From: Tom Werner <pubsub@...>
Date: 2007-09-27T03:49:29+09:00
Subject: Re: Best way to create Bignum from C extension

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