From: Nilesh Trivedi <nilesh.tr@...>
Date: 2009-08-25T15:24:10+09:00
Subject: Re: doubling numbers with Object#object_id

AFAIK, FixNums are implemented as direct values and not C structs that
represent objects. Small integers - which means integers that can be
held in (sizeof(unsigned long int) * 8 -1 = 31 bits). 1 bit is used
for sign and 30 bits for the value.

Here is the INT2FIX macro from Ruby source:
 #define INT2FIX(i) ((VALUE)(((long)(i))<<1 | FIXNUM_FLAG))

Shift 1 bit to the left, and bitwise OR it with 1. In other words,
object_id = 2n + 1. That explains why your double method works.

cheers
nilesh

On Tue, Aug 25, 2009 at 11:36 AM, EbFFA<eflatffa@gmail.com> wrote:
> I was playing with Ruby and wrote an interesting method:
>
>    def double n
>      n.object_id - 1
>    end
>
>    irb> double 5
>      => 10
>    irb> double 1
>      => 2
>    irb> double 0
>      => 0
>    irb> double -101
>      => -202
>    irb> double 100000001
>      => 200000002
>    irb> double 10000000000    # into Bignum territory now
>      => -606548539
>
> Anyone know why this works? Why Ruby was implemented this way?
>
> I tried creating an object_id collision based on this doubling rule,
> but it seems that all Fixnum object_id`s are odd, and all other
> object_id`s are even (in fact they all seem to end in 8). Clever.
>
>