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digitalmars.D.bugs - [Issue 6531] New: assertion failure in std.range.iota

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           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



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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Aug 19 2011
next sibling parent d-bugmail puremagic.com writes:
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Yao Gomez <yao.gomez gmail.com> changed:

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



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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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



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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10:34:42 PDT ---


 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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 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



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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08:13:26 PDT ---


 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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 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
----------------------------------------------------------------------------
                 CC|                            |safety0ff.bugz gmail.com



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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Martin Nowak <code dawg.eu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |acehreli yahoo.com



*** Issue 9877 has been marked as a duplicate of this issue. ***

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 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.


Yes, casting is optimized away by dmd.
Walter suggested to use an opaque function or inline asm to enforce rounding to
lower precision.
It seems like C99 addresses this by specifying that casts and assignments need
to be rounded to lower precision.



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Martin Nowak <code dawg.eu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull



https://github.com/D-Programming-Language/phobos/pull/1673

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