[erlang-questions] changes in native functions for the next otp release.

Angel Alvarez <>
Mon Jan 25 16:42:56 CET 2010


El Lunes, 25 de Enero de 2010 Richard O'Keefe escribió:
> 
> On Jan 23, 2010, at 11:01 PM, Angel wrote:
> > Iet ab=N
> > let M=(a+1)(b+1);
> >
> > M=ab+a+b+1  that is N +a +b +1  so M  > N
> >
> > Ok but in erlang...
> >
> > Erlang R13B01 (erts-5.7.2) [source] [rq:1] [async-threads:0] [hipe]  
> > [kernel-
> > poll:false]
> >
> > Eshell V5.7.2  (abort with ^G)
> > 1> N=71641520761751435455133616475667090434063332228247871795429.
> > 71641520761751435455133616475667090434063332228247871795429
> >
> > 2> R0=trunc(math:sqrt(N)).
> > 267659337146589062070609641472
> 
> At this point you have said
>     "please, Erlang, calculate a 53-bit APPROXIMATION to the square
>      root of my 197-bit[%] integer, and then chop that back to the
>      next smaller integer."
> [%] I haven't counted the bits exactly, this is ceiling(59*(10/3)).
> 
> The integer square root of N is
> 267659337146589069735395147283    and Erlang returned
> 267659337146589062070609641472
>                   ^^^^^^^^^^^^^
> which is NOT the integer square root of N, because YOU NEVER ASKED
> FOR THE INTEGER SQUARE ROOT OF N.  Convert the integer square root
> of N to a floating point number and you get
> 26765933714658906.............
> which is as accurate as math:sqrt/1 can possibly make it,
> because that's as precise as an IEEE double-precision floating
> point number (which is what math:sqrt/1 is DEFINED to return)
> can possibly be.
> 
> > where is the problem? That's erlang or the gmp library?
> 
> Neither.  The problem is that you are confusing a DOUBLE PRECISION
> FLOATING POINT square root (which is what math:sqrt/1 is) with an
> INTEGER SQUARE ROOT (the integer-sqrt function in Gambit Scheme).
> Erlang is correctly doing *precisely* what you told it to do.
> >
> 
> > this was on R13B1 i think (have to re-test on current versión) is  
> > still
> > broken.
> 
> Nothing is broken.  As far as I know, Erlang doesn't offer, and
> nowhere claims to offer, an integer square root function.  What is
> broken is the belief that math:sqrt/1 is such a function.
> 
> You will have to program your own integer square root function.
> >
> >
> > Im probing some supeesecrete variations of "fermat little's theorem"  
> > on RSA
> > numbers clearly is a problem if native math:sqrt still broken and port
> > latencies (serialitation and parsing) of my actual code begin to  
> > annoy me.
> 
> You have not yet shown that an integer square root function programmed
> in Erlang is too inefficient to use.  While truncate(math:sqrt(N)) may
> not be the right answer, it is surely a good start for a (careful!)
> Newton-style iteration.
> >
> >
> 
> 
Ok! Ok! My fault!!

Sometime we dont see some BIG elefants in front of us... 

C Port was easy at that moment, a kind "Welcome Back to your old habits!!"
for the insecure newbie across the erlang lands... and a lazy gmp use. 


Thanks!!


/angel
-- 
Este correo no tiene dibujos. Las formas extrañas en la pantalla son letras.
__________________________________________

Clist UAH a.k.a Angel
__________________________________________
Sex is a battle, love is war.


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