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From Tim Ellison <t.p.elli...@gmail.com>
Subject Re: [classlib] Some further study on hashcode and HashMap
Date Wed, 04 Jul 2007 11:23:13 GMT
Jimmy,Jing Lv wrote:
> 2007/7/4, Xiao-Feng Li <xiaofeng.li@gmail.com>:
>> Well, maybe I should read some random number theory, since I think
>> although >>2  removes the meaningless bits in LSB, it introduces two
>> meaningless bits in MSB. If you treat the 32 bit equally when encoding
>> their info into log(num_bucket) bits, the bits in LSB and MSB give you
>> the same effect. Or you assume the lower bits are more significant by
>> default?
> 
> Yes,  the second hypothesis I made here is that the lower bits are
> more important than higher bits,

How about: the lower bits are *equally as important* as the higher bits
if the hashCode algorithm is producing an even distribution of values.

> we focus on lower bits more than higher bits (though not throw higher
> bits away) however it is imposible for them to be equal in indexing,
> especially in small buckets.

Yes, and of course we keep the full hashCode around too (to use for a
fast check of potential equals() and for rehashing).

> We choose lower bits for most of time hashcodes alter in lower bits.

Why?

> And move meaningless bits to MSB here fits this algorithm
> :)

We just don't consider them when computing the index, so they could be
meaningful or meaningless, we don't care.

Regards,
Tim


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