http://d.puremagic.com/issues/show_bug.cgi?id=5229

           Summary: Inaccurate parsing of floating-point literals
           Product: D
           Version: D1 & D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: wrong-code
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bugzilla@kyllingen.net


--- Comment #0 from Lars T. Kyllingstad <bugzilla@kyllingen.net> 2010-11-17 03:29:34 PST ---
80-bit reals give you roughly 19 decimal digits of precision.  Thus, for a given number, 20 digits should usually be enough to ensure that the literal gets mapped to the closest representable number.

The following program shows that this is not always the case.  Here, 23 digits is needed to get the closest representable number to pi^2, even though the approximation to pi^2 itself is only accurate to 18 digits!

Test case:

    void main()
    {
        // Approximations to pi^2, accurate to 18 digits:
        real closest = 0x9.de9e64df22ef2d2p+0L;
        real next    = 0x9.de9e64df22ef2d3p+0L;

        // A literal with 23 digits maps to the correct
        // representation.
        real dig23 = 9.86960_44010_89358_61883_45L;
        assert (dig23 == closest);

        // 22 digits should also be (more than) sufficient,
        // but no...
        real dig22 = 9.86960_44010_89358_61883_5L;
        assert (dig22 == closest);  // Fails; should pass
        assert (dig22 == next);     // Passes; should fail
    }

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Don <clugdbug@yahoo.com.au> changed:

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--- Comment #1 from Don <clugdbug@yahoo.com.au> 2010-11-17 04:23:43 PST ---
Actually 19 digits works. The thing that's wrong is that the compiler uses _all_ provided digits. Instead, according IEEE754, it should only take the first 19 digits, performing decimal rounding of the l9th digit if more digits are provided.

It's a problem in DMC's standard library implementation of strtold().

I once found a case where adding more decimal digits made the number smaller(!)

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Maxime Chevalier-Boisvert <maximechevalierb@gmail.com> changed:

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--- Comment #2 from Maxime Chevalier-Boisvert <maximechevalierb@gmail.com> 2013-02-14 19:08:40 PST ---
Floating-point parsing also fails on this much shorter number:

4294967295.0
parses to:
4294967296.0

When parsed using:
double val = to!(double)(numStr);

Also fails with formattedRead.

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--- Comment #3 from Lars T. Kyllingstad <bugzilla@kyllingen.net> 2013-02-15 00:44:29 PST ---
Maxime: What you're describing looks like a bug in Phobos, while this report is about a bug in DMD.  You should probably create a new report for that one.

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David Nadlinger <code@klickverbot.at> changed:

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--- Comment #4 from David Nadlinger <code@klickverbot.at> 2013-05-20 14:30:02 PDT ---
@Lars: If she is using DMD on Windows, the root cause might in fact be the same.

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--- Comment #5 from Maxime Chevalier-Boisvert <maximechevalierb@gmail.com> 2013-05-20 14:51:28 PDT ---
(In reply to comment #4)
> @Lars: If she is using DMD on Windows, the root cause might in fact be the same.

It turned out to be a problem with my code. I apologize for not testing thorougly enough before commenting. My comment should be removed from this thread if possible.

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