Issue #6065 has been updated by Martin Bosslet.


Yusuke Endoh wrote:
>  I agree with akr; Martin's proposal depends on the type long, which
>  is not "portable" enough.  For designing the APIs, I think we should
>  refer gmplib, which is a giant in this area.
>  
>  http://gmplib.org/manual/Integer-Import-and-Export.html
>  

Having something like that would of course be pure luxury :) I really
like it because it is friendly to clients in the sense that
they can simply specify any format they need in their particular
case and won't have to bother any further with endianness or 
raw data type. If you need longs, you get longs, if you need unsigned
char, you can also have that - sounds very appealing to me.

Although this will be more work to implement I would assume this
is the perfect solution indeed? What do you think?

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Feature #6065: Allow Bignum marshalling/unmarshalling from C API
https://bugs.ruby-lang.org/issues/6065

Author: Martin Bosslet
Status: Assigned
Priority: Normal
Assignee: Kenta Murata
Category: core
Target version: 2.0.0


Currently, there's no public C API to create a Bignum. 
There is rb_big_pack and rb_big_unpack that will do the
job, but they are not portable.

Could we offer public functionality that is independent 
of the internal representation for the task of 
marshaling/unmarshalling a Bignum to raw C data types?

I'd like to propose something like

- creating a bignum:

  VALUE rb_big_from_ary(unsigned long *longs, size_t num_longs, int signed)

- retrieving a representation of a Bignum (longs are allocated):

  size_t rb_big_to_ary(VALUE big, unsigned long **longs, int *signed)

For getting a representation, rb_big2str could also be used,
but the above would simplify things when developing an extension
that is in need of Bignum support.

Names and signatures are of course open for discussion,
the example should just serve as an indication of what 
I'm aiming at.

To avoid ambiguity, it would have to be defined how 
the longs are ordered and how the signed flag is to be
interpreted - I would suggest a very simple
representation: Let "longs" be the representation of
the absolute value of the bignum in little- or big-endian
order, where each of the longs themselves should probably
be in the same order, in order to eliminate ambivalence.
Signed is either 0 or 1, so no two's complement or anything
involved. 

I would volunteer to provide a patch for this if we would
agree on something.


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