Remove booster dependency #49
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gustavosr8
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Sep 6, 2024
| auto hash_t = hasher_t(x.first); | ||
| auto hash_u = hasher_u(x.second); | ||
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| hash_u ^= hash_t + 0x9e3779b9 + (hash_u<<6) + (hash_u>>2); |
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Where did this 0x9e3779b9 value comes from?
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It's the magic value used by boost themselves: https://stackoverflow.com/questions/35985960/c-why-is-boosthash-combine-the-best-way-to-combine-hash-values
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Do you mind to mention it in the commit message? Magic numbers can easily become the reason for a misunderstanding or doubt, IMHO.
gustavosr8
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This avoids depending on boost for anything, which simplifies the list of dependencies. Instead, we inline their hash_combine implementation, including the magic constants, even if it isn't the best possible hash combinator [1]. The hasher class organization was taken from [2]. Since the hash of std::optional<unsigned> should be the same as the hash of unsigned when it contains a value [3], and on libstdc++ and libc++ that hash is simply the identity function [4], it felt more appropriate to use hash_u (the second member of the pair) as the seed, so it gets some additional mixing. [1] https://stackoverflow.com/q/35985960/9512804 [2] https://stackoverflow.com/a/20602159/9512804 [3] https://en.cppreference.com/w/cpp/utility/optional/hash [4] https://tessil.github.io/2016/08/29/benchmark-hopscotch-map.html
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henriquesimoes
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Sep 9, 2024
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