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Re: Shift, Pop, Unshift and Push with Impunity!

by Dominus (Parson)
on Jan 05, 2001 at 07:46 UTC ( #49991=note: print w/ replies, xml ) Need Help??


in reply to RE: Shift, Pop, Unshift and Push with Impunity!
in thread Shift, Pop, Unshift and Push with Impunity!

Says nobody in particular:

There is an example of unsuccessful order of data that will make Perl's implementation of hashes to take O(n) for searching.
When Hashes Go Wrong


Comment on Re: Shift, Pop, Unshift and Push with Impunity!
Re: Re: Shift, Pop, Unshift and Push with Impunity!
by tilly (Archbishop) on Jan 05, 2001 at 15:50 UTC
    The following is a slight modification of your program. It (ab)uses the fact that Perl only checks whether to split buckets when you use a new bucket. It does not make any assumptions about Perl's hashing algorithm. It is therefore able to generate the special keys much more quickly than yours does.
    my $HOW_MANY = shift || 100_000; my ($s, $e1, $e2); $s = time; for ($i=0; $i<$HOW_MANY; $i++) { $q{$i}=1; } $e1 = time - $s; print qq{ Normally, it takes me about $e1 second(s) to put $HOW_MANY items into +a hash. Now I'm going to construct $HOW_MANY special keys and see how long it takes to put those into a hash instead. }; undef %q; print "Generating $HOW_MANY keys\n"; my @keys; my $i = 0; while (@keys < $HOW_MANY) { $i++; my %hash = (0 => 0, $i => 0); if (%hash =~ /1/) { # Goes in the same bucket as 0 push @keys, $i; } } print "Putting the $HOW_MANY special keys into the hash.\n"; $i = 0; $lasttime = $s = time; foreach $key (@keys) { $h{$key} = 1; $i++; if (time() - $lasttime > 5) { my $h = %h + 0; print "I have put $i keys into the hash, using $h bucket(s)\n" ; $lasttime = time; } } $e2 = time - $s; print qq{ The $HOW_MANY special keys took $e2 seconds to put into the hash, instead of $e1 seconds. };

    Update: (Much later.) Perl changed when it splits buckets so this program no longer runs slowly. Furthermore Perl's hashing algorithm is now random, so Dominus' program no longer runs slowly. Shucks.

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