http://perl5.git.perl.org/perl.git?a=search&h=HEAD&st=grep&s=HeHASH

HeHASH Returns the computed hash stored in the hash entry. U32 HeHASH(HE* he)

Which is fine ... Er, actually, it is terminally stupid.

In order to obtain the HE* for a key, I first need to look it up; and in order to lookup the key, I need to calculate the hash value!

Reminds me of the old joke: Anti-thirst pills; take two in a glass of water.

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)

In the absence of evidence, opinion is indistinguishable from prejudice.

I'm with torvalds on this Agile (and TDD) debunked I told'em LLVM was the way to go. But did they listen!

hv_exists_ent()

But if I supply the hash value, why does it need the keysv?

If all I have is the string and its length (the normal case), it means I have to construct an SvPV which is surely more computationally and time costly than simply recalculating the hash; as it requires a several memory allocations.

heh :)

http://perl5.git.perl.org/perl.git/blob?f=hv.h#l457

 457 #define hv_exists_ent(hv, keysv, hash)                           
+       \
458     (hv_common((hv), (keysv), NULL, 0, 0, HV_FETCH_ISEXISTS, 0, (h
+ash)) \
459      ? TRUE : FALSE)
460 #define hv_fetch_ent(hv, keysv, lval, hash)                       
+      \
461     ((HE *) hv_common((hv), (keysv), NULL, 0, 0,                  
+      \
[download]

Seems all the hash functions are built on http://perldoc.perl.org/perlapi.html#hv_common which is undocumented and subject to change, at least its Only 579 lines http://perl5.git.perl.org/perl.git/blob?f=hv.c#l337

http://perl5.git.perl.org/perl.git?a=search&h=HEAD&st=grep&s=PERL_HASH_WITH_SEED

hello? https://metacpan.org/pod/Hash::Util#hash_value

hello? https://metacpan.org/pod/Hash::Util#hash_value

Nice find! But, it doesn't help :(

I can use the PERL_HASH macro to obtain the hash value; but even if I set up an SV with the key for hv_exists_ent() to ignore; it complains: Use of uninitialized value in subroutine entry; for every iteration; and apparently does nothing?

HV* countColors( SV *img ) {
    STRLEN l;
    U32 *rgba = (U32*)SvPVx( img, l ), i;
    HV* counts = newHV();
    SV *dummy = newSV( 4 );

    l /= 4;
    for( i = 0; i < l; ++i ) {
        U32 hash;
        PERL_HASH( (U32)hash, (char*)&rgba[ i ], 4 );
        SvPV_set( dummy, (char*)&rgba[ i ] );

        if( hv_exists_ent( counts, dummy, hash  ) ) {
            SV **val = hv_fetch( counts, (char*)&rgba[ i ], 4, hash );
            SvIV_set( *val, SvIV( *val ) + 1 );
        }
        else {
            SV *val = newSViv( 1 );
            hv_store( counts, (char*)&rgba[ i ], 4, val, hash );
        }
    }
    return counts;
}
[download]

Even if I could work out (why do I have to guess? Why isn't this stuff documented?) how to persuade hv_exists_ent() to use the pre-calculated hash value and ignore the SV it requires, setting that SV up costs more than recalculating the hash does -- which is what I was trying to avoid.

What a dumb api.

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)

In the absence of evidence, opinion is indistinguishable from prejudice.

I'm with torvalds on this Agile (and TDD) debunked I told'em LLVM was the way to go. But did they listen!