what's the reasonale of loghelper() in mathmodule.c

[rockins]

On Apr 9, 9:11 pm, Mark Dickinson <dicki... at gmail.com> wrote:
> On Apr 9, 4:38 am,rockins<ybc2... at gmail.com> wrote:
>
> > I cannot understand it well, can anyone explain me why and how
> > loghelper() can compute any base logarithm? Or could anyone give me
> > some reference(such as, books or papers)?
>
> loghelper is there so that log(n) can be computed for any positive
> integer n---it's nothing to do with computing logs to an arbitrary
> base.
>
> All of the other math functions convert an integer argument to a float
> first.  That conversion fails if the integer is larger than the
> largest
> representable float (around 1.7e308 on most systems).  For example:
>
> >>> from math import sqrt, log
> >>> sqrt(10**600)
>
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> OverflowError: long int too large to convert to float>>> log(10**600)
>
> 1381.5510557964274
>
> The sqrt call first tries to convert 10**600 to a float, giving an
> OverflowError (even though the actual square root *is* representable
> as a float).  The log call goes through loghelper instead, which
> doesn't try to convert 10**600 to a float, but instead computes
> the log based on the top few bits of 10**600 (in its internal
> binary representation) and on the number of bits required to
> represent 10**600.
>
> You're not going to learn much about math function implementations
> from mathmodule.c:  all it does it wrap the platform libm functions.
>
> Mark

Thanks Marks, your explanation is very clear. Yes, I do not need to
learn much about the math functions' implementation of libm, I just
need to know why there exists loghelper() in mathmodule.c; I have
checked Python's longobject.c, I think I have understood why
loghelper() is needed now. Since Python can represent arbitrary
precision long integer object, so for logarithm it indeed need to deal
with very huge number. Doesn't it? If there's any misunderstanding of
mine, please point out. Thank you very much.

-rockins