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digitalmars.D - hash function

reply "Adam D. Ruppe" <destructionator gmail.com> writes:
druntime has a hash implementation in module rt.util.hash that 
looks pretty useful, but isn't publicaly exported by the library 
anywhere.

Does phobos or druntime offer a public hash function like that? 
If no, can it expose this one?
Feb 14 2013
next sibling parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Fri, Feb 15, 2013 at 03:26:28AM +0100, Adam D. Ruppe wrote:

 druntime has a hash implementation in module rt.util.hash that looks
 pretty useful, but isn't publicaly exported by the library anywhere.
 
 Does phobos or druntime offer a public hash function like that? If
 no, can it expose this one?


+1. This is a very good and useful hash function that should be usable
by user code. No need to force the user to reinvent the wheel or
copy-n-paste code.


T

-- 
The diminished 7th chord is the most flexible and fear-instilling chord. Use it
often, use it unsparingly, to subdue your listeners into submission!
Feb 14 2013
prev sibling next sibling parent reply "Zach the Mystic" <reachBUTMINUSTHISzach gOOGLYmail.com> writes:
On Friday, 15 February 2013 at 02:26:31 UTC, Adam D. Ruppe wrote:

 druntime has a hash implementation in module rt.util.hash that 
 looks pretty useful, but isn't publicaly exported by the 
 library anywhere.

 Does phobos or druntime offer a public hash function like that? 
 If no, can it expose this one?


This might be a good time to bring up something that was 
bothering me when I examined the current druntime hash 
implementation. Just out of curiosity I was researching hash 
functions, and I found the hash function expert Bob Jenkins' FAQ 
about hashes:

http://burtleburtle.net/bob/hash/hashfaq.html

He says specifically that any hash function requiring a modulo 
prime at the end is bad (inefficient), but if I'm not mistaken, 
the druntime does just that. At line 50 of the associative array 
module there is a list of primes, and I believe the 
implementation assumes them to be necessary.

https://github.com/D-Programming-Language/druntime/blob/master/src/rt/aaA.d

I don't know enough to be sure this is a problem. There might be 
some other reason for the modulo prime, but I'd like to put my 
mind at ease on it.
Feb 14 2013
parent "Zach the Mystic" <reachBUTMINUSTHISzach gOOGLYmail.com> writes:
On Friday, 15 February 2013 at 03:33:13 UTC, Zach the Mystic 
wrote:

 This might be a good time to bring up something that was 
 bothering me when I examined the current druntime hash 
 implementation. Just out of curiosity I was researching hash 
 functions, and I found the hash function expert Bob Jenkins' 
 FAQ about hashes:

 http://burtleburtle.net/bob/hash/hashfaq.html

 He says specifically that any hash function requiring a modulo 
 prime at the end is bad (inefficient), but if I'm not mistaken, 
 the druntime does just that. At line 50 of the associative 
 array module there is a list of primes, and I believe the 
 implementation assumes them to be necessary.

 https://github.com/D-Programming-Language/druntime/blob/master/src/rt/aaA.d

 I don't know enough to be sure this is a problem. There might 
 be some other reason for the modulo prime, but I'd like to put 
 my mind at ease on it.


Without objection, so filed...

http://d.puremagic.com/issues/show_bug.cgi?id=9522
Feb 16 2013
prev sibling parent reply Jacob Carlborg <doob me.com> writes:
On 2013-02-15 03:26, Adam D. Ruppe wrote:

 druntime has a hash implementation in module rt.util.hash that looks
 pretty useful, but isn't publicaly exported by the library anywhere.

 Does phobos or druntime offer a public hash function like that? If no,
 can it expose this one?


TypeInfo has a "getHash" method.

-- 
/Jacob Carlborg
Feb 14 2013
parent reply "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Fri, Feb 15, 2013 at 08:28:34AM +0100, Jacob Carlborg wrote:

 On 2013-02-15 03:26, Adam D. Ruppe wrote:

druntime has a hash implementation in module rt.util.hash that looks
pretty useful, but isn't publicaly exported by the library anywhere.

Does phobos or druntime offer a public hash function like that? If no,
can it expose this one?


 
 TypeInfo has a "getHash" method.


[...]

Which has a lot of bugs and inconsistencies. Like the fact that hashing
a char[], a const(char)[], and a string (immutable(char)[]) gives
different results.


T

-- 
Blunt statements really don't have a point.
Feb 15 2013
parent Jacob Carlborg <doob me.com> writes:
On 2013-02-15 16:13, H. S. Teoh wrote:


 Which has a lot of bugs and inconsistencies. Like the fact that hashing
 a char[], a const(char)[], and a string (immutable(char)[]) gives
 different results.


Didn't know that. I just knew it existed.

-- 
/Jacob Carlborg
Feb 15 2013