Re: isEqual: and hash in NSDate

[Adrian Robert]

On Jul 22, 2005, at 12:44 PM, Richard Frith-Macdonald wrote:

> On 2005-07-22 15:57:03 +0100 Adrian Robert <address@hidden> 
> wrote:
>
>
> > This really seems like a hack (not that it's any worse than the 
> > current state :).  Couldn't the implementations of -hash and -isEqual 
> > be aligned without this loss of information?  E.g. something like 
> > [warning, sloppy first-attempt code here]:
> > -isEqual:other
> >   return abs(selfVal - other->selfVal) < epsilon;
> > -hash
> >   return selfVal / epsilon;
> > (or maybe (selfVal + epsilon/2.0) / epsilon)
>
> No ... each NSDate has to have its own independent hash ... but 
> (following the OpenStep spec) dates are equal if they are within a 
> second of each other.
>
> That means that with dates d1, d2, d3 which have offsets from the 
> reference date of 0.0, 0.7 and 1.4 seconds,
> d1 is eqal to d2 and d2 is equal to d3 but d1 is not equal to d3 ... a 
> rather counterintuitive behavior!

Right, bad.  Thisd could be fixed by changing -isEqual: to simply

return [self hash] == [other hash]

I don't see anything wrong conceptually with stipulating that dates 
within some epsilon are isEqual and retrieve the same values as keys in 
a hash table, though this might preclude some practical uses.  However 
OS X compatibility seems the better way to go at this point.

(While we're on this subject, could we change NSEvent timestamps to be 
in seconds like OS X (and OpenStep for that matter), instead of 
milliseconds?)