From: Tom Werner <pubsub@...>
Date: 2007-09-26T02:32:08+09:00
Subject: Best way to create Bignum from C extension

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

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