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Changed the original QuickSort to QuickSelect #8

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Changed the original QuickSort to QuickSelect #8

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54 changes: 28 additions & 26 deletions Kdtree/Source/Kdtree/Private/KdtreeInternal.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -24,46 +24,48 @@ void Swap(T* Var1, T* Var2)
}

template <typename T>
void QuickSort(T* First, T* Last, TFunctionRef<int(T, T)> Comparator)
void QuickSelect(T* First, T* Nth, T* Last, TFunctionRef<int(T, T)> Comparator)
{
T* Left = First;
T* Right = Last;
T Pivot = *First;

while (true)
{
while (Left <= Last && Comparator(*Left, Pivot) == 1)
T* Pivot = First;
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I think that using median is better choice instead of first element.
How do you think?

T* Left = First + 1;
T* Right = Last - 1;

while (Left <= Right)
{
Left++;
while (Left <= Right && Comparator(*Left, *Pivot) == 1) Left++;
while (Left <= Right && Comparator(*Right, *Pivot) == -1) Right--;
if (Left <= Right)
{
Swap(Left, Right);
Left++;
Right--;
}
}
while (Right >= First && Comparator(*Right, Pivot) == -1)

Swap(Pivot, Right);

if (Right == Nth)
{
Right--;
break;
}
if (Left > Right)
else if (Nth < Right)
{
break;
Last = Right;
}
else
{
First = Left;
}
Swap(Left, Right);
Left++;
Right--;
}

if (First < Left - 1)
{
QuickSort(First, Left - 1, Comparator);
}
if (Left < Last)
{
QuickSort(Left, Last, Comparator);
}

}

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Please delete this line.

template <typename T>
void NthElement(T* First, T* Nth, T* Last, TFunctionRef<int(T, T)> Comparator)
{
// TODO: Use faster algorithm such as introselect.
QuickSort(First, Last - 1, Comparator);
QuickSelect(First, Nth, Last, Comparator);
}

FKdtreeNode* BuildNode(const FKdtreeInternal& Tree, int* Indices, int NumData, int Depth)
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