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AK: Make single pivot quick_sort guarantee a max stack depth of log(n)

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- The change to quick_sort requires SimpleIterator to support
  assignment.
- Rename quick_sort to single_pivot_quick_sort to make it easier
  to choose a specific implementation (not based on signature).
- Ensure single_pivot_quick_sort does not copy the pivots
- Expand sorts_without_copy test case to cover both single and dual
  pivot implementations.
This commit is contained in:
Mart G 2021-01-31 13:13:49 +01:00 • committed by Andreas Kling
parent 40da077f6c
commit 9068398f6b
3 changed files with 105 additions and 19 deletions
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51
AK/QuickSort.h
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@ -86,35 +86,50 @@ void dual_pivot_quick_sort(Collection& col, int start, int end, LessThan less_th
}
template<typename Iterator, typename LessThan>
void quick_sort(Iterator start, Iterator end, LessThan less_than)
void single_pivot_quick_sort(Iterator start, Iterator end, LessThan less_than)
{
int size = end - start;
if (size <= 1)
return;
for (;;) {
int size = end - start;
if (size <= 1)
return;
int pivot_point = size / 2;
auto pivot = *(start + pivot_point);
int pivot_point = size / 2;
if (pivot_point)
swap(*(start + pivot_point), *start);
if (pivot_point)
swap(*(start + pivot_point), *start);
auto&& pivot = *start;
int i = 1;
for (int j = 1; j < size; ++j) {
if (less_than(*(start + j), pivot)) {
swap(*(start + j), *(start + i));
++i;
int i = 1;
for (int j = 1; j < size; ++j) {
if (less_than(*(start + j), pivot)) {
swap(*(start + j), *(start + i));
++i;
}
}
swap(*start, *(start + i - 1));
// Recur into the shorter part of the remaining data
// to ensure a stack depth of at most log(n).
if (i > size / 2) {
single_pivot_quick_sort(start + i, end, less_than);
end = start + i - 1;
} else {
single_pivot_quick_sort(start, start + i - 1, less_than);
start = start + i;
}
}
swap(*start, *(start + i - 1));
quick_sort(start, start + i - 1, less_than);
quick_sort(start + i, end, less_than);
}
template<typename Iterator>
void quick_sort(Iterator start, Iterator end)
{
quick_sort(start, end, [](auto& a, auto& b) { return a < b; });
single_pivot_quick_sort(start, end, [](auto& a, auto& b) { return a < b; });
}
template<typename Iterator, typename LessThan>
void quick_sort(Iterator start, Iterator end, LessThan less_than)
{
single_pivot_quick_sort(start, end, move(less_than));
}
template<typename Collection, typename LessThan>