/// <summary>
/// <para>
/// Replacement for list.OrderBy(?).Take(n)
/// </para>
/// <para>
/// 1- Low Memory usage (since this method requires only ~"2*n" elements be in memory)
/// </para>
/// 2- Speed up by an order of magnitude especially when the input is large.
/// </summary>
public static IEnumerable <T> TakeOrdered <T, TKey>(this IEnumerable <T> list, int n, Func <T, TKey> keySelector, bool ascending = true) where TKey : IComparable <TKey>
{
var pq = new MyPriorityQueue <T>(n,
ascending
?
(Func <T, T, bool>)((a, b) => keySelector(a).CompareTo(keySelector(b)) >= 0)
:
(Func <T, T, bool>)((a, b) => keySelector(a).CompareTo(keySelector(b)) < 0)
);
Stack <T> stack = new Stack <T>();
int count = 0;
foreach (T item in list)
{
pq.InsertWithOverflow(item);
count++;
}
int min = Math.Min(count, n);
for (int i = 0; i < min; i++)
{
stack.Push(pq.Pop());
}
for (int i = 0; i < min; i++)
{
yield return(stack.Pop());
}
}
