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- Subject: Re: Lua 5.1 (beta) now available
- From: Rici Lake <lua@...>
- Date: Mon, 21 Nov 2005 13:26:00 -0500
On 21-Nov-05, at 10:45 AM, Mike Pall wrote:
I don't care so much whether NaN can be inserted or not or
whether it can be retrieved or not. But breaking table traversals
is not a good idea IMHO.
I agree with this.
NaN's seem to cause problems for almost all languages, I doubt whether
there is a really good solution which would be acceptable to everyone.
My reading of IEEE-754r is that it does not prohibit the implementation
of an "is identical to" operator which would return true when comparing
a NaN to itself. Unfortunately, such an operator would also return
false when comparing 0 to -0, amongst other cases. That would be
confusing, so a reasonable compromise would probably be to do the
equality test in hash lookups something like this:
if ((a == b) || representation_equal(a, b)) ...
where representation_equal(a, b) is implemented similarly to the
hashing algorithm (i.e. word by word comparison).
As a side-node, the normalization step (a = a + 1.0) in the hashing
algorithm is certainly correct for ieee-754 binary floating point
(although I believe it may fail to normalize decimal floating point
numbers) but it is amazingly slow on Pentiums if given a NaN or an