[MacRuby-devel] Strange NSDate behavior building 32 bit v 64 bit (Richard Sepulveda)

[macruby at djc.net]

Thanks for looking into that Vincent.
My 2 cents, for what it is worth:

- NEVER use floating point for equality checks. Ever.
    Not only is the precision misleading (Radix 2 - isms/ etc.), but 32 vs. 64 
... etc. get in the way, quite often, in fact
- If NSDate in particular, "always use NSDate methods of time calculations 
whenever possible"
else
- to_i (as Vincent lists)


On Wed, 2 Feb 2011 08:20:40 +0900
  Vincent Isambart <vincent.isambart at gmail.com> wrote:
>> My conclusions,
>> - 0.7 and 0.9 fix return the correct time now, 0.9 fix can't do necessary 
>>time arithmetic
>> - Ruby Time.to_f is broken in all 3 versions
>> - Bignum.to_f is broken in all 3 versions
>> - I believe the to_f method is behind all of these problems but i haven't 
>>had time to find the problem nor
>> a solution.
> 
> Your problems with #to_f are due to the fact that 32-bit MacRuby stores 
>floats in 30 bits (MRI stores them in objects in 64 bits,  64-bit MacRuby 
>stores them in 62 bits). The reason is mainly speed related (and we also 
>don't have to garbage-collect them). At some time I think we had 32-bit mode 
>using objects for storing them like MRI, and 64-bit mode using 62 bits but we 
>decided to handle them the same way in both modes to have less code to 
>maintain.
> 
> Seeing that very few people using 32-bit (and we will probably stop 
>"supporting" it after 0.9), you will never get the precision you want from 
>them. What you can do with MacRuby's current state though:
> - if you don't need sub-second precision, use #to_i and do computations in 
>integers (you can us Time.at to bring them back in Time instances),
> - if you really need sub-second prevision, you would have to use BigDecimal. 
>To convert a Time into BigDecimal: BigDecimal.new("%d.%06d" % [time.tv_sec, 
>time.tv_usec]). To convert a BigDecimal back to time: Time.at(bd.to_i, (bd * 
>1000000).to_i % 1000000)
> _______________________________________________
> MacRuby-devel mailing list
> MacRuby-devel at lists.macosforge.org
> http://lists.macosforge.org/mailman/listinfo.cgi/macruby-devel