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


On Wed, 2014-07-23 at 11:37 +0200, Richard Biener wrote:
> 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).

Isn't that what boost / std hash is actually doing?  Maybe something
like the attached example could be an improvement?

As with boost / std hash, the hash function is a template functor that
can be specialized on various types.  The 'incremental_hash' simply
plugs together a hash value combiner and a hash function.  Types that
are not implemented by the hash function (in this example) are caught at
compile time, since implicit conversions do not take place.  However, it
should also possible to write hash functions with a bunch of operator ()
overloads to utilize implicit conversions.

One advantage of this construct is that new types can be added along
with hash function specializations, without the need to touch any of the
existing hashing facilities.

I don't know how/whether this would fit with the already existing hash
map stuff (hash-table.h, hash-map.h).  It seems those don't support user
specified hash functions.

Cheers,
Oleg

typedef unsigned int hashval_t;
typedef long long HOST_WIDE_INT;
typedef unsigned int size_t;

// -----------------------------
// hash function and some specializations

struct raw_data
{
  const void* data;
  size_t len;

  raw_data (const void* d, size_t l) : data (d), len (l) { }

  template <typename T>
  explicit raw_data (const T& val) : data (&val), len (sizeof (T)) { }
};

template <typename T> struct my_hash;

template <> struct my_hash<unsigned int>
{
  hashval_t operator () (unsigned int val);
};

template <> struct my_hash<HOST_WIDE_INT>
{
  hashval_t operator () (HOST_WIDE_INT val);
};

template <> struct my_hash<raw_data>
{
  hashval_t operator () (raw_data val);
};

template <> struct my_hash<const void*>
{
  hashval_t operator () (const void* val);
};

template <typename T> struct my_hash<T*>
{
  hashval_t operator () (T* val)
  {
    return my_hash<const void*> () (val);
  }
};

template <typename T> struct my_hash<const T*>
{
  hashval_t operator () (const T* val)
  {
    return my_hash<const void*> () (val);
  }
};

// -----------------------------
// a possible hash combiner
struct my_hash_combiner
{
  hashval_t operator () (hashval_t a, hashval_t b);
};


// -----------------------------
// generic incremental hash

template <template<typename> class HashFunction, class HashValueCombiner>
class incremental_hash
{
public:
  incremental_hash (void) : m_value (0) { }
  incremental_hash (hashval_t seed) : m_value (seed) { }

  hashval_t value (void) const { return m_value; }

  template <typename T> incremental_hash& add (const T& val)
  {
    m_value = HashValueCombiner () (m_value, HashFunction<T> () (val));
    return *this;
  }

private:
  hashval_t m_value;
};

// hash function of an incremental_hash is its current value, regardless
// of the actual hash function used.
template <template<typename> class Func, class Combiner>
struct my_hash< incremental_hash<Func, Combiner> >
{
  hashval_t operator () (const incremental_hash<Func, Combiner>& val)
  {
    return val.value ();
  }
};


// -----------------------------
// use case

struct my_object
{
  int a, b, c, d;
};

typedef incremental_hash<my_hash, my_hash_combiner> my_inc_hash;

int main (int argc, const char* argv[])
{
  my_object some_obj;

  my_inc_hash hash;

  hash.add (5U)
      .add<unsigned int> (234)     // explicit hash func, implicit conversion.
      .add (HOST_WIDE_INT (2135))
      .add (raw_data (argv, argc)) // raw data hash
      .add (raw_data (some_obj))   // iterative_hash_object macro
      .add (argv)                  // pointer hash
      .add (argv[0]);              // pointer hash

  my_inc_hash hash2 (0x5533);
  hash2.add (123U)
       .add (hash);  // 'merge' two incremental hashes.

  return hash2.value ();
}

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