peter_k wrote:

> Please, help me, i'm new to this.

>

> I want to do hash_map, where key will be unique structure...

>

> struct key {

> long data[3];

> }

>

> ...and the value will be unsigned char;

>

> How to do efficient hash function which will return first 20 bits of

> data[3]??
(I hope you meant 20 bits of whole data as there is no data[3] element

.

Why do you want just 20 bits of data? When generating hash, you should

always try to combine as much of key as possible.

Now to combine data of key, I am using this kind of operations:

unsigned hash = 123456789; //initial seed

hash = ((hash << 4) + hash) ^ data[0];

hash = ((hash << 4) + hash) ^ data[1];

hash = ((hash << 4) + hash) ^ data[2];

(actually, I have helper template class to do this). Not perfect maybe,

but good enough.

> So my hash will have table of 1048657 elements...

>

> *) Is standard hash_map very fast?? I wanted to store here about 128 MB

> of elements...
Well, number of keys is more important than total amount of data....

Generally, hash maps are faster than binary trees, especially for very

large data sets, but have nasty characteristics to behave oddly when

your hash function has some problems or sometimes (but it is very rare)

when your hash function is exposed to specific data (resulting in too

many collisions).

Anyway, I use hash-maps exclusively...

Mirek