more numeric constants please (especially EPSILON)

Jeff Walden jwalden+es at MIT.EDU
Tue Jul 9 12:13:05 PDT 2013


I'm only commenting on the proposals that seem to be in the current draft, because I'm reviewing a patch that adds only those particular constants.  :-)  Just to be clear why I'm saying nothing about the other constants, neither to praise nor to disparage.

On 03/09/2012 08:00 PM, Roger Andrews wrote:
> Number.EPSILON == 2^-52
> The difference between 1 and the smallest value >1 that is representable as a floating-point number.

Why pick this particular epsilon?  Why not, say, 2**-1074 instead, as the difference between 0 and the next largest number?  Seeing only the name I'd have guessed 2**-1074.

> Number.MAX_INTEGER == 2^53 - 1
> The maximum integer value that can be stored in a number without losing precision.
> (OK, so technically 2^53 can be stored, but that's an anomaly.)

Why discount the anomaly?  Looking at SpiderMonkey's source code, we have <http://mxr.mozilla.org/mozilla-central/search?string=%3C%3C%2053> as vaguely representative of most of the places using a number like this, I think -- could be others not using the "<< 53" string, but that's probably a fair sample.  Ignore the RNG_DSCALE one, that's a red herring.  But all the others use 2**53 as the pertinent value.  (The dom/bindings/PrimitiveConversions.h hits using 2**53 -1 is a bug, I'm told, due to recent spec changes.)  So if this constant is to exist, and I think it's a fair constant to add, why would it not be 2**53?

Jeff


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