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Re: Robin Hood Hashing for the perl core

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From:
Dave Mitchell
Date:
August 30, 2021 12:52
Subject:
Re: Robin Hood Hashing for the perl core
Message ID:
YSzUZm43psOE0Z/8@iabyn.com
On Wed, Aug 18, 2021 at 01:09:47PM +0000, Ed Avis wrote:
> I have asked before whether references could always be preserved in hash
> keys

Just a general observation here about proposals to change or improve hash
semantics. I've been meaning for many years (but lacked tuits) to introduce
hash vtables, which would allow for differing hash implementations to be
called on a per-hash basis (so you could declare a hash as being of a
key-order-preserving or ref-as-key-preserving type for example).

Implementing this would then allow proposals such as yours to implemented
as optional extras without risking breaking backwards compatibility or
internals efficiency.

I even reserved a flag bit for it:

#define SVf_FAKE	0x01000000  /* ...
                                       3: HV: informally reserved by DAPM
                                          for vtables

Having such a mechanism would also allow us to remove all the
special-cased code in hv.c for %ENV, tied etc.

-- 
"There's something wrong with our bloody ships today, Chatfield."
    -- Admiral Beatty at the Battle of Jutland, 31st May 1916.

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