From: GOTO Kentaro <gotoken@...>
Date: 2002-09-16T10:49:47+09:00
Subject: Re: Time#+ and usec

At Sun, 15 Sep 2002 09:02:57 +0900,
Michael Campbell wrote:
> > (Time.at(0) + .1).usec # => 100000
> > (Time.at(0) + .2).usec # => 200000
> > (Time.at(0) + .3).usec # => 299999
> > (Time.at(0) + .4).usec # => 400000
> 
> Looks to me like your standard "base 10 numbers can't necessarily be
> represented exactly in a base 2 system" issue.

Yep.  For example, 

% ruby -rieee754hack -e'[0.1,0.2,0.3,0.4,0.5].each{|i| puts i.human_readable}'
+1 * 0b1.1001100110011001100110011001100110011001100110011010 * 2**(1019-1023)
+1 * 0b1.1001100110011001100110011001100110011001100110011010 * 2**(1020-1023)
+1 * 0b1.0011001100110011001100110011001100110011001100110011 * 2**(1021-1023)
+1 * 0b1.1001100110011001100110011001100110011001100110011010 * 2**(1021-1023)
+1 * 0b1.0000000000000000000000000000000000000000000000000000 * 2**(1022-1023)
% 

But my FreeBSD box reports 100000, 200000, 300000 and 400000 for
(Time.at(0)+0.1).usec etc.  The result seems to depend on the
implementation of modf(3) which is used in time.c:time_plus().

-- Gotoken