Devel::Peek -- hash "Quality" query

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Lower values (closer to 100%) are better:

A random hash should distribute the keys equally to all buckets, so it would need the minimal number of comparisons. Real world
distributions will have some buckets fuller than others. Since the number of comparisons (for visiting each element exactly once) is
quadratic with the number of elements in a bucket (linear search), the additional comparisons for the longer chains will not be
offset by the savings of comparisons from shorter chains.

But don't worry about it too much, as you can't really do anything about it anyways. The only reason for Devel::Peek to print this
metric is to allow Perl5 Porters to play with different implementations of the hashing function.

Cheers,

-Jan

From: activeperl-bounces [at] listserv.ActiveState.com [mailto:activeperl-bounces [at] listserv.ActiveState.com] On Behalf Of
Deane.Rothenmaier [at] walgreens.com
Sent: Tuesday, January 19, 2010 1:19 PM
To: activeperl [at] listserv.ActiveState.com
Subject: Devel::Peek -- hash "Quality" query

Dear gurus,

>From the perldoc for Devel::Peek:

The "quality" of a hash is defined as the total number of comparisons needed to access
every element once, relative to the expected number needed for a random hash. The value
can go over 100%.

The total number of comparisons is equal to the sum of the squares of the number of
entries in each bucket. For a random hash of "n" keys into "k" buckets, the expected
value is:

n + n(n-1)/2k

I'm working on some code documentation and have gotten to this point. The above is vaguely worded. Or maybe I should just say that
it doesn't explicitly state what I'm looking for... So, do I want a higher percentage for hash "quality," or a lower percentage?
That is, which direction of change signals improvement?

TIA,

Deane Rothenmaier
Programmer/Analyst
Walgreens Corp.
224-542-5150

You cannot strengthen the weak by weakening the strong. -- Abraham Lincoln

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(filtered medium)"><style></style></head><body lang=3DEN-US link=3Dblue =
vlink=3Dpurple><div class=3DWordSection1>Lower values (closer to 100%) are better:<o:p></o:p><o:p> </o:p>A random hash should distribute the keys equally to all buckets, so =
it would need the minimal number of comparisons. Real world =
distributions will have some buckets fuller than others. Since the =
number of comparisons (for visiting each element exactly once) is =
quadratic with the number of elements in a bucket (linear search), the =
additional comparisons for the longer chains will not be offset by the =
savings of comparisons from shorter chains.<o:p></o:p><o:p> </o:p>But don’t worry about it too much, as you can’t really do =
anything about it anyways. The only reason for Devel::Peek to =
print this metric is to allow Perl5 Porters to play with different =
implementations of the hashing function.<o:p></o:p><o:p> </o:p>Cheers,<o:p></o:p>-Jan<o:p></o:p><o:p> </o:p><div =
style=3D'border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in =
4.0pt'><div><div style=3D'border:none;border-top:solid #B5C4DF =
1.0pt;padding:3.0pt 0in 0in 0in'>From:=
 =
activeperl-bounces [at] listserv.ActiveState.com =
[mailto:activeperl-bounces [at] listserv.ActiveState.com] On Behalf Of =
Deane.Rothenmaier [at] walgreens.com Sent: Tuesday, January 19, =
2010 1:19 PM To: =
activeperl [at] listserv.ActiveState.com Subject: Devel::Peek -- =
hash "Quality" query<o:p></o:p></div></div><o:p> </o:p> Dear =
gurus, From the =
perldoc for Devel::Peek: The =
"quality" of a hash is defined as the total number of =
comparisons needed to access every =
element once, relative to the expected number needed for a random hash. =
The value can go over 100%. The =
total number of comparisons is equal to the sum of the squares of the =
number of entries in each bucket. For a random hash of =
"n" keys into "k" buckets, the expected =
 value is: =
n + n(n-1)/2k =
 I'm working =
on some code documentation and have gotten to this point. The above is =
vaguely worded. Or maybe I should just say that it doesn't explicitly =
state what I'm looking for... So, do I want a higher percentage =
for hash "quality," or a lower percentage? That is, =
which direction of change signals improvement? TIA, =
 Deane =
Rothenmaier Programmer/Analyst Walgreens =
Corp. 224-542-5150 You cannot strengthen the weak by weakening =
the strong. -- Abraham =
Lincoln<o:p></o:p></div></div></body></html>
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Devel::Peek -- hash "Quality" query

RE: Devel::Peek -- hash "Quality" query