http://d.puremagic.com/issues/show_bug.cgi?id=4271

           Summary: drop/pop methods for std.algorithm.BinaryHeap
           Product: D
           Version: future
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2010-06-04 15:28:50 PDT ---
During the usage of a Heap one of the most commonly used operations is to pop an item from a heap and then to use it. But in std.algorithm.BinaryHeap the pop() doesn't return the item, probably for performance reasons (it requires the copy of the item).

(So to pop an use one item you can use pop(1)[0] or take the top and then pop,
but the second possibility is not an expression.)

There are some ways to solve this problem, but the simpler is probably to just use two different names for two functions. So the pop() can return the popped item. And then the heap can define another method that just pops the top item and doesn't return it, this method can be called for example "drop".

The following untested code gives is an idea of how it can be implemented:

/**
Pops the largest element (according to the predicate $(D less))
and returns it.
*/
    ElementType!Range pop()
    {
        enforce(_length);
        auto tmp = _store.front;
        enforce(_length);
        if (_length > 1)
            swap(_store.front, _store[_length - 1]);
        --_length;
        percolateDown(_store[0 .. _length]);
        return tmp;
    }

/**
Drops the largest element (according to the predicate $(D less)).
*/
    void drop()
    {
        enforce(_length);
        if (_length > 1)
            swap(_store.front, _store[_length - 1]);
        --_length;
        percolateDown(_store[0 .. _length]);
    }

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Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrei@metalanguage.com


--- Comment #1 from Andrei Alexandrescu <andrei@metalanguage.com> 2010-06-04 15:30:36 PDT ---
What's wrong with top()?

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--- Comment #2 from bearophile_hugs@eml.cc 2010-06-04 15:40:21 PDT ---
In most of my usages of a heap I don't have just to use the top item or just to pop the top item, I have to pop the top item and then use the popped out item.

To perform this operation I can currently use something like:

item = heap.top();
heap.pop();

But this is not handy because that is not a single expression, so I can't put it inside other expressions. So I have to use:

somefunction(heap.top(1)[0]);

But being this operation so common, I'd like to write simpler code:

somefunction(heap.top());

I have suggested to add the drop() method too to not decrease the performance
of the (uncommon) code that just needs to remove the top item with no need to
use it (or uses it using top() in other points of the code).

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--- Comment #3 from bearophile_hugs@eml.cc 2010-06-04 16:00:04 PDT ---
Sorry, that was a partially wrong comment, I meant:

So I have to use:

somefunction(heap.pop(1)[0]);

But being this operation so common, I'd like to write simpler code:

somefunction(heap.pop());

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Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
         AssignedTo|nobody@puremagic.com        |andrei@metalanguage.com


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