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Re: how to allow integer overflow for calculating hash code of a string?

From: Craig James <cjames(at)emolecules(dot)com>
To: Haifeng Liu <liuhaifeng(at)live(dot)com>
Cc: "pgsql-admin(at)postgresql(dot)org" <pgsql-admin(at)postgresql(dot)org>
Subject: Re: how to allow integer overflow for calculating hash code of a string?
Date: 2012-09-21 15:21:04
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-admin
On Thu, Sep 20, 2012 at 7:56 PM, Haifeng Liu <liuhaifeng(at)live(dot)com> wrote:

> On Sep 20, 2012, at 10:34 PM, Craig James <cjames(at)emolecules(dot)com> wrote:
> On Thu, Sep 20, 2012 at 1:55 AM, Haifeng Liu <liuhaifeng(at)live(dot)com> wrote:
>> I want to write a hash function which acts as String.hashCode() in java:
>> hash = hash * 31 + s.charAt(i)... but I got integer out of range error. How
>> can I avoid this? I saw java do not care overflow of int, it just make the
>> result negative.
> Use the bitwise AND operator to mask the hash value with 0x3FFFFFF before
> each iteration:
>   hash = (hash & 67108863) * 31 + s.charAt(i);
> Craig
> Thank you, I believe your solution is OK for a hash function, but I am
> aiming to create a hash function that is consistent with the one
> applications use. I know postgresql 9.1 has a hash function called
> hashtext, but I don't know what algorithm it use,  and I also see that it's
> not recommended to relay on it. So I am trying to create a hash function
> which behaves exactly the same as java.lang.String.hashCode().  The later
> one may generate negative hash value. I guess when the number is
> overflowing, the part out of range will be ignored, and if the highest bit
> get 1, the hash value turn to negative value.

You are probably doing something where you want the application and the
database to implement the exact same function, but if you stick to the Java
built-in function, you will only have control over one implementation of
that function.  What happens if someone working on Java changes the how the
Java internals work?

A better solution would be to implement your own hash function in Postgres,
and then once you know exactly how it will work, re-implement it in Java
with your own code.  That's the only way you can ensure consistency between
the two.


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