Ah, right you are. I missed that.
I think the collisions possible with PersistentHashSet and
PersistentHashMap were 'well known', at least to a subset of people. What
is probably less well known is a nice small program that caused many
collisions with today's hash functions, and an explanation of why. Thanks
for that.
Andy
On Wed, Oct 23, 2013 at 10:13 AM, Paul Butcher <p...@paulbutcher.com> wrote:
> On 23 Oct 2013, at 17:43, Andy Fingerhut <andy.finger...@gmail.com> wrote:
>
> Paul, your function solve returns a set of solutions, but there is nothing
> on the program that seems to rely upon being able to quickly test whether a
> particular solution is in such a set. Returning a sequence from solve is
> much faster, since it avoids the PersistentHashSet hash collision issue
> entirely.
>
> Just replace #{#{}} with [#{}], and remove the call to 'set' in solve.
>
>
> The problem with that, Andy, is that it will result in a lot of
> duplicates. For the 4x4 problem, for example, that approach raises the size
> of the returned collection from 8 to 384.
>
> My guess is that for the 6x9 problem it will just remove the set handling
> problem and replace it with a memory/time issue as the search space grows
> (and I only have 16G in my MacBook Pro :-)
>
> I don't need to get this working - I only knocked it together as an
> interesting exercise. And if it's helped to track down an issue in the
> Clojure runtime, it's already achieved far more than I expected it to :-)
>
> Thanks again,
>
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>
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