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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??

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!

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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,
};

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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