Re: From HWaddress to long

James Kanze <>
Mon, 17 Jan 2011 11:28:40 -0800 (PST)
On Jan 17, 2:09 pm, "Kevin P. Fleming" <> wrote:

On 01/17/2011 07:45 AM, Andrea Crotti wrote:

Andrea Crotti<> writes:

In short I need an hash function that gives me a long from
an hardware address.

What could be an easy solution that still gives me a good

I thought I could simply do

(((16 * 16) % LONG_MAX) * 2) % LONG_MAX...

But probably it's a stupid idea.
A mac address is basically 24bytes, and a long is maybe max 8 bytes, so
of course I have to do something to squeeze it...

Of course something already done would be also good.
A generic string hash I'm not sure, since my strings only uses 16
possible characters.

(for example I found this
But I guess an md5/sha would be also fine, right?

A MAC address is 6 bytes (48 bits). It will fit in a 64-bit
unsigned integer (which might be a 'long' on some platforms
and not on others) just fine.

And it's guaranteed to fit in a long long. Depending on table
size and how MAC addresses are allocated, however, it's possible
that converting the MAC address to size_t might not result in
a very good hash. If that's the case, using a good string hash
over the six bytes could reduce collisions significantly.

James Kanze

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