implement a hash function for Enums #30500
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andyferris
reviewed
Dec 25, 2018
Co-Authored-By: KristofferC <kristoffer.carlsson@chalmers.se>
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I rewrote this slightly to make it a bit easier to understand (IMO). This should be good to go now. cc @JeffBezanson if you want to take another look. |
jmert
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May 21, 2022
Duplicates improvement made to Base's Enums: JuliaLang/julia#30500
jmert
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Duplicates improvement made to Base's Enums: JuliaLang/julia#30500 **Before:** ```julia julia> @bitflag Flag1 flag1=0 flag2 flag3 flag4 julia> @benchmark hash(flag1) BenchmarkTools.Trial: 10000 samples with 1000 evaluations. Range (min … max): 11.035 ns … 52.801 ns ┊ GC (min … max): 0.00% … 0.00% Time (median): 15.714 ns ┊ GC (median): 0.00% Time (mean ± σ): 16.003 ns ± 1.305 ns ┊ GC (mean ± σ): 0.00% ± 0.00% ▁▃ █▁▄ ▁ ▂▂▂▁▁▁▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▄▂▃▄██▆███▃▃▂▂▂▂▆▅█▆▆█▅▃▂ ▃ 11 ns Histogram: frequency by time 17.5 ns < Memory estimate: 0 bytes, allocs estimate: 0. ``` **After:** ```julia julia> @benchmark hash(flag1) BenchmarkTools.Trial: 10000 samples with 1000 evaluations. Range (min … max): 1.397 ns … 33.455 ns ┊ GC (min … max): 0.00% … 0.00% Time (median): 4.819 ns ┊ GC (median): 0.00% Time (mean ± σ): 4.907 ns ± 1.042 ns ┊ GC (mean ± σ): 0.00% ± 0.00% ██ ▅▅ ▂▁▁▁▁▂▂▂▁▁▁▁▂▃▂▁▁▁▁▂▂▁▁▁▂▃▃▂▁▁▁▂▂▂▁▁▁▁▂▂▂▁▁▃███▆▁▂▄███▃▁▂▂ ▃ 1.4 ns Histogram: frequency by time 5.66 ns < Memory estimate: 0 bytes, allocs estimate: 0. ```
jmert
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May 21, 2022
Duplicates improvement made to Base's Enums: JuliaLang/julia#30500 **Before:** ```julia julia> @bitflag Flag1 flag1=0 flag2 flag3 flag4 julia> @benchmark hash(flag1) BenchmarkTools.Trial: 10000 samples with 1000 evaluations. Range (min … max): 11.035 ns … 52.801 ns ┊ GC (min … max): 0.00% … 0.00% Time (median): 15.714 ns ┊ GC (median): 0.00% Time (mean ± σ): 16.003 ns ± 1.305 ns ┊ GC (mean ± σ): 0.00% ± 0.00% ▁▃ █▁▄ ▁ ▂▂▂▁▁▁▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▄▂▃▄██▆███▃▃▂▂▂▂▆▅█▆▆█▅▃▂ ▃ 11 ns Histogram: frequency by time 17.5 ns < Memory estimate: 0 bytes, allocs estimate: 0. ``` **After:** ```julia julia> @benchmark hash(flag1) BenchmarkTools.Trial: 10000 samples with 1000 evaluations. Range (min … max): 1.397 ns … 33.455 ns ┊ GC (min … max): 0.00% … 0.00% Time (median): 4.819 ns ┊ GC (median): 0.00% Time (mean ± σ): 4.907 ns ± 1.042 ns ┊ GC (mean ± σ): 0.00% ± 0.00% ██ ▅▅ ▂▁▁▁▁▂▂▂▁▁▁▁▂▃▂▁▁▁▁▂▂▁▁▁▂▃▃▂▁▁▁▂▂▂▁▁▁▁▂▂▂▁▁▃███▆▁▂▄███▃▁▂▂ ▃ 1.4 ns Histogram: frequency by time 5.66 ns < Memory estimate: 0 bytes, allocs estimate: 0. ```
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Going via the fallback
object_idis slow and I have also seen it make my hash function start to allocate when implementinghashfor structs containing enums.This will create hash-collisions if different enums with the same name are used in the same hashmap but that seems quite unlikely(?). Suggestions for better implementations are welcome.