Replace std::hash with our custom hash function #2952
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Codecov ReportAttention: Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## master #2952 +/- ##
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Coverage 93.51% 93.51%
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Files 1121 1121
Lines 42913 42919 +6
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+ Hits 40131 40137 +6
Misses 2782 2782 ☔ View full report in Codecov by Sentry. |
benjaminwinger
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ray6080
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Feb 27, 2024
| @@ -133,31 +134,52 @@ inline void Hash::operation( | |||
| template<> | |||
| inline void Hash::operation( | |||
| const double& key, common::hash_t& result, common::ValueVector* /*keyVector*/) { | |||
| result = std::hash<double>()(key); | |||
| // 0 and -0 are not byte-equivalent, but should have the same hash | |||
| if (key == 0) { | |||
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I wonder if adding 0.0 can also solve the "0 and -0 are not byte-equivalent" problem without an if/else branch.
auto newKey = key + 0.0;
result = murmurhash64(*reinterpret_cast<const uint64_t*>(&newKey));
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That does appear to also work.
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@benjaminwinger I merged this, so it can be in our dev build to be verified by the bug reporter. |
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std::hash is implementation dependent and not guaranteed to be the same between different stdlibs, which means our current hash index layout is not always portable.
std::hashalso apparently is not required to produce the same result for the same input in different runs of the same program, and while there hasn't been any evidence of that yet, it could cause issues in future.From https://en.cppreference.com/w/cpp/utility/hash:
Fixes #2943.
I did a quick benchmark on a long string (first five paragraphs of lorem ipsum), and this took 123ns/iteration on average, compared to 117ns/iteration with std::hash, which is fairly reasonable.