# Best way to create Bignum from C extension **URL:** \ **Category:** ruby-talk\ **Created:** [25 September 2007 17:32 UTC](https://rubytalk.org/t/best-way-to-create-bignum-from-c-extension/40913 "2007-09-25T17:32:08Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Tom\_Werner](https://avatars.discourse-cdn.com/v4/letter/t/e9c0ed/32.png) [@Tom\_Werner](https://rubytalk.org/u/Tom_Werner)\ **Post date:** [25 September 2007 17:32 UTC](https://rubytalk.org/t/best-way-to-create-bignum-from-c-extension/40913/1 "2007-09-25T17:32:08Z")
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](http://chronic.rubyforge.org)) >     God ([god.rubyforge.org](http://god.rubyforge.org)) >     Fuzed ([fuzed.rubyforge.org](http://fuzed.rubyforge.org)) > \* Site: >     [rubyisawesome.com](http://rubyisawesome.com) ---
**Author:** ![Tom\_M](https://avatars.discourse-cdn.com/v4/letter/t/839c29/32.png) [@Tom\_M](https://rubytalk.org/u/Tom_M)\ **Post date:** [26 September 2007 18:40 UTC](https://rubytalk.org/t/best-way-to-create-bignum-from-c-extension/40913/2 "2007-09-26T18:40:14Z")
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... > **···** > > On Sep 25, 1:32 pm, 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. > > > > -- > > Tom Preston-Werner > > > > \* Libraries: > >     Chronic (chronic.rubyforge.org) > >     God (god.rubyforge.org) > >     Fuzed (fuzed.rubyforge.org) > > \* Site: > >     rubyisawesome.com ---
**Author:** ![Paul\_Brannan](https://avatars.discourse-cdn.com/v4/letter/p/76d3ee/32.png) [@Paul\_Brannan](https://rubytalk.org/u/Paul_Brannan)\ **Post date:** [28 September 2007 15:03 UTC](https://rubytalk.org/t/best-way-to-create-bignum-from-c-extension/40913/3 "2007-09-28T15:03:33Z")
Take a look at the rb\_big\_\* functions declared in intern.h. I think you can work something out using them. Paul > **···** > > 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. ---
**Author:** ![Tom\_Werner](https://avatars.discourse-cdn.com/v4/letter/t/e9c0ed/32.png) [@Tom\_Werner](https://rubytalk.org/u/Tom_Werner)\ **Post date:** [26 September 2007 18:49 UTC](https://rubytalk.org/t/best-way-to-create-bignum-from-c-extension/40913/4 "2007-09-26T18:49:29Z")
Tom M wrote: > **···** > > > On Sep 25, 1:32 pm, 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. > > > > > > -- > > > 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 ---
**Author:** ![Tom\_M](https://avatars.discourse-cdn.com/v4/letter/t/839c29/32.png) [@Tom\_M](https://rubytalk.org/u/Tom_M)\ **Post date:** [26 September 2007 19:45 UTC](https://rubytalk.org/t/best-way-to-create-bignum-from-c-extension/40913/5 "2007-09-26T19:45:03Z")
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... > **···** > > On Sep 26, 2:49 pm, Tom Werner \ wrote: > > > Tom M wrote: > > \> On Sep 25, 1:32 pm, 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. > > > > \>\> -- > > \>\> 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