round trip issue

[matunia]

during tests on random values I've found several cases where the generated string won't do the round trip to the original value e.g.:

431.45061948654927 -> 431.45061948654933
27.201309524416448 ->27.201309524416452
...

[rotanov]

Indeed those two do not pass.
I also stumbled upon this with 0.51211952535068594.

I fixed running nunit tests with vs2022 here https://github.com/rotanov/grisu.net/tree/fix_nunit
and added your (@matunia) test cases plus mine here https://github.com/rotanov/grisu.net/tree/nonpassing_test_cases

Also last time I noted this it seemed that it's was only possible to reproduce on:

• Ryzen 7 3700X
• Ryzen 9 3900X

But it didn't fail on Ryzen 7 2700X.

And now it fails again on i7-11800H.

Also at one point I thought maybe if something wrong with Double.Parse, maybe something CPU dependent. Now I don't remember why I thought so and no longer sure if issue is CPU dependent or not. Just in case I tried compiling everything with .NET7 and that didn't fix the issue.

Judging by pull requests may be @shunter can help somehow?

[kring]

It's probably worth checking the implementation against the original C++ implementation, especially if there were changes since it was ported. I'm afraid I haven't used this project for several years now, so I don't have any particular insight into what might be wrong.

[rotanov]

Alright, this seems to be the valid mirror for the original C++ implementation (https://github.com/stlab/double-conversion). I'll return with a pull request if I manage to locate the issue when I have time for that. Thanks!

[rotanov]

I compiled original C++ grisu, run my test case and it's exhibits the same behavior as C# port:

But! strtod parses 0.512119525350686 back to 0.51211952535068594 and the first one is indeed shorter. And Double.Parse in C# does not:

[rotanov]

Maybe the behavior in Double.Parse is related to this issue: dotnet/runtime#4406.

edit: I 95% sure it's the source of the issue. Resolution is to use IEEE compliant double parser instead of C#'s Double.Parse.