From: Nilesh Trivedi <nilesh.tr@...>
Date: 2009-08-25T15:26:21+09:00
Subject: Re: doubling numbers with Object#object_id

All other Ruby objects are allocated on addresses that are multiples
of 4 to avoid clash with FixNum. So Object ID is always even for them.

cheers
nilesh

On Tue, Aug 25, 2009 at 11:54 AM, Nilesh Trivedi<nilesh.tr@gmail.com> wrote:
> 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.
>>
>>
>