libstore/derivations: speed up DerivationOutput JSON parsing #14871
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Radvendii
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Jan 1, 2026
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I like this change a lot. Thanks for putting it together! I'm too tired tongiht, but when I get a chance I'll test to confirm the speedup on my machine.
| auto [method, hashAlgo] = methodAlgo(); | ||
| return DerivationOutput::Impure{ | ||
| .method = std::move(method), | ||
| .hashAlgo = hashAlgo, |
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This should presumably also be std::move(hashAlgo). I realise this is unrelated to the main change of this PR
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That's an enum, so that's pointless.
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adl_serializer<DerivationOutput> previously built a std::set<std::string_view> of keys and compared against multiple temporary std::sets to select the output variant. Replaced this with a json.size() switch and direct key presence checks (optionalValueAt(...)) to avoid per-parse allocations and tree balancing.
The measured speedup is typically around 25%-80%, with Deferred case being 350% faster.
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