Default ordering of Hash keys

[Martin D Kealey]

On Mon, 8 Aug 2011, Luben Karavelov wrote:
> For some types of keys (that have zero/NULL as valid value, i.e.
> integer, ptr, cstring) parrot iterates keys in index order, not
> ordered by creation.
>
> For the rest of key types it iterates them in memory order, that is
> the same as create order, if there were no key deleted. This is faster
> because it has less computation and a lot less data cache fails,
> especially with big hashes.
>
> The change arround 2.9 was part of optimization of parrot hashes done
> by me and chromatic. We were optimizing for the common case that is
> having String hash keys.

I suggest that pseudo-randomness would be worth retaining in order to
prevent order dependencies from occurring in future code.

To implement it, and yet remain efficient, it would suffice to return the
keys in some simple function of storage order; say, reverse storage order,
or perhaps still in storage order, but not starting at the lowest memory
location; along the lines of
	( $RandomOffset .. $#HashBackingArray, 0 .. $RandomOffset-1 )

Or even just permuting the bottom few bits of the index:

	( 0 .. ($#HashBackingArray .| 7) ) . map { $_ .^ 5 }

-Martin