http://d.puremagic.com/issues/show_bug.cgi?id=6531

           Summary: assertion failure in std.range.iota
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Severity: major
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: andrej.mitrovich@gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2011-08-19 07:49:29 PDT ---
import std.range;
import std.stdio;

void main()
{
   auto r1 = iota(0.0, 4.0, 0.03);  // ok
   auto r2 = iota(0.0, 3.0, 0.03);  //
core.exception.AssertError@std.range(4001): Assertion failure
}

I want a range in steps of 0.03, beginning at 0.0 and ending at the closest point to 3.0.

Line 4001 is:
assert(start + count * step >= end);

There's no documentation, and no custom assert message, so I don't know what was the intention of the author.

It doesn't help that this fails in debug mode but works fine in release mode.

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Yao Gomez <yao.gomez@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |yao.gomez@gmail.com


--- Comment #1 from Yao Gomez <yao.gomez@gmail.com> 2012-02-06 12:48:21 PST ---
With DMD 2.058HEAD the assertion is now at line 4136.

Doing a little bit of debugging, it seems that the line at 4135:
---
if (pastEnd < end) ++count;
---
is the culprit. With the r1 range, the count variable is correctly updated, as both pastEnd and end are different.

But with the failling range (r2), pastEnd and end have the same value!, and thus the count is not correctly updated.

Changing the previous code to:
---
if (pastEnd <= end) ++count;
---
seems to do the trick, and passes all the std.range unit tests. But I don't know if that's the correct fix.

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--- Comment #2 from Yao Gomez <yao.gomez@gmail.com> 2012-02-06 12:51:13 PST ---
I have a patch with this solution, but I haven't pulled it yet, because I need more input in whether this is the right fix or not.

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dawg@dawgfoto.de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dawg@dawgfoto.de
         Depends on|                            |7455


--- Comment #3 from dawg@dawgfoto.de 2012-02-06 20:27:53 PST ---
The correct fix is to narrow the lhs of the comparison.
assert(cast(Value)(start + count * step) >= end)

It is a common source of bugs that the intermediate 80-bit result is used for temporaries.

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--- Comment #4 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-04-05 10:34:42 PDT ---
(In reply to comment #3)
> The correct fix is to narrow the lhs of the comparison.
> assert(cast(Value)(start + count * step) >= end)

That doesn't fix the issue.

Yao Gomez'es solution does, but I don't know if it's a proper solution.

Yao, you could try making a pull request and see if you get more comments on github. Issue 9877 might be a duplicate too.

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--- Comment #5 from Martin Nowak <code@dawg.eu> 2013-04-06 01:47:07 PDT ---
> if (pastEnd <= end) ++count;

This is definitely wrong. There are only 100 steps in [0, 0.03 ..., 3.0). The problem was that assert(0.0 + 100 * 0.03 >= 3.0) fails because of floating-point excess precision.

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drug007 <drug2004@bk.ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |drug2004@bk.ru


--- Comment #6 from drug007 <drug2004@bk.ru> 2013-04-11 07:13:10 PDT ---
I propose the following:

    auto pastEnd = start + count * step;
    if (step > 0)
    {
        if (pastEnd < end) {
            ++count;
            pastEnd = start + count * step;
        }
        assert(pastEnd >= end);
    }
    else
    {
        if (pastEnd > end) {
            ++count;
            pastEnd = start + count * step;
        }
        assert(pastEnd <= end);
    }

Martin Nowak's fix is the better (simpler and more clear) but I just don't like
cast.

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--- Comment #7 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-04-11 08:13:26 PDT ---
(In reply to comment #6)
> Martin Nowak's fix is the better (simpler and more clear) but I just don't like
> cast.

I've tried his fix and it didn't work for me.

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--- Comment #8 from drug007 <drug2004@bk.ru> 2013-04-11 08:21:21 PDT ---
(In reply to comment #7)
> (In reply to comment #6)
> > Martin Nowak's fix is the better (simpler and more clear) but I just don't like
> > cast.
> 
> I've tried his fix and it didn't work for me.

Try mine. It works for me (with your example).

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safety0ff.bugz@gmail.com changed:

           What    |Removed                     |Added
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                 CC|                            |safety0ff.bugz@gmail.com


--- Comment #9 from safety0ff.bugz@gmail.com 2013-04-11 09:34:33 PDT ---
Indeed, casting does not seem sufficient to force correct rounding of
intermediate results.
Seems like the solution is to either assign and force rounding, or use
approxEqual with appropriate constants for the error terms.

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