From: Tom M <thomas.macklin@...>
Date: 2007-09-27T04:45:03+09:00
Subject: Re: Best way to create Bignum from C extension

On Sep 26, 2:49 pm, Tom Werner <pub...@rubyisawesome.com> wrote:
> Tom M wrote:
> > On Sep 25, 1:32 pm, Tom Werner <pub...@rubyisawesome.com> wrote:
>
> >> I've been hand-translating some Ruby code into C for an extension and
> >> have come across the need to create and return a Bignum from the
> >> extension. I see that there is a macro INT2NUM that will produce a
> >> Bignum from long or long long, but what is the best way to create an
> >> arbitrarily sized Bignum from a byte array packed as:
>
> >> [a, b, c] -> (a * (256 ** 0)) + (b * (256 ** 1)) + (c * (256 ** 2))
>
> >> in other words, reversing the array and flatting it would produce the
> >> correct binary representation of the number.
>
> >> I tried creating a Fixnum with INT2FIX(0) and then using rb_funcalls to
> >> incrementally construct a Bignum, but that approach 1) didn't seem to
> >> work past WORD sized numbers and 2) seems like it would be awfully slow.
>
> >> --
> >> Tom Preston-Werner
>
> >> * Libraries:
> >>     Chronic (chronic.rubyforge.org)
> >>     God (god.rubyforge.org)
> >>     Fuzed (fuzed.rubyforge.org)
> >> * Site:
> >>     rubyisawesome.com
>
> > dumb question probably, but could you use scientific notation style
> > numbers; i.e., 1.076 * 10^10?  You'd return a base as a float and an
> > exponent as a fixnum, and then you could calculate it on the fly...
>
> No, the reason I need to do this is to read in Erlang's packed data
> representation. Interestingly enough, that *is* how Erlang transmits floats!
>
> --
> Tom Preston-Werner
>
> * Libraries:
>     Chronic (chronic.rubyforge.org)
>     God (god.rubyforge.org)
>     Fuzed (fuzed.rubyforge.org)
> * Site:
>     rubyisawesome.com

something like breaking up the string into substrings and then calling
unpack on each substring, then reassemling... think 'get_more_digits'
since you're already inclined towards Erlang...