PerlApi: hashes

BrowserUk has asked for the wisdom of the Perl Monks concerning the following question:

in Re: Perl Hashes in C?, I construct a hash using the perlapi from Inline::C.

Two of the apis, hv_fetch() and hv_store() optionally take a numeric hash value which if supplied is used directly rather than the key string and length being (re)hashed.

I assume that idea is that once you've hashed a key once, it saves rehashing it; but the fly in that ointment is that there does not seem to be any api to obtain the hash value?

Also, there does not appear to be an equivalent option to supply the hash value to hv_exists()?

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!

Comment on PerlApi: hashes

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Re: PerlApi: hashes by Anonymous Monk on Aug 12, 2015 at 02:32 UTC
http://perldoc.perl.org/perlapi.html#hv_exists_ent bool hv_exists_ent(HV hv, SV keysv, U32 hash)	[reply]
Re^2: PerlApi: hashes by BrowserUk (Patriarch) on Aug 12, 2015 at 04:28 UTC
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. 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!	[reply]
Re^3: PerlApi: hashes by Anonymous Monk on Aug 12, 2015 at 06:40 UTC
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	[reply] [d/l]
Re^4: PerlApi: hashes by BrowserUk (Patriarch) on Aug 12, 2015 at 12:38 UTC
Re^5: PerlApi: hashes by NERDVANA (Priest) on Jul 11, 2018 at 04:02 UTC
Re^2: PerlApi: hashes by Anonymous Monk on Aug 12, 2015 at 02:38 UTC
http://perldoc.perl.org/perlapi.html#HeHASH Returns the computed hash stored in the hash entry. U32 HeHASH(HE* he) http://perl5.git.perl.org/perl.git?a=search&h=HEAD&st=grep&s=HeHASH	[reply]
Re^3: PerlApi: hashes by BrowserUk (Patriarch) on Aug 12, 2015 at 04:32 UTC
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!	[reply]