Re: Numeric key collision related bug in Lua 5.3

[Tim Hill <drtimhill@...>]

> On Apr 21, 2015, at 1:23 PM, Liam Devine <liamdevine@oolua.org> wrote:
> 
> On 21/04/15 21:04, Andrew Starks wrote:
> 
>> It would add tremendously to my understanding if someone someone would
>> make up a story that includes a user in a real-world scenario hitting
>> this edge case. ...
>> 
>> 
>> 
>> -Andrew
>> 
> 
> Correction to my earlier email a this does not just effect rawequal.
> 
>> a, b = math.maxinteger + 2.1, math.maxinteger + 3.5
>> print(math.type(a), math.type(b))
> float	float
>> return a == b
> true
> 
> 

This is expected behavior. math.maxinteger is 2^63-1. An IEEE754 double-precision float has 53 bits for the mantissa. Since your integer has a magnitude of approx 2^63, only the most significant 53 bits are stored (along with an appropriate exponent) and the lower 10 bits are lost to rounding errors (again, as expected). So the smallest delta that will actually generate a different float value is approx 2^10, or 1024..

mi = math.maxinteger
a, b = mi + 1024, mi + 2048
print(a==b)	-> false

—Tim