On Jun 29, 3:09 pm, Nicolas Oury <nicolas.o...@gmail.com> wrote:
> Dear all,
>
> I am coding a (very) small hash consing library for clojure.
>
> For those we don't happen to know what hash consing is, it is a way of
> allowing equal data structures to be shared in memory.
> This leverages the purity (as in "immutability") of data structures to
> reduce memory footprint (no duplication of data for just a constant
> overhead per allocation) and makes hashing and equality tests faster.
> (You can use java non overloaded hashCode, and == to test equality,
> because of the sharing. So both operations are O(1), with a very small
> constant, whatever is the complexity of the hash consed data
> structures.)
I don't know anything about hash consing. Based on my
limited understanding of the description I am just wondering
if this is different from structural sharing that Clojure collections
have.
Quoting the docs[1]:
"
In particular, the Clojure collections support efficient creation of
'modified'
versions, by utilizing structural sharing, and make all of their
performance
bound guarantees for persistent use.
"
Do you have something different in mind with hash consing?
Regards,
Parth
[1] http://clojure.org/data_structures#toc12
> This can then be used, in combination with referential transparency to
> make memoized function faster.
>
> I have a java file and a clojure interface that seem to work, at least
> on my example. I plan to put something somewhere someday, but before
> spending too much time in making this releasable, i Have a few
> questions:
>
> - does something already exists in contrib that I have missed?
> - is someone else working on that?
> - are there any "features that I need and that you must implement" that
> you think of?
>
> My current plans are:
> - using a java concurrent hash map to soft referenced objects for the
> hash consing table.
> - only two fields in an hash consed object: the value it represents,
> and a generic field called "cached_data" used to store anything you
> want to memoize about this data structure. (Keeping this cached data a
> bit longer is the reason I plan to use soft references and not weak
> references)
>
> I am not a clojure expert and I am not a java coder at all, so don't
> hesitate to tell me if my plans are somehow wrong.
>
> Bets regards,
>
> Nicolas.
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