Re: [PATCH 1/4] Add an abstract incremental hash data type

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Jul 18, 2014 at 3:08 AM, Trevor Saunders <tsaunders@mozilla.com> wrote:
> On Thu, Jul 17, 2014 at 06:36:31AM +0200, Andi Kleen wrote:
>> On Wed, Jul 16, 2014 at 10:40:53PM -0400, Trevor Saunders wrote:
>> >
>> > > + public:
>> > > +
>> > > +  /* Start incremential hashing, optionally with SEED.  */
>> > > +  void begin (hashval_t seed = 0)
>> > > +  {
>> > > +    val = seed;
>> >
>> > why isn't this the ctor?
>>
>> It's standard for hash classes to have explicit begin()/end().
>> All the existing ones I've seen work this way.
>
>  I only know of one vaguelly similar thing
>  http://mxr.mozilla.org/mozilla-central/source/mfbt/SHA1.h#37  which
>  doesn't do that, and a bunch of people doing something doesn't
>  necessarily mean it makes sense.  Now there may be a good reason it
>  does make sense, but unless these other people need begin() to be
>  fallible I don't see it.

I agree with Trevor here.  Please make begin() the constructor.
Btw, what will be the way to plug in an alternative hash function?
That is, there doesn't seem to be a separation of interface
and implementation in your patch (like with a template or a base-class
you inherit from).

Richard.

>> > > +  /* Add unsigned value V.  */
>> > > +  void add_int (unsigned v)
>> > > +  {
>> > > +    val = iterative_hash_hashval_t (v, val);
>> > > +  }
>> >
>> > istm this is a great spot to provide a bunch of overloads of just add()
>> > and let the compiler pick the appropriate one for your type.
>>
>> Sorry I'm not into code obfuscation. With hashing it's far better
>> to work with explicit visible types instead of invisible magic.
>
> if that were true I'd expect you'd see lots of cases of people using a
> different hash function than the type of the expression being passed,
> but a quick look at the later patches didn't show me any of those.
> Not repeating the type of something is hardly obfiscation, and in most
> cases there's only one sane function to call for a given expression.
>
> but I guess its easy enough to change later if somebody gets really
> annoyed by it so whatever.
>
> Trev
>
>>
>> -Andi