Heap Queue in C#

A minimal re-implementation of the Python heapq module.

public class HeapQ<T> where T : IComparable
{
    List<T> items;

    public HeapQ ()
    {
        items = new List<T> ();
    }

    public bool Empty {
        get { return items.Count == 0; }
    }

    public T First {
        get {
            if (items.Count > 1) {
                return items[0];
            }
            return items[items.Count - 1];
        }
    }

    public void Push (T item)
    {
        items.Add (item);
        SiftDown (0, items.Count - 1);
    }

    public T Pop ()
    {
        T item;
        var last = items[items.Count - 1];
        items.RemoveAt (items.Count - 1);
        if (items.Count > 0) {
            item = items[0];
            items[0] = last;
            SiftUp (0);
        } else {
            item = last;
        }
        return item;
    }

    void SiftDown (int startpos, int pos)
    {
        var newitem = items[pos];
        while (pos > startpos) {
            var parentpos = (pos - 1) >> 1;
            var parent = items[parentpos];
            if (parent.CompareTo (newitem) <= 0)
                break;
            items[pos] = parent;
            pos = parentpos;
        }
        items[pos] = newitem;
    }

    void SiftUp (int pos)
    {
        var endpos = items.Count;
        var startpos = pos;
        var newitem = items[pos];
        var childpos = 2 * pos + 1;
        while (childpos < endpos) {
            var rightpos = childpos + 1;
            if (rightpos < endpos && items[rightpos].CompareTo (items[childpos]) <= 0)
                childpos = rightpos;
            items[pos] = items[childpos];
            pos = childpos;
            childpos = 2 * pos + 1;
        }
        items[pos] = newitem;
        SiftDown (startpos, pos);
    }
    
}