Take top n items from IEnumerable<> efficiently

TakeDescending.cs

public static IReadOnlyCollection<T> TakeDescending<T, TKey>(IEnumerable<T> source, Func<T, TKey> keySelector, int count)
	where TKey : IComparable<TKey>
{
	var index = 0;
	var keys = new TKey[count];
	var values = new T[count];
	TKey min = default;
	var minIndex = 0;
	
	foreach (var item in source)
	{
		var key = keySelector(item);
		if (index < keys.Length)
		{
			keys[index] = key;
			values[index] = item;
			
			if (index == 0)
			{
				min = key;
			}
			else
			{
				if (key.CompareTo(min) < 0)
				{
					min = key;
					minIndex = index;
				}
			}
			
			index++;
		}
		else if (key.CompareTo(min) > 0)
		{
			keys[minIndex] = key;
			values[minIndex] = item;
			
			// sequential search for new min
			min = key;
			for (var i = 0; i < keys.Length; i++)
			{
				if (keys[i].CompareTo(min) < 0)
				{
					min = keys[i];
					minIndex = i;
				}
			}
		}
	}
	
	if (index < values.Length)
	{
		Array.Resize(ref values, index);
	}
	
	return values;
}