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adjusted hysteresis factor in unchecked_emplace_with_rehash

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joaquintides
2022-10-29 12:33:19 +02:00
parent 11a2f42714
commit 51a2c434bc
1 changed files with 7 additions and 3 deletions
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10
include/boost/unordered/detail/foa.hpp
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@@ -1654,11 +1654,15 @@ private:
/* Due to the anti-drift mechanism (see recover_slot), new_arrays_ may be /* Due to the anti-drift mechanism (see recover_slot), new_arrays_ may be
* of the same size as the old arrays; in the limit, erasing one element at * of the same size as the old arrays; in the limit, erasing one element at
* full load and then inserting could bring us back to the same capacity * full load and then inserting could bring us back to the same capacity
* after a costly rehash. We introduce a 10% level of hysteresis to avoid * after a costly rehash. To avoid this, we jump to the next capacity level
* that (the size_/10 addendum). * when the number of erased elements is <= 10% of total elements at full
* load, which is implemented by requesting additional F*size elements,
* with F = P * 10% / (1 - P * 10%), where P is the probability of an
* element having caused overflow; P has been determined to be ~0.187 via
* simulation, yielding F ~ 0.019 ~ 1/52.
*/ */
auto new_arrays_=new_arrays(std::size_t( auto new_arrays_=new_arrays(std::size_t(
std::ceil(static_cast<float>(size_+size_/10+1)/mlf))); std::ceil(static_cast<float>(size_+size_/52+1)/mlf)));
iterator it; iterator it;
BOOST_TRY{ BOOST_TRY{
/* strong exception guarantee -> try insertion before rehash */ /* strong exception guarantee -> try insertion before rehash */