From: Paul Brannan <pbrannan@...>
Date: 2007-09-29T00:03:33+09:00
Subject: Re: Best way to create Bignum from C extension

On Wed, Sep 26, 2007 at 02:32:08AM +0900, Tom Werner 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.

Take a look at the rb_big_* functions declared in intern.h.  I think you
can work something out using them.

Paul