# Re: Perfect Hashfunctions

From:
luser- -droog <mijoryx@yahoo.com>
Newsgroups:
comp.programming,comp.lang.c++,comp.lang.c
Date:
Fri, 12 Nov 2010 22:42:06 -0800 (PST)
Message-ID:
On Nov 12, 9:44 pm, "Johannes Schaub (litb)" <schaub-johan...@web.de>
wrote:

I want to learn how to create perfect hash functions for switch statement=

s

over strings. So given

switch(str) {
case str1...
case str2...
...
case strN...

}

I don't understand the example. Why are you worried about switch
statements? As far as I know, the term 'perfect hash' refers to
a mapping between strings and integers. And the switch statement
gives you a mapping between integers and code sequences.
Would this pseudocode more accurately reflect what you want?

enum { str1, str2, str3, ..., strN };
ph = makeperfecthash(set of strings)
....
switch (applyhash(ph, str)) {
case str1: ...
case str2: ...
...
case strN: ...
}

There are at least 2 pieces: building (or picking) the hash
function which will perform perfectly over a known set of
strings, and actually executing that hash for a specific
string to get a specific integer.

I want to create a perfect hash function that gives me an unique integer =

for

each of those strI, in the range of [0 ... M] (M should preferably be
small).

and candy-coated. ;)

However, I can't for the life of me find any tutorial or documentation on
how to build up such a function. All I find are statements that I can use=

a

two-stage hashing system, and use a certain function for the first stage =

and

then for the second stage finding the function from a set of "universal
hashing functions" by randomly testing whether a given function will be
perfect. This sounds like "let's try randomly, and if there is no functio=

n

that works, we are just shit-outta-luck"

Well, yeah. What did you expect?

I have no idea what an universal hashing function really is about and
whether that is the only way of creating a perfect hash function, too: I
read about CHD, BDZ, BMZ, CHM, BRZ and FCH on the page of cmph, but I've =

no

clue how they relate to the big picture.

This part is out of my league.

Can anyone shed some light on this, please? Is it really difficult to do =

a

perfect hashing?

If I haven't shed any light on the real issue,
hopefully I've helped clarify the question.

--
Do not trouble me with Faramir,
I know his uses and they are few.

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