空堆上的容器/堆Pop（）

Question

I have used the container/heap package to implement a priority queue. One thing bothers me though. What should the behaviour of the interface.Pop() method be if the heap is empty? I don't see anything mentioned in the documentation and the source code doesn't seem to be expecting this situation:

// Pop removes the minimum element (according to Less) from the heap
// and returns it. The complexity is O(log(n)) where n = h.Len().
// It is equivalent to Remove(h, 0).
//
func Pop(h Interface) interface{} {
        n := h.Len() - 1
        h.Swap(0, n)
        down(h, 0, n)
        return h.Pop()
}

Clearly, if h.Len() is 0 this is not going to work well. Is this simply meant to panic or is the user expected to always check whether there are any items left?

Answer 1

The natural behaviour is to panic. This is what the IntHeap example does.

As pointed out in the comments, control will not reach h.Pop() if h.Swap() panics. However, h.Pop() can still be called on an empty heap if heap.Remove() is given -1 as the index:

// Remove removes the element at index i from the heap.
// The complexity is O(log(n)) where n = h.Len().
//
func Remove(h Interface, i int) interface{} {
    n := h.Len() - 1
    if n != i {
        h.Swap(i, n)
        down(h, i, n)
        up(h, i)
    }
    return h.Pop()
}

If h.Swap() panics on negative indices, h.Pop() should also panic for consistency.

Having h.Swap() silently ignore negative indices and h.Pop() return a default value like nil is consistent, but other programmers would find that surprising so I don't recommend it.