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Re^3: Hash order randomization is coming, are you ready?

by kst (Initiate)
on Mar 29, 2013 at 05:25 UTC ( #1026088=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Hash order randomization is coming, are you ready?
in thread Hash order randomization is coming, are you ready?

Hmm. I tried this test script:

#!/usr/bin/perl use strict; use warnings; my %hash = map { $_ => 1 } (0..5); my %copy = %hash; my $hash_keys = join(' ', keys %hash); my $copy_keys = join(' ', keys %copy); print "$hash_keys --> $copy_keys, ", $hash_keys eq $copy_keys ? "Same" : "Different", " order \n";

with several different versions of Perl. It prints "Different order" with all the versions I tried (5.8.9, 5.10.1, 5.12.5, 5.14.4, 5.16.3, 5.17.10). With all but 5.17.10, the output is the same each time the program is run:

4 1 3 0 2 5 --> 1 4 0 3 2 5, Different order

With 5.17.10, I get different orders each time I run the program.

So, if I understand correctly, the code you posted is visibly broken and has been for a very long time.


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Re^4: Hash order randomization is coming, are you ready?
by demerphq (Chancellor) on Mar 29, 2013 at 13:56 UTC

    So, if I understand correctly, the code you posted is visibly broken and has been for a very long time.

    Well yes, it is broken, that was my point for bringing it up :-). Whether it is visibly broken or not depends on whether you have any collisions. In the case you posted there are two sets of collisions, 4 and 1, and 3 and 0. (You can tell because they reverse each copy in earlier perls.) However if changed it to be 1,2,3,5,7,8 you would not have any collisions and the order would appear the same. Eg try:

    my %hash = map { $_ => 1 } (1,2,3,5,7,8);

    In 5.18 the order will be different for every hash. No exceptions. Here is what perlfunc will say in 5.18:

    Hash entries are returned in an apparently random order. The actual r +andom order is specific to a given hash; the exact same series of operations on two hashes may result in a different order for each hash. Any inser +tion into the hash may change the order, as will any deletion, with the exc +eption that the most recent key returned by C<each> or C<keys> may be deleted without changing the order. So long as a given hash is unmodified you +may rely on C<keys>, C<values> and C<each> to repeatedly return the same o +rder as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for details on why hash order is randomized. Aside from the guarantees provided here the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl.
    ---
    $world=~s/war/peace/g

      Ok, I just didn't expect that many collisions for such a small set of keys. I've never paid much attention to how Perl computes hashes for %hashes; I've been content so far to assume that It Just Works.

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