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### Re: Minimum Hash keys allocated?

by BrowserUk (Pope)
 on Sep 26, 2010 at 18:25 UTC ( #862105=note: print w/replies, xml ) Need Help??

in reply to Minimum Hash keys allocated?

The first time you assign a value to a new hash, it gets allocated 8 slots. When the number of slots used reaches 6 (which may be more than 6 keys if you get hash collisions), it expands to 16 slots. And it doubles again when 3/4s of the slots (12/16) are used. And so on.

You can see this if you run the following code:

```perl -wE"my %h;\$h{\$_}=\$_, say \$_, ':',scalar %h for 1 .. 1025" | more
1:1/8
2:2/8
3:3/8
4:3/8  ## collision
5:4/8
6:5/8
7:6/8
8:7/16
9:8/16
10:9/16
11:10/16
12:10/16 ## collision
13:10/16 ## collision
14:11/16
15:11/16 ## collision
16:14/32
17:14/32 ## collision
...

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

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Re^2: Minimum Hash keys allocated?
by flexvault (Monsignor) on Sep 26, 2010 at 21:32 UTC
WOW . . .

That was even a better answer than I expected!

For the interactive application I've written/using, pre-allocating the hashes, has improvements of 8-24% using hashes with 256 to 2048 keys on AIX and even better on Linux. But I didn't know about how the algorithm worked on smaller hashes. With your great and simple script, I can test larger combinations as well.

Thank you very much...Ed

PS: Is the algorithm the same on perl 5.8.8?
PS: Is the algorithm the same on perl 5.8.8?

To my knowledge, it hasn't changed since before 5.6 days. Possibly never, but that's as far as I go back with Perl.

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