On Jul 31, 2013, at 7:55 AM, Dicebot <public@dicebot.lv> wrote:

> On Wednesday, 31 July 2013 at 14:43:21 UTC, Daniel Kozak wrote:
>> is there a way for AA to behave same as PHP?
> 
> I doubt it. This snippet suggests that AA's in PHP are not simply AA's and do additionally track insertion order (or use some similar trick). Data in associative arrays is organized in a way that allows fast key lookup and iterating in an original order requires tracking extra state. Contrary to PHP, D does care about performance so I see no way this can happen automatically.

This seems more likely to be a library type.  I have something like this that I use very frequently at work where the behavior is pluggable, so it can be used as a cache that automatically does LIFO/FIFO eviction according to an age window, number of entries, etc.  Or none of those if all you want is a plain old hashtable.  It's not a tremendous amount of work to adapt an existing implementation to work this way.  The only annoying bit I ran into (at least with C++) is that the symbol lookup rules pretty much prevent overrides of existing functions to be mixed in via the policies.  Does alias this work around this problem in D?  I've never tried.

On Fri, Aug 23, 2013 at 11:22:47AM -0700, Sean Kelly wrote:
> On Jul 31, 2013, at 7:55 AM, Dicebot <public@dicebot.lv> wrote:
> 
> > On Wednesday, 31 July 2013 at 14:43:21 UTC, Daniel Kozak wrote:
> >> is there a way for AA to behave same as PHP?
> > 
> > I doubt it. This snippet suggests that AA's in PHP are not simply AA's and do additionally track insertion order (or use some similar trick). Data in associative arrays is organized in a way that allows fast key lookup and iterating in an original order requires tracking extra state. Contrary to PHP, D does care about performance so I see no way this can happen automatically.
> 
> This seems more likely to be a library type.  I have something like this that I use very frequently at work where the behavior is pluggable, so it can be used as a cache that automatically does LIFO/FIFO eviction according to an age window, number of entries, etc. Or none of those if all you want is a plain old hashtable.  It's not a tremendous amount of work to adapt an existing implementation to work this way.  The only annoying bit I ran into (at least with C++) is that the symbol lookup rules pretty much prevent overrides of existing functions to be mixed in via the policies.  Does alias this work around this problem in D?  I've never tried.

In http://d.puremagic.com/issues/show_bug.cgi?id=10733 bearophile and I discussed code reuse between the current AA and an ordered AA implementation. It appears that a more fundamental structure is the Set, where the key is the value. An AA can be implemented in terms of a Set, by making the set element a struct containing a key and value field, with a custom toHash() and opCmp() that only hashes/compares the key part.

By adding a pair of pointers to this customized set element, we can make an ordered AA in terms of Set.

Plus, a Set is something that's very useful on its own as well.


T

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