StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

PO
text
Body
copied!<p>There's no such thing as a “good hash function” for universal hashes (ed. yes, I know there's such a thing as “universal hashing” but that's not what I meant). Depending on the context different criteria determine the quality of a hash. Two people already mentioned SHA. This is a cryptographic hash and it isn't at all good for hash tables which you probably mean.</p> <p>Hash tables have very different requirements. But still, finding a good hash function universally is hard because different data types expose different information that can be hashed. As a rule of thumb it is good to consider <em>all</em> information a type holds equally. This is not always easy or even possible. For reasons of statistics (and hence collision), it is also important to generate a good spread over the problem space, i.e. all possible objects. This means that when hashing numbers between 100 and 1050 it's no good to let the most significant digit play a big part in the hash because for ~ 90% of the objects, this digit will be 0. It's far more important to let the last three digits determine the hash.</p> <p>Similarly, when hashing strings it's important to consider all characters – except when it's known in advance that the first three characters of all strings will be the same; considering these then is a waste.</p> <p>This is actually one of the cases where I advise to read what Knuth has to say in <em>The Art of Computer Programming</em>, vol. 3. Another good read is Julienne Walker's <a href="http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx" rel="noreferrer">The Art of Hashing</a>.</p>

Querying!

Guidance

An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload