Deep recursion on subroutine with bignum and Astro::Units

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I'm doing some stuff with the module Astro::Units whose pod suggests to explicitly use bignum. The program works fine until I try doing some simple arithmetic. I can't reproduce the error with an SSCE so here are the errors and the calculations causing them:

Deep recursion on subroutine "Math::BigFloat::bdiv" at /5.26.2/Math/BigInt.pm line 1867.
Deep recursion on subroutine "Math::BigInt::Calc::_lsft" at /5.26.2/Math/BigFloat.pm line 1948.
Deep recursion on anonymous subroutine at /5.26.2/Math/BigInt/Calc.pm line 1412.
Deep recursion on subroutine "Math::BigInt::bdiv" at /5.26.2/Math/BigInt.pm line 54.

Here's the math and the results when loading and not loading (but not directly using) both bignum and Astro::Units:

perl -le '$Da=406700; $Dp=356400; $Dm=361918; $pct = ($Da - $Dm) / ($D
+a - $Dp); print $pct'
[download]

0.890298210735586

perl -Mbignum -le '$Da=406700; $Dp=356400; $Dm=361918; $pct = ($Da - $
+Dm) / ($Da - $Dp); print $pct'
[download]

0.8902982107355864811133200795228628230616

perl -Mbignum -MAstro::Units -le '$Da=406700; $Dp=356400; $Dm=361918; 
+$pct = ($Da - $Dm) / ($Da - $Dp); print $pct'
[download]

0.8902982

perl -MAstro::Units -le '$Da=406700; $Dp=356400; $Dm=361918; $pct = ($
+Da - $Dm) / ($Da - $Dp); print $pct'
[download]

0.890298210735586

Can anyone see what's going on here? That third result is puzzling. What could be causing such errors? Thanks!

Comment on Deep recursion on subroutine with bignum and Astro::Units Select or Download Code

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Re: Deep recursion on subroutine with bignum and Astro::Units by hv (Prior) on Jan 21, 2023 at 12:02 UTC
On a general note, I consider it unwise of the module author to use `bignum` and doubly-unwise for them to recommend that everyone using their module should do so. Each of `bignum, bigint, bigrat` can be fine and useful for simple one-liners, but for anything more complex they are very likely to cause problems. For this specific issue, note that the current Math::BigInt maintainer has made wide-reaching changes multiple times in recent months in an attempt to make the interactions between the different classes behave more sanely in the context of `use bignum`; it is not clear to me whether it has now reached a stable point. So you may want to try both more recent and older versions of the distribution - check the changelog for references to "upgrading and downgrading". For getting the calculations you are using to work reliably, I suggest ditching `bignum`; instead instantiate your numbers as `Math::BigFloat`, and hope that the use of `bignum` inside `Astro::Units` doesn't result in unexpected conversions to `Math::BigInt` or `Math::BigRat`. Note that you should also be able to change the `Math::BigFloat` configuration after loading `Astro::Units`, if the latter's choices are not suitable for your needs.	[reply] [d/l] [select]
Re^2: Deep recursion on subroutine with bignum and Astro::Units by Anonymous Monk on Jan 21, 2023 at 17:38 UTC
Thank you Monks! I'm still struggling to isolate the problem to an SSCE but did make an interesting discovery. Turns out the math is not causing the error. I though it was because commenting that line out eliminated it. Just before doing the math I use "int" on one of the numbers. If I don't use "int" the program works! It also works if "int" is used and "use bignum" is commented out AND this is done: `Math::BigFloat->config({ div_scale => 1 });` (don't need to use Math::BigFloat, cause I guess Astro::Units already loaded it). I have no idea what is going on but will just not use "int". Hope this makes sense. Thanks again!	[reply]
Re: Deep recursion on subroutine with bignum and Astro::Units by soonix (Chancellor) on Jan 21, 2023 at 10:10 UTC
I added an issue to https://github.com/spajai/Astro-Units/issues/1 assuming that this is the original developer's repo.	[reply]
Re: Deep recursion on subroutine with bignum and Astro::Units by karlgoethebier (Abbot) on Jan 21, 2023 at 13:52 UTC
I always thought it was like converting miles to kilometers. You don't really need a Perl module for that. Does not make a big difference whether AU or nautical miles, light years or whatever - except the size of the numbers. But I am only a hobby astronomer and anyway very bad in math. ŤThe Crux of the Biscuit is the Apostropheť	[reply]
Re^2: Deep recursion on subroutine with bignum and Astro::Units by Anonymous Monk on Jan 22, 2023 at 13:09 UTC
I always thought it was like converting miles to kilometers. You don't really need a Perl module for that. Thanks for the reality check. Converting AU to KM is easier than using the module. Multiply by 149597870.7 and done.	[reply]
Re: Deep recursion on subroutine with bignum and Astro::Units by Anonymous Monk on Jan 21, 2023 at 07:35 UTC
See bug report, this Astro::Units won't even install with modern Math::BigInt. I don't know what's the purpose of line 12, and why setting div_scale to 0 with older Math::BigInt results in 7 digits you observe in your third case, but that's direct consequence. I'd suggest installing latest Math::BigInt, fixing line#12 (change value to 1? don't know) and see if deep recursion problem goes away.	[reply]
Re: Deep recursion on subroutine with bignum and Astro::Units by Anonymous Monk on Jan 21, 2023 at 05:07 UTC
OP again. Perl 5.26.2 with module versions: Astro::Units 1.10 (No other versions) bignum 0.47 Math::BigInt 1.999816	[reply]
Re^2: Deep recursion on subroutine with bignum and Astro::Units by smarthacker67 (Beadle) on Feb 06, 2023 at 17:23 UTC
I will fix the lib, keep the thread active.	[reply]
Re: Deep recursion on subroutine with bignum and Astro::Units by Anonymous Monk on Jan 21, 2023 at 03:36 UTC
OP here. Forgot to mention after the errors Perl uses a lot of CPU for about 10 seconds and dies with: Segmentation fault: 11	[reply]