private static void AssertQuickSortResult <TItem>(QuickselectSortOrder order, TItem[] items, int i, TItem result)
{
var cmp = Comparer <TItem> .Default;
if (order == QuickselectSortOrder.Ascending)
{
var minimum = items.Skip(i).Min();
foreach (var item in items.Take(i))
{
cmp.Compare(item, minimum).Should().BeLessOrEqualTo(0);
}
cmp.Compare(minimum, result).Should().Be(0);
}
else
{
var maximum = items.Skip(i).Max();
foreach (var item in items.Take(i))
{
cmp.Compare(item, maximum).Should().BeGreaterOrEqualTo(0);
}
cmp.Compare(maximum, result).Should().Be(0);
}
}
public void Should_work_in_trivial_cases(QuickselectSortOrder order)
{
var items = new [] { 1, 7, 4, 15, 34, -54, 5 };
items.QuickSelect(0, order: order).Should().Be(order == QuickselectSortOrder.Ascending ? items.Min() : items.Max());
items.QuickSelect(items.Length, order: order).Should().Be(order == QuickselectSortOrder.Ascending ? items.Max() : items.Min());
items.QuickSelect(2, order: order).Should().Be(order == QuickselectSortOrder.Ascending ? 4 : 7);
}
public void Should_select_fast_on_sameData(QuickselectSortOrder order)
{
var itemsCount = 8000;
var items = Enumerable.Repeat(new string('c', 10), itemsCount).ToArray();
var comparer = new CountedComparer <string>(Comparer <string> .Default);
var result = items.QuickSelect(6000, order: order, comparer: comparer);
comparer.Called.Should().BeLessThan(itemsCount * 7); // Avg Performace O(n)
AssertQuickSortResult(order, items, 6000, result);
}
public void Should_select_fast_on_similarData(QuickselectSortOrder order)
{
var itemsCount = 800;
var items = new[] { "c", "a", "b", "zf", "e", "f", "w", "we", "z", "gg" }
.SelectMany(prefix => Enumerable.Repeat(prefix + new string('c', 10), itemsCount))
.ToArray();
var comparer = new CountedComparer <string>(Comparer <string> .Default);
var result = items.QuickSelect(6000, order: order, comparer: comparer);
comparer.Called.Should().BeLessThan(items.Length * 7); // Avg Performace O(n)
AssertQuickSortResult(order, items, 6000, result);
}
private static int IndexOfMin <T>(this T[] list, IComparer <T> comparer, QuickselectSortOrder order)
{
var minIndex = 0;
for (var i = 0; i < list.Length; i++)
{
if (comparer.CompareWithOrder(list[i], list[minIndex], order) < 0)
{
minIndex = i;
}
}
return(minIndex);
}
public void Should_move_top_items_to_the_beginning_of_given_array(QuickselectSortOrder order)
{
var random = new Random();
var itemsCount = random.Next(100, 200);
var items = new int[itemsCount];
for (var i = 1; i <= itemsCount - 1; i++)
{
for (var j = 0; j < itemsCount; j++)
{
items[j] = random.Next(10);
}
var result = items.QuickSelect(i, order: order);
AssertQuickSortResult(order, items, i, result);
}
}
private static int Partition <T>(this T[] list, int startIndex, int endIndex, int pivotIndex, IComparer <T> comparer, QuickselectSortOrder order)
{
Swap(list, pivotIndex, startIndex);
var pivot = list[startIndex];
var i = startIndex;
var j = endIndex + 1;
while (true)
{
do
{
i++;
} while (i <= endIndex && comparer.CompareWithOrder(list[i], pivot, order) < 0);
do
{
j--;
} while (comparer.CompareWithOrder(list[j], pivot, order) > 0);
if (i >= j)
{
Swap(list, startIndex, j);
return(j);
}
Swap(list, i, j);
}
}
private static int CompareWithOrder <T>(this IComparer <T> comparer, T first, T second, QuickselectSortOrder order) => comparer.Compare(first, second) * (int)order;
public static T QuickSelect <T>(this T[] list, int top, IComparer <T> comparer = null, QuickselectSortOrder order = QuickselectSortOrder.Ascending)
{
if (top > list.Length || list.Length == 0)
{
throw new InvalidOperationException($"Requested to select top {top} items from array with size {list.Length}.");
}
if (comparer == null)
{
comparer = Comparer <T> .Default;
}
if (top == list.Length)
{
list.Swap(list.IndexOfMax(comparer, order), top - 1);
return(list[top - 1]);
}
if (top <= 0)
{
list.Swap(list.IndexOfMin(comparer, order), 0);
return(list[0]);
}
var startIndex = 0;
var endIndex = list.Length - 1;
var pivotIndex = top;
while (endIndex > startIndex)
{
pivotIndex = list.Partition(startIndex, endIndex, pivotIndex, comparer, order);
if (pivotIndex == top)
{
break;
}
if (pivotIndex > top)
{
endIndex = pivotIndex - 1;
}
else
{
startIndex = pivotIndex + 1;
}
pivotIndex = Next(startIndex, endIndex);
}
return(list[pivotIndex]);
}
private static int IndexOfMax <T>(this T[] list, IComparer <T> comparer, QuickselectSortOrder order) => list.IndexOfMin(comparer, (QuickselectSortOrder)((int)order * -1));
