From: bbiker <renard@...>
Date: 2007-10-24T01:40:03+09:00
Subject: Re: pop/push, shift/unshift

On Oct 23, 10:32 am, Bob Hutchison <hu...@recursive.ca> wrote:
> On 23-Oct-07, at 10:02 AM, Jesús Gabriel y Galán wrote:
>
> Hi,
>
>
>
>
>
> > On 10/23/07, richard.j.d...@gmail.com <richard.j.d...@gmail.com>  
> > wrote:
>
> >> Maybe 'enque' and 'deque' could be used as names for methods to put
> >> items on the front of a Queue and remove items from the back  
> >> somewhere
> >> in Ruby - then you wouldn't need push/pop and shift/unshift depending
> >> on which end of the queue you were operating on.
>
> > I like this:
>
> > class Array
> >    alias :enqueue :push
> >    alias :dequeue :shift
> > end
>
> > Then you get Queue semantics and the start of the queue is the first
> > element in the Array :-). (you could do enqueue --> unshift, dequeue
> > --> pop if you rather have the first element of the queue the last in
> > the array).
>
> You have to be really careful here. Push/pop and shift/unshift are  
> not the same functions working on opposite ends of the array, no  
> matter what it sounds like from the documentation.
>
> There is an issue with shift that I think amounts to a bug. Shift/
> unshift work at the beginning of the array, so shift conceptually  
> requires moving array elements around. Ruby (and other programming  
> languages too, specifically some implementations of Common Lisp)  
> optimise shift so as to not have to actually move anything in memory.  
> What it does is, more or less, to move the start of the array 'right'  
> -- so no movement but there is now some part of the array before the  
> start of the array. The bug is that Ruby doesn't stomp on the cell of  
> the array that is being shifted before the start, and so that cell  
> still contains a reference to some object (and IT IS INVISIBLE).
>
> You say this will never happen? or rarely? Well, it's not 'never' for  
> sure, and 'rarely' doesn't help much when you get caught by it. How  
> did I find out about it? Implementing a cache (the uncached stuff was  
> hanging around in memory, intermittently since unshift will re-use  
> the parts of the array before the start). I also found (and reported,  
> maybe even supplied a patch for) a problem in Mongrel's thread  
> management code that was using shift. How did I debug it the first  
> time? Don't ask.
>
> I believe/hope that this will be fixed in some future version of Ruby.
>
> This monkey patch fixes the problem, if this is how you want to solve  
> it...
>
> class Array
>    alias :clingy_shift :shift
>
>    def shift
>      self[0] = nil
>      clingy_shift
>    end
> end
>
> Cheers,
> Bob
>
> > Jesus.
>
> ----
> Bob Hutchison                  -- tumblelog at http://www.recursive.ca/so/
> Recursive Design Inc.          -- weblog athttp://www.recursive.ca/
> hutchhttp://www.recursive.ca/      -- works onhttp://www.raconteur.info/
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It was my understanding that unshift was the leaky method. There was a
monkey patch but I have lost the reference to that thread.