>> Lua 5.3.4 Copyright (C) 1994-2017 Lua.org, PUC-Rio

>>> print(1e97 % 97)

>> 3.0

>

> Lua 5.1/5.2 computed the module (a%b) with (a - floor(a/b) * b),

> which may introduce other rouding errors. Lua 5.3 computes the module

> with 'fmod'; at least in my machine, the only error is the one in the

> representation of 1e97 itself (see my other messages).

>

> -- Roberto

I wondered how math.fmod work without arbitrary precision math.

Is above example just lucky ?

Just find out ... it is not luck !

If there is no strtod error, math.fmod is correct.

The algorithm is elegant, without multiplication or division:

https://eli.thegreenplace.net/2018/computing-remainders-by-doubling/