes nearly double the time to execute for 100000 Things.

indexing array with closure ▿
    Execution time: 0.082 s
    Number of tests: 20

indexing array with keypath ▿
    Execution time: 0.143 s
    Number of tests: 20

This seems slower than expected. I wanted to bring the results of my test to light, as I found it surprising. Perhaps there is room to optimize the implementation of KeyPaths?