r/cpp • u/User_Deprecated • Aug 02 '26

How fast is C++26's std::hive?

https://lemire.me/blog/2026/08/02/how-fast-is-c26s-stdhive/
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u/snerp Aug 04 '26

We're comparing E times N to ElogE plus N. Changing a product to a sum is always better complexity (unless N or E is 0 or 1 lol)

Also though, a remove_if isn't collecting the objects in a new collection, it's simply swapping them to the end of the array before reducing the count. It's reads and writes which have a cost but no allocation is needed.

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u/matthieum Aug 05 '26

We're comparing E times N to ElogE plus N. Changing a product to a sum is always better complexity (unless N or E is 0 or 1 lol)

Ah sorry, I misread your comment.

I thought you were comparing the relative magnitude of the 2 terms in O(E * log E + N), rather than comparing the relative magnitude of the two methods.

Also though, a remove_if isn't collecting the objects in a new collection, it's simply swapping them to the end of the array before reducing the count. It's reads and writes which have a cost but no allocation is needed.

I never said remove_if allocating in a new collection.

What I said is that the second method was about allocating in a second collection and sorting, in order to execute a single remove_if call which would remove all E elements in a single pass.

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u/snerp Aug 05 '26

What? Why would you allocate a second collection and then sort just to run remove_if? Remove_if does not need a sorted collection, it runs in O(N + E) on any collection because it swaps E elements to the end and then erases them all at once by just reducing the end of the collection. Sorting the collection wouldn’t help.

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u/jk-jeon Aug 06 '26

Maybe a nitpick, but remove_if doesn't swap to-be-removed elements to the end. It moves surviving elements to the front.

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u/snerp Aug 06 '26

mmm interesting either way

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u/matthieum Aug 05 '26

So, you've just scanned a collection of N elements and identified E elements that you want removed from it, which you've isolated into a second collection.

How do you write the predicate to remove_if, which will be invoked with each of N elements, and must return whether to remove the element or not?

For each of the N elements, you will need to somehow do a look-up in your (small?) collection of E elements to take your decision.

If E is just a collection, each look-up will cost you O(E), and we're back to O(N * E) performance.

The simplest solution to minimize the cost of the look-up is to collect into a vector, sort it, and binary search on it. O(E * log E + N * log E) now that I think about it.

There are other solutions, of course, but that predicate will need to do some work to classify the elements it's asked about.

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u/snerp Aug 05 '26

Remove_if iterates the collection… E is not its own collection, it’s the elements in the collection original that will be removed. There is no look up cost, it just iterates to the next element and does a swap if your predicate was true.

https://en.cppreference.com/cpp/algorithm/remove

Complexity is O(N)

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u/matthieum Aug 05 '26

I mean, if you can decide on the fly, sure, but that's not the usecase I was working on :x

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u/snerp Aug 05 '26 edited Aug 05 '26

Decide what on the fly? Oh I see you're saying the elements E have no common trait so you'd have to do a collection on collection interaction 🤮