From: ptkwt@... (Phil Tomson)
Date: 2005-02-07T20:25:11+09:00
Subject: math changes from 1.8.1 to 1.8.2?


I've got some code that does simulated fixed-point math and things like 
exp(-x).  I find that my simulations work fine on a Windows XP box, 
Pentium IV, Ruby 1.8.2.  However, when I run the same exact Ruby code on my 
Powerbook G4 running Ruby 1.8.1 I get lots of errors from my unit tests.

I suspect that what I'm doing is really stressing the floating point 
operations so it could just be a difference between the G4 and Pentium 
FPUs, but I'm also wondering if there were changes between Ruby 1.8.1 and 
Ruby 1.8.2 that might account for this (even changes to the way Bignum 
works, might account for it).  I'll load up Ruby 1.8.2 on the Powerbook 
tonight to see if that makes any difference...


For the curious, I created a class called FixedPt which allows you 
simulate fixed point math (useful for hardware design).  So you can do 
things like:

fp = FixedPt.new(Math.exp(-5),18,18)

Which defines an 18bit register with the binary point at bit 18 and stuffs 
the result of Math.exp(-5) into it (shifting the result to the left 18 
times).

fp.to_f #=> 0.00673675537109375
fp.raw  #=> 1766  
fp.to_i #=> 0


Phil