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BAN: Optimize quick sort

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We now use the middle element as the pivot and do three way partitioning
to improve sorting with equivalent elements
This commit is contained in:
Bananymous
2026-05-13 02:03:24 +03:00
parent 5f237abc3b
commit d181f9e553
1 changed files with 39 additions and 18 deletions
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57
BAN/include/BAN/Sort.h
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@@ -21,26 +21,47 @@ namespace BAN::sort
namespace detail
{
template<typename It, typename Comp>
It partition(It begin, It end, Comp comp)
template<typename It>
struct partition_pair
{
It pivot = prev(end, 1);
It lt;
It gt;
};
It it1 = begin;
for (It it2 = begin; it2 != pivot; ++it2)
template<typename It, typename Comp>
partition_pair<It> partition(It begin, It end, Comp comp)
{
It pivot = next(begin, distance(begin, end) / 2);
It lt = begin;
It eq = begin;
It gt = end;
while (eq != gt)
{
if (comp(*it2, *pivot))
if (comp(*eq, *pivot))
{
swap(*it1, *it2);
++it1;
swap(*eq, *lt);
if (pivot == lt)
pivot = eq;
++lt;
++eq;
}
else if (comp(*pivot, *eq))
{
--gt;
swap(*eq, *gt);
if (pivot == gt)
pivot = eq;
}
else
{
++eq;
}
}
swap(*it1, *pivot);
return it1;
return { lt, gt };
}
}
template<typename It, typename Comp = less<it_value_type_t<It>>>
@@ -48,9 +69,9 @@ namespace BAN::sort
{
if (distance(begin, end) <= 1)
return;
It mid = detail::partition(begin, end, comp);
quick_sort(begin, mid, comp);
quick_sort(++mid, end, comp);
const auto [lt, gt] = detail::partition(begin, end, comp);
quick_sort(begin, lt, comp);
quick_sort(gt, end, comp);
}
template<typename It, typename Comp = less<it_value_type_t<It>>>
@@ -85,9 +106,9 @@ namespace BAN::sort
return insertion_sort(begin, end, comp);
if (max_depth == 0)
return heap_sort(begin, end, comp);
It mid = detail::partition(begin, end, comp);
intro_sort_impl(begin, mid, max_depth - 1, comp);
intro_sort_impl(++mid, end, max_depth - 1, comp);
const auto [lt, gt] = detail::partition(begin, end, comp);
intro_sort_impl(begin, lt, max_depth - 1, comp);
intro_sort_impl(gt, end, max_depth - 1, comp);
}
}